Why is the Evaluation Order of Expressions with User Variables Undefined in SQL?

Undefined Evaluation Order for Expressions with User Variables in SQL

In the world of SQL, the order of evaluation for expressions is typically straightforward. However, an exception arises when user variables are involved. In such scenarios, the order becomes undefined, leading to unpredictable query results.

The reason behind this behavior lies in the flexibility given to database systems by the SQL standard. The standard does not impose any specific evaluation order, leaving it to the discretion of individual database optimizers. As a result, different DBMSs handle the evaluation of expressions involving user variables in varying ways.

In the provided example query:

SET @a := 0;

SELECT 
@a AS first,
@a := @a + 1 AS second,
@a := @a + 1 AS third,
@a := @a + 1 AS fourth,
@a := @a + 1 AS fifth,
@a := @a + 1 AS sixth;

The manual states that the order of evaluation for the @a user variable is undefined. This means that the database optimizer is free to choose an arbitrary order for incrementing and using the variable within the expression. This lack of predictability can result in inconsistent query outputs across different database systems or even different executions within the same system.

Therefore, it is crucial to be aware of this undefined evaluation order when dealing with user variables in SQL expressions. Developers should avoid relying on specific evaluation sequences and ensure their queries are designed to handle varying evaluation orders to guarantee reliable and consistent results.

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