Interactive Browsing and Navigation in Relational Databases

# Interactive Browsing and Navigation in Relational Databases

## Abstract

Although researchers have devoted considerable attention to helping database users formulate queries, many users still find it challenging to specify queries that involve joining tables. To help users construct join queries for exploring relational databases, we propose ETable, a novel presentation data model that provides users with a presentation-level interactive view. This view compactly presents one-to-many and many-to-many relationships within a single enriched table by allowing a cell to contain a set of entity references. Users can directly interact with this enriched table to incrementally construct complex queries and navigate databases on a conceptual entity-relationship level. In a user study, participants performed a range of database querying tasks faster with ETable than with a commercial graphical query builder. Subjective feedback about ETable was also positive. All participants found that ETable was easier to learn and helpful for exploring databases.

## 1Introduction

A considerable challenge for non-technical users of relational databases is constructing join queries [?]. The join operation is required for even simple data lookup queries since relational databases store information in multiple separate normalized tables. Although database normalization provides many benefits for managing data (e.g., avoiding update anomalies), it significantly decreases the usability of database systems by forcing users to write many join queries to explore databases.

Constructing join queries is difficult for several reasons. The main reason is that users find it difficult to determine which relations to join among many relations. Understanding the role of each relation that represents a relationship of interest and finding the right join attributes are not trivial tasks, even when a schema diagram is given. To tackle this challenge, users often write complex queries by starting with a simpler query and iteratively adding operators [?]. Although this iterative strategy is helpful, it is still challenging because the format of join query results is hard to interpret. For example, consider a query that joins two relations in many-to-many relationships (e.g., Papers and Authors in Figure 3). A result of this query produces a large number of duplications (e.g., the title of each paper repeated as many times as the number of its authors). People represent the same information differently when they use a spreadsheet. For instance, they might create a cell containing multiple values separated by commas. Relational databases cannot represent data in this way because the relational model (as implemented in most relational DBMSs) requires that data be at least in the first normal form.

The usability challenge of writing complex queries has been studied by many researchers. Although visual query builders help people formulate SQL queries [?], they separate query construction and result presentation parts [?], introducing a usability gap between users’ actions and their results [?]. To overcome this limitation, researchers argue that database interfaces need to adopt the direct manipulation principle [?], a well-known concept in the human-computer interaction (HCI) area [?]. It enables users to iteratively specify operators by directly interacting with result instances using simple interactions [?]. Researchers also argue that join query results should be represented in an easier-to-understand format that improves the interpretation of query results. Jagadish et al. [?] proposed the notion of the presentation data model, which they defined as a full-fledged layer above the logical and physical schema. This presentation layer allows users to better understand the query results without requiring full awareness of the schema. All this research strongly suggests the need for developing database interfaces that are usable, interactive, and interpretable.

We present ETable, a novel presentation data model with which users can interactively browse and navigate databases on an entity-relationship level without writing SQL. ETable presents a query result as an enriched table in which each cell can contain a set of entity references. By deliberately relaxing the first normal form, we compactly represent one-to-many and many-to-many relationships within a single table — a novel capability that enables users to more easily browse and interpret query results consisting of multiple relations. Figure 1 illustrates how ETable effectively presents a list of SIGMOD papers containing the keyword “user” from an academic paper database collected from DBLP and the ACM Digital Library (see Figure 3 for schema). Each row in ETable shows the base attributes and relevant entities of a paper, such as its authors and cited papers. If a relational database were used to obtain the same information, 9 tables would need to be joined, and the results produced would be hard to interpret (e.g., many duplicated cells).

To discover which relevant entities should be shown for each row, ETable uses a novel graph-based model called the typed graph model (TGM), which frees users from concerning themselves with the complexity of the logical schema; users may instead focus on exploring and understanding the data set at the conceptual (or entity-relationship) level. The typed graph model stores relational data as graphs in which nodes represent entities (e.g., authors, papers) and edges represent relationships (e.g., those that relate authors to papers). This transformation enables ETable to retrieve other related entities through simple graph operations. For example, a given paper’s authors, stored as direct neighbors, can be retrieved through a quick neighbor-lookup.

