Visual query languages to design complex queries: a systematic literature review

Structured query language (SQL) is a widely used language for accessing both relational and non-relational databases. SQL is the standard form of access in relational databases, while in non-relational databases, SQL is becoming increasingly available and consolidating itself as an access interface for querying data in cluster environments. Despite its declarative syntax, the specification of SQL queries is not a trivial task, even for experts, because some queries demand complex constructs (i.e., subqueries, joins, set operations, conditional expressions, grouping restrictions, and recursion). Visual query languages (VQLs) are an alternative to reduce this complexity. However, although several VQLs have been proposed, they are not widely used in practice. By identifying and analyzing the support provided by VQLs that make it possible to design complex SQL queries, this study collected evidence that helps discover the strengths and weaknesses of each VQL, providing useful feedback for other research initiatives that seek to propose improved VQLs. For this purpose, a systematic literature review was carried out. After analyzing 22 relevant studies and performing 462 inspections, this review points to the need for more expressive VQLs, computer-aided software engineering (CASE) tools available to end users, and more rigorous evaluations to investigate the VQL syntax and semantics.