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

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

In SQL, the sequence of evaluating expressions in a query is generally not defined, particularly when user variables are involved. This can lead to unexpected results, as demonstrated in the example provided from the MySQL Manual:

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;

In this query, the values assigned to user variable @a are used to generate subsequent values. However, the order in which the expressions involving @a are evaluated may vary, leading to different results each time the query is executed.

This ambiguity in evaluation order arises because the SQL standard does not specify the sequence of operations for expressions with user variables. As a result, database management systems (DBMSs) are free to implement their own evaluation strategies, typically delegating this decision to the query optimizer.

Optimizers may decide to prioritize evaluating expressions based on performance considerations, such as the need for intermediate results or the potential for parallel execution. This flexibility allowsDBMSs to optimize query execution based on the specific characteristics of each query.

However, the lack of a defined evaluation order can introduce inconsistencies when expressions involve user variables that are updated within the query. As seen in the example above, the values of such variables may be modified in an unanticipated order, leading to erroneous results.

To ensure consistent results, it is advisable to explicitly control the order of evaluation using temporary variables or subqueries. By isolating the evaluation of user variables from dependent expressions, you can guarantee the desired sequence of operations and avoid potential ambiguities.

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