As the construction of complex queries and the exploration of data are inherently iterative processes, database exploration tools should provide easy-to-use operations to help users incrementally revise queries [?]. ETable’s direct manipulation interface enables users to directly work with and modify an existing enriched table to update its associated queries. For example, imagine a user, Jane, who would like to further explore the result in Figure 1. To see the detailed information about the authors of a particular paper, she clicks on its “author count” button (Figure Figure 2-b). This simple interaction of tapping the button is translated into a series of primitive operators behind the scene, such as Select, as in selecting the row associated with a paper; and Add, as in adding and joining the Authors table with the Papers table. With a few rounds of similar interactions, Jane can incrementally build complex queries.

ETable’s novel ideas work together to address an important, often overlooked problem in databases. The seminal vision paper by Jagadish et al. [?] introduced the notion of the presentation data model and argued the importance of direct manipulation interface. However, designing an easy-to-use system that meets these requirements is challenging. ETable is one of the first instantiations of this important idea, filling a critical research gap, by effectively integrating HCI principles to greatly improve database usability. To enable the creation of such a usable tool, ETable tightly integrates: (1) a novel hybrid data model representation, which advances over the relational and nested-relational models, to naturally represent entities and relationships; and (2) a novel set of interactions that closely work with the representation to enable users to specify expressive queries through direct manipulation. With ETable’s user interface, non-experts can easily and naturally explore databases without writing SQL, while ETable internally performs queries under the hood.

Through ETable, we contribute:

• A novel presentation data model that presents a query result as an enriched table for users to easily browse and explore relational databases (Section Section 3, Section 5);

• A graph-based model, called typed graph model (TGM) that provides an abstraction of relational databases, for users to explore data in ETable at a conceptual level (Section Section 4);

• A set of user-level actions, operations that users can directly apply to an enriched table to incrementally construct complex queries and navigate databases (Section Section 6.1);

• The usable interface of ETable that outperforms a commercial graphical query builder in a user study, in both speed and subjective ratings across a range of database querying tasks (Section Section 6, Section 7).

## 2Related Work

### 2.1Database Usability & Query Specifications

Since Query-by-Example (QBE) was developed in 1970s [?], database researchers have studied fairly extensively the usability aspect of database systems [?]. Usability is important, especially because not all database users have expertise in writing complex queries; many non-technical users find it challenging to write even very simple join queries [?]. Many existing approaches are aimed at assisting users with formulating queries. One representative method is the visual query builder, which enables users to visually manipulate schema elements on a graphical interface [?]. However, most visual querying systems require that users have precise knowledge of a schema, which makes it difficult for non-experts to use. This limitation can be relieved in keyword search systems, studied extensively in the last decade [?], or natural language interfaces [?]. However, most of existing approaches [?] separate queries and results so that users cannot directly refine query results, which decreases the usability of the systems. Nandi and Jagadish [?] argued that users’ querying process is often iterative, so database systems should guide users toward interactively formulating and refining queries.

### 2.2Direct Manipulation & Iterative Querying

Several database researchers argued that the usability of database querying systems can improve by adopting the direct manipulation paradigm [?], a well-established design principle in the HCI and information visualization areas. Acknowledging that users’ needs are often ambiguous rather than precisely specifiable, researchers have developed many tools that enable users to interactively browse and explore databases [?]. Although they are not specifically designed for relational databases, a number of interactive visualization systems for entity-relationship data have been developed by information visualization researchers [?]. For example, NetLens [?] visualizes relationships between two selected entity types in many-to-many relationships, and GraphTrail [?] visually summarizes each entity type and enables users to switch between entities. Although these visualization systems provide an overview of data sets, they are not suited for examining database instances along with attributes. In exploring and analyzing instance-level information, tabular interfaces, including spreadsheets, are better suited and often preferred by database users [?]. Tyszkiewicz [?] argued that spreadsheets can play a role as a database engine by using functions and macros. Liu and Jagadish [?] formally defined operators that interactively perform grouping operations within a spreadsheet. However, since the rigid tabular structure does not effectively present many-to-many relationships, the spreadsheet suffers from the same problems that relational databases have (i.e., a large number of duplications). To overcome this limitation, Jagadish et al. [?] proposed using a presentation view layer on top of underlying databases, which is the notion of the presentation data model, defined as a full-fledged layer on top of the logical and physical models. The challenge is to design presentation data models that help people easily understand join query results and interact with them.

