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

Undetermined Evaluation Order of Expressions with User Variables in SQL

The MySQL documentation indicates that the evaluation order of expressions involving user variables is not deterministic. Consider the following 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 expected output would be a sequence of increasing integers:

first   second  third   fourth  fifth   sixth
  0      1      2      3      4      5

However, as the documentation notes, the order of evaluation for user variables is undefined. This means that different database implementations or even different executions of the same query may produce different results.

The reason behind this undefined evaluation order lies in the flexibility afforded by the SQL standard. The standard does not specify how expressions should be evaluated, leaving this decision open to individual database systems. Consequently, each database optimizer can choose its own approach for evaluating expressions, including the order of evaluation.

This lack of a standard evaluation order can have implications when relying on specific variable values in subsequent calculations or when comparing results across different database instances. To ensure consistency and avoid unpredictable behavior, it is best practice to minimize the use of user variables in complex expressions and to explicitly define their evaluation order when necessary.

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