Can a Systematic Approach Translate Human-Readable Query Descriptions into SQL?

Constructing SQL Queries from Human-Readable Descriptions

Problem:

Whenever faced with a human-readable description of a query, developers typically rely on heuristics and brainstorming to translate it into a SQL query. However, is there a systematic and mathematical approach to this translation process?

Answer:

Yes, there is a systematic approach to constructing SQL queries from human-readable descriptions. It involves understanding the correspondence between natural language expressions, logical expressions, relational algebra expressions, and SQL expressions.

Steps to Translate Human-Readable Descriptions to SQL

1. Identify the predicate of each table: A table's predicate is a natural language statement template that describes the rows in that table.
2. Express the query in terms of the table predicates: Use relational operators (JOIN, WHERE, IN, etc.) to combine and filter table predicates to express the desired rows.
3. Translate to SQL: Use the SQL syntax for each relational operator to translate the relational algebra expression into a SQL query.

Relational Operators in SQL

• JOIN: Combines rows from multiple tables based on common columns (e.g., INNER JOIN, LEFT JOIN).
• WHERE: Filters rows based on a condition (e.g., WHERE COLUMN = VALUE).
• IN: Checks if the value of a column matches a list of values (e.g., WHERE COLUMN IN (VALUE1, VALUE2)).
• UNION: Combines rows from multiple tables or subqueries (e.g., UNION, UNION CORRESPONDING).
• VALUES: Creates a table with a specific set of rows and columns (e.g., VALUES (VALUE1, VALUE2)).

Example

Consider the following human-readable description:

Find all people who are liked by someone but who don't like Ed.

Predicate of the Likes table:

[person] likes [liked]

Relational Algebra Expression:

FOR SOME x, Likes(person, x) AND Likes(x, liked) AND person = 'Bob' AND NOT Likes(x, 'Ed')

SQL Query:

SELECT DISTINCT l1.liker AS person, l2.liked AS liked
FROM Likes l1
INNER JOIN Likes l2 ON l1.liked = l2.liker
WHERE l1.liker = 'Bob'
AND NOT (l1.liked, 'Ed') IN (SELECT * FROM Likes)

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