### 2.3Data Models for Effective Presentation

To develop an intuitive structure for presentation data models, we review a number of data models that conceptualize the mini-world represented in databases. One such example is the nested relational model, studied in the 1980s, which allows each cell to contain another table that presents one-to-many relationships in a single table [?]. The nested model has been used in several studies for designing database interfaces. Bakke et al. [?] recently designed a direct manipulation interface for nested-relational databases, and DataPlay [?] also used the nested model for presenting query results. However, the model suffers from scalability issues because the sizes of the nested tables often become huge when an inner table contains a large number of associated rows or columns [?]. One way to tackle this problem is to replace the inner table with a set of pointers. For example, the object-relational model lets attributes be user-defined types that include pointers [?]. We adapt this idea by introducing an entity reference which compactly represents related entities. Another class of the data models that effectively conceptualize the real-world is the graph data model [?]. It represents entities as nodes and relationships as edges based on the entity-relationship model [?]. Catarci et al., [?] used a graph-style translation layer for their visual querying system. To provide users with an easy-to-understand view at an entity-relationship level, we also maintain a graph-style model, transformed from relational databases, under the presentation view.

## 3Introducing ETable

Before we describe the technical details of the proposed data models, we introduce ETable by describing what users see and how they can interact with it.

Representation.

Figure 1 illustrates an enriched table that we call Etable. As mentioned earlier, it presents a list of SIGMOD papers containing the keyword “user” from our collected database (see Figure 3 for schema). Each row of Etable represents a single entity of the selected entity type (i.e., Papers); its column represents either a base attribute of the entity (e.g., year) or a set of relevant entities (e.g., authors, keywords). This representation is formed by pivoting a query result of a join of multiple tables (e.g., Papers, Paper_keywords, Authors) to a user-selected entity type (e.g., Papers). One advantage of this representation is that it can simultaneously present all relevant information about an entity in a single row (e.g., authors, keywords, citations). The relational model cannot represent all of this information in a single relation without duplications because every attribute value must be atomic. For instance, when the Papers table is joined with the Authors table, the paper information is repeated as many times as the number of authors, which prevents users from quickly interpreting the results. We integrate information spread across multiple tables into a single table by allowing each cell to contain a set of references to other entities.

Interactions.

Users can interact with Etable to explore further information. For instance, to examine further information about the authors of the papers in Figure 1, users can create a new Etable that lists authors in several ways, as depicted in Figure 2: (1) If users are interested in one of the authors (e.g., Arnab Nandi), they can click on his name to create a new Etable consisting of one row that presents its attributes; (2) if users want to list the complete set of authors (e.g., all seven authors of the paper titled “Making database systems usable”), they can click on the author count in the right corner of the cell (i.e., 7); and (3) if users want to list and sort the entities across the entire rows in a column (e.g., Who wrote the most papers about “user” in SIGMOD?), they can click on the pivot button on the column menu, which groups and sorts the authors based on the number of papers they have written. By gradually applying these operations, users can incrementally make sense of data and build complex queries.

## 4Typed Graph Model

In this section, we define a typed graph model (TGM) which enables users to explore relational databases on a conceptual entity-relationship level without having to know a logical schema. A relational schema and instances are translated into a database schema graph and database instance graph as a preprocessing step, and all operations specified by users on the ETable interface are executed over these graphs, not relational databases.

We represent entities and relationships as a graph with types and attributes. Each entity forms a node, and relationships among the entities become edges. A typed graph database (TGDB) consists of a TGDB schema graph, , and a TGDB instance graph, .

The typed graph model, similar to many graph data models [?], is much more effective for conveying a conceptual understanding of the mini-world represented in databases than the relational model. As it abstracts relational databases, users can ignore the logical and physical representation of data. Users can also easily understand the structure of data, since nodes always represent entities and edges represent relationships, Unlike TGM, the relational model is a mixture of entities, relationships, and multivalued attributes. Although some existing graph models are more expressive for representing a variety of relationships (e.g., hierarchical parent-child relationships among entities), we simply use nodes and edges to focus on making the semantics of the underlying relations more explicit by mapping to entities and relationships that they represent in the real world.

Relational databases can be translated into the TGDB schema and instance graphs in a near-automatic process. We adapt the reverse engineering literature pertaining to translating relational databases into several graph-style models [?]. A detailed procedure presented in Appendix ? includes an analysis of a relational schema based on primary keys, foreign keys, and cardinalities for classifying tables into several categories, and a series of actions that create the schema graph. Table 1 summarizes the categories of node and edge types based on how they are determined from relational schema. Figures Figure 4 and Figure 5 illustrate a schema graph and a part of the instance graph constructed from an academic publication database whose schema is shown in Figure 3.

## 5ETable Presentation Data Model

We present our ETable presentation data model for usable exploration of entities and relationships in databases.

### 5.1Enriched Table

A query result in the ETable model is presented as an enriched table, which we also call ETable. An ETable has a set of columns and consists of a set of rows . The columns are categorized into two types: single-attribute columns and entity-reference columns. The value of the single-attribute column is atomic as it is in the relational model. The value of the entity-reference column contains a single or a set of entity references. The entity reference refers to another node in the database instance graph. Unlike a foreign key in the relational model, each entity reference is shown as a clickable label, similar to a hyperlink on a webpage. Just like how a hyperlink’s hypertext describes the webpage that the link points to (instead of its URL), for example, ETable represents an author’s entity reference by the author name (instead of the author ID).

The entity-reference columns present rich information spread across multiple relations within a single enriched table. While a foreign key attribute in the relational model contains only a single reference for a many-to-one relationship because of the first normal form, an entity-reference column can represent one-to-many relationships, many-to-many relationships, or multivalued attributes in a single column. Furthermore, the entity-reference column has advantages over the nested relational model which requires much screen space as it squeezes another table into cells, leading to inefficient browsing. Unlike the nested model, ETable presents clickable labels that compactly show information and allow users to further explore relevant information.

### 5.2ETable Specification

An ETable can be specified by selecting specific elements of the TGDB database schema and instance graphs introduced in the previous section.

A query pattern can be represented as an acyclic graph where one of the nodes is marked as a primary node type and any node can have selection conditions. For example, the query pattern in Figure 6 represents a query that produces a list of researchers who have published papers at SIGMOD after 2005 and are currently working at institutions in Korea.

### 5.3Incremental Query Building with Primitive Operators

In ETable, a query pattern can be constructed and updated by primitive operators. Each operator builds on an existing ETable query to generate a new, updated ETable query. In this subsection, we describe these operators in detail. In Section 6.1, we will describe how users’ actions performed on the ETable user interface will invoke these operators. Formally, given an ETable specification , each of the following operator creates a new specification , except the Initiate operator which creates a new ETable from scratch.

1. Initiation.

A new ETable can be created by selecting one of the node types in the schema graph. Its result lists the corresponding nodes.

2. Selection.

ETable rows can be filtered based on their columns, similar to the selection operator in the relational model. Applying a selection condition to the primary node type filters the rows of the current ETable.

Another node type can be added to a query pattern to examine how it is related to the current primary node type. It corresponds to adding a join operator in the relational model. Selecting one of the node types that are linked to the primary node type by an edge type (i.e., ), adds it to the participating node types in the current query .

4. Shifting focus to another participating node type.

The primary node type can be changed to one of the other participating node types . It can be thought of as representing the current join result from a different angle.

The above primitive operators enable us to build any complex queries by incrementally specifying the operators one-by-one. Figure 7 (left) illustrates the query construction process consisting of 8 operators. A new query pattern can be created with Initiate; Selection conditions can be added with Select, just like writing expressions in WHERE clauses in SQL; and node types can be added with Add, just like adding relations to FROM clauses and setting one of them as a GROUP BY attribute. Also, the primary node type can be changed with Shift, similar to changing the GROUP BY attribute. A sequence of these operators specified constitutes a query pattern in the ETable model. These operators can be invoked by users on the user interface with user-level actions, which we will describe details in Section 6.1. The right side of Figure 7 shows how users can specify the same query through the user interface.

### 5.4Query Execution

A query pattern is executed to produce a result in the ETable format. The execution process is divided into two steps: instance matching and format transformation. The first step extracts matched node instances from the TGDB instance graph, and the second step transforms a result from the first step into the ETable format.

#### Instance Matching

The instance matching process finds a set of matched instances for a given query pattern. Formally, it returns a graph relation , which consists of a set of tuples, each of which contains a list of node instances in the database instance graph. The graph relation is generated with an instance matching function , which consists of a series of operations. The operations constitute primitives which make up a graph relation algebra.

A graph relation , similar to a relation in the relational model, consists of a set of tuples with a set of attributes. The schema of the graph relation is defined as a set of node types where . In other words, each attribute corresponds to a node type. The node type determines the domain of the attribute (i.e., ). A base graph relation is defined as a graph relation with a single attribute. In other words, each node type produces a base graph relation . A non-base graph relation can be created by applying the following graph relation operators to the base graph relations.

1. Selection.

It filters tuples of a graph relation using a selection condition applicable to one of the node types .

2. Join.

It joins two graph relations and using edge types . The attributes of the created graph relation is a concatenation of the attributes of the two graph relations.

We use a symbol, , to differentiate it from the relational correspondence, , and not to be confused with natural join.

3. Projection.

It removes all attributes of the graph relations except the given attribute. Duplicated rows are eliminated.

These operators enable us to define an instance matching function . In fact, this function only requires the Selection and Join operators: the Projection operator will be used later in the format transformation step.

Figure 8 (left) illustrates the instance matching process. It returns a graph relation, which is an intermediate format to be transformed into the ETable format.

#### Format Transformation

A graph relation obtained from the instance matching function is transformed into the ETable format. We describe how rows and columns of ETable are determined from it.

The rows of ETable consist of nodes of the primary node type, filtered by all selection conditions in the query pattern. They are extracted from the instance matching result:

Given the result of the instance matching function, all attributes except the attribute representing the primary node type are discarded, and then, each of distinct node in that column becomes a row.

ETable has three types of columns to present rich information for each row. In addition to the attributes of the primary node types, which we call base attributes , we introduce two other types of columns for presenting a set of entity references: participating node columns, , and neighbor node columns, .

1. List of base attributes

: It is a full set of the attributes of the primary node type . The value of the column would be a single value:

2. List of participating node types

: It is a set of all the node types in the query pattern, except the primary node type , i.e., . The value of the column would be a set of entity references:

3. List of neighbor node types

: It is a set of all the neighboring node types of the primary node type in the schema graph regardless of the query pattern, i.e., . The value of the column would be a set of nodes references:

Figure 8 (right) illustrates the results produced from the format transformation process. The first two columns are base attributes, and the rest of the columns are participating node columns. We omit neighbor node columns as some of these columns are the same as the participating node columns.

By transforming the graph relation into the ETable format, we compactly present join query results without duplications. Each row of ETable is uniquely determined by a node of a primary node type. The participating node columns show all the other entity types in the query pattern with respect to the primary node type. This transformation process is similar to setting one of the relations as a GROUP BY attribute in SQL, but while GROUP BY aggregates the corresponding instances into a single value (i.e., COUNT, AVG), ETable presents a list of the corresponding instances as entity references. The neighbor node columns are also useful for describing the rows of the ETable, although information in these columns is not obtained from the graph relation. These columns enable users to browse one-to-many or many-to-many relationships. Moreover, they provide users with a preview of possible new join operations as it presents all the join candidates. For instance, a ETable in Figure 1 consists of many neighbor node columns (e.g., Authors) that helps users browse rich information about each paper.

## 6Interface & System Design

ETable’s interface (Figure Figure 9) consists of four components: (1) the default table list, (2) the main view, (3) the schema view, and (4) the history view. The default table list presents a list of entity types in the schema graph. Users can pick one from the list to initiate a query. The main view presents an ETable executed based on a query pattern which is graphically shown over the schema view. Users can directly interact with the main view to update the current query. The list of actions specified by users is presented on the history view, which allows users to revert to a previous state.

### 6.1User-Level Actions

Users can update the current query pattern by directly interacting with ETable via user-level actions. As shown in Figure 7, these actions in turn invoke their corresponding primitive operators (discussed in Section 5.3).

1. Open a new table.

Users can open a new table by clicking a node type on the default table list. The action invokes the operator (Fig Figure 7: action U1).

2. Filter.

Users can filter the rows of the current ETable by inducing selection conditions via a popup window at the column header (Fig Figure 7: action U3). Besides the base attributes, users can also filter rows by the labels of the neighbor nodes columns (e.g., authors’ names), which is translated into subqueries. We currently provide only a conjunction of predicates, but it is straightforward to provide disjunctions and more operations. The action invokes the Select operator.

3. Pivot.

Users can change the primary node type by clicking the pivot button on the context menu for neighbor or participating node columns. It calls the Add operator if the column is the neighbor node type (Fig Figure 7: action U4); it performs the Shift operator if it is the participating node type.

4. See a particular node.

When users are interested in one of the entity references, they can click it to create a new ETable consisting of a single row presenting the clicked entity. Unlike the above actions, it invokes two primitive operators: it initiates a new ETable, and then perform the Select operator to show the single node. For the clicked node :

5. See all related nodes.

When users are interested in a full list of entity references, they can click a number (i.e., entity reference count) in the right corner of a cell (Fig Figure 7: action U2). It also encapsulates two primitive operators. The operators invoked are different depending on whether the selected column is neighbor or participating node column. For the neighboring node column of :

and for the participating node column :

ETable supports additional actions that help with database exploration, such as: (1) Sort rows based on the values in a column; (2) Hide/show columns to reduce visual complexity in the interface; and (3) Revert to previous queries via the left history panel.

### 6.2Architecture

ETable system uses a three-tier architecture, consisting of (1) an interactive user interface front-end that can run in any modern web browsers, written in HTML, JavaScript, and D3.js1; (2) a Python-based application server; and (3) a PostgreSQL database backend. The PostgreSQL database stores TGDB schema and instance graphs in four relational tables: nodes, edges, node_types, and edge_types. A query pattern for ETable is translated into SQL queries that operate on the PostgreSQL database. To efficiently perform queries, we partition a long SQL query into multiple queries consisting of a fewer number of relations to be joined (i.e., each for a single entity-reference column) and merge them.

## 7Evaluation: User Study

To evaluate the usability of ETable, we conducted a user study that tests whether users can construct queries quickly and accurately. We compared ETable with Navicat Query Builder.2 Navicat is one of the most popular commercial database administration tools with a graphical query building feature. Graphical builders such as Navicat Query Builder have been commonly used as baseline systems in database usability research [?].

### 7.1Experimental Design

Participants.

We recruited 12 participants from our university through advertisements posted to mailing lists at our institution. All were graduate students who had taken at least one database course or had industry experience using database systems. The participants rated their experience in SQL, averaging at a score of 4.67 using a 7-point Likert scale (ranged from 3 to 6) with 1 being “having no knowledge” and 7 being “expert”, which means most participants considered themselves non-expert database users. None of them had used the graphical query builder before. Each participant was compensated with a \$15 gift card.

Data set.

We used an academic publication data set used throughout this paper, which we collected from DBLP3 and ACM Digital Library.4 It contains about 38,000 papers from 19 top conferences in the areas of databases (e.g., SIGMOD), data mining (e.g,. KDD), and human-computer interaction (e.g., CHI), since 2000. A relational schema was designed using standard design principles, resulting in 7 relations with 7 foreign keys as depicted in Figure 3. As the main focus of this evaluation is on ETable’s usability, this data set creates a sufficiently large and complex database for such purpose.

Procedure.

Our study followed a within-subjects design with two conditions: the ETable and Navicat conditions. Every participant first completed six tasks in one condition and then completed another six tasks in the remaining condition. The orders of the conditions were counterbalanced, resulting in 6 participants in each ordering. We generated two matched sets of tasks (6 tasks in each set) differing only in their specific values used for parameters such as the title of the paper. Before the participants were given the tasks to carry out for each condition, they went through a 10-minute tutorial for the tool they would use. For each task, the participants could ask clarifying questions before starting, and they had a maximum of 5 minutes to complete each task. After the study, they completed a questionnaire for subjective ratings and qualitative feedback. Each study lasted for about 70 minutes. Participants completed the study using Chrome browser, running on a Windows desktop machine, with a 24-inch monitor at a 1920x1200 resolution.

We carefully generated two matched sets of 6 tasks that cover many database exploration and querying tasks. Table 2 shows one set (the other set is similar). The tasks fall into three categories: finding attribute values (Tasks 1 & 2); filtering (Tasks 3 & 4); aggregation (Tasks 5 & 6). The tasks were designed based on prior research studies and their categorization of tasks. Specifically, our categories are based on those used in database and HCI research [?], and our tasks vary in difficulty as in [?].

Measurements.

We measured participants’ task completion times. If a participant failed to complete a task within 5 minutes, the experimenter stopped the participant and recorded 300 seconds as the task completion time. After completing tasks for both conditions, the participants filled out a post-questionnaire that asked for their subjective ratings about ETable (10 questions) and their subjective preference between two conditions (7 questions).

### 7.2Results

The average task times for ETable were faster than those for Navicat for all six tasks. Figure 10 summarizes the task time results. We performed two-tailed paired t-tests. The differences were statistically significant for Tasks 1, 3, 5, and 6 () and marginally significant for Tasks 2 and 4 (, , respectively). The results of Task 2 may be explained by an outlier participant who did not understand the requirement that each row of the final results must represent a different keyword. Although Task 4 involves the highest number of operations that require participants to spend significant time in interpreting intermediate results before applying the next operators, ETable helped participants complete this task over 30% faster than Navicat.

The task completion times for ETable generally have low variance. The larger variance in Navicat is mainly due to syntax errors that the participants faced. Many participants, who are non-database experts, could not recall some SQL syntax and had trouble debugging errors. In particular, they had trouble specifying GROUP BY queries in Navicat. For example, many participants did not specify a GROUP BY attribute in their SELECT clauses in their first attempts. We also observed that many Navicat participants were overwhelmed by the complexity of the syntax of join queries [?] and preferred to specify new SQL queries from scratch instead of debugging existing ones when their original queries failed. Unlike graphical query builders such as Navicat, ETable helps nonexperts gradually build complex queries without having to know the exact query syntax.

Subjective ratings.

We asked participants to rate various aspects of ETable using 7-point Likert scales (7 being “strongly agreed”). Their subjective ratings were generally very positive (see Table 3). In particular, all participants found ETable easy to learn (i.e., rated 6 or 7), and almost all participants (11/12) found ETable easy to use and helpful for browsing data in databases. They also enjoyed using ETable (10/12) and would like to use software like ETable in the future (11/12). In response to the “helpful to interpret and understand results” question, one participant commented that “there are too many attributes ..., which is not easy to interpret.” To address this, as future work, we plan to develop techniques to rank and select the most important columns to show whenever a table has a large number of columns [?].

We also asked participants to compare ETable and Navicat in 7 aspects. All participants indicated that ETable was easier to learn and was more helpful in browsing and exploring data. A majority of participants liked ETable more (11/12) and found it easier to use (10/12). They would choose to use ETable in the future (10/12) and felt more confident using it (8/12). Half of the participants answered that ETable is more helpful in finding specific data than Navicat. This result was expected because ETable’s innovation focuses more on supporting data exploration.

Qualitative feedback.

We asked participants about the features they liked about ETable. Many participants (9/12) explicitly mentioned the “pivot” feature. They said that the pivot feature enabled them to easily specify complex join queries. One participant said “I also loved the pivot feature ... having multiple pivots throughout the course of forming a query. I messed up a query, but could still find the right answer by doing an appropriate pivot.” In addition, many participants said that ETable provides an intuitive view to users. One said “It is easy to see data from the perspective of what the users want to see/retrieve ...” Another said “Visually, I was able to see ... the effects of the SQL operations, which made it easier to use and verify intermediate results.

## 8Expressiveness

This section discusses the expressiveness of the ETable model. We will first express the overall functionality of the ETable queries as a general SQL query pattern. By doing so, we will show how typical join queries can be translated into ETable queries, through multiple steps (similar to [?]), demonstrating ETable’s expressiveness. Any join queries involving only FK-PK relationships on a relational database schema that meets ETable’s assumptions (detailed in Appendix) can be translated into an ETable query that operates on TGDB.

The overall functionality of ETable queries can be expressed as the following general SQL query pattern:

SELECT .*, ent-list(), ent-list(),
FROM
WHERE AND
AND AND AND AND
GROUP BY ;

where ent-list presents a list of corresponding entity references, similar to the json_agg operator in PostgreSQL.5 Each of the four components in an ETable query (i.e., primary node type , node types , edge types , and selection conditions ) maps to a clause in SQL: primary node type to GROUP BY clause; node_types to FROM clause; edge_types to join conditions; selection conditions to WHERE clause.

Following the above mappings, we now follow the approach similar to that in [?] to show that ETable can expressively handle typical join SQL queries, through a step-by-step translation. That is, for any SQL join query following the above pattern, there exists an equivalent ETable query.

1. Transforms a relational algebra join expression () to a graph relation correspondence (described in Section 5.4) by analyzing the list of relations in the FROM clause, and the join conditions in the WHERE clause. (Each is a node type; each an edge type.)

2. Applies the original selection conditions to the TGDB’s node types;

3. If there is a group by attribute, transform it to the graph’s primary node type; otherwise, if no group by attribute exists, arbitrarily set a primary node type.

ETable can express typical join queries consisting of the core relational algebra (i.e., relational algebra expression that does not contain set operations), which accounts for a large number of the database workloads. ETable additionally lets users choose a primary node type from the list of selected relations, and introduces the entity-reference columns (i.e., represented as ent-list in the above SQL pattern) to effectively present join queries. This paper focuses on the critical usability challenge that arises when joining several tables. In our future work, we plan to further increase ETable’s expressiveness of the presentation model to the full set of operators in the relational algebra, through introducing additional operators to support more complex queries (e.g., set operations, complex aggregations, etc.).

## 9Conclusions

We proposed ETable, a new presentation data model for interactively exploring relational databases. The enriched table representation of ETable generates a holistic, interactive view of databases that helps users browse relevant information at an entity-relationship level. By directly interacting with the interface, users can iteratively specify operators, enabling them to incrementally build complex queries and navigate databases. ETable outperformed a commercial graphical query builder in a user study, in both speed and subjective ratings across a range of database querying tasks.

This work takes a first step towards developing a practically usable, interactive interface for relational databases, and opens up many interesting opportunities. Future research directions include: (1) incorporating more operations to further improve expressive power (e.g., set operations); (2) accelerating the execution speed of updated queries (e.g., by reusing intermediate results); (3) leveraging machine learning techniques to rank and select important columns to display. The above ideas could usher a new generation of interactive database exploration tools that will benefit all database users.

## 10Acknowledgments

This material is based upon work supported by the NSF Graduate Research Fellowship Program under Grant No. DGE-1148903 and the NSF under Grant No. IIS-1563816.

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