How Can I Safely Define and Use a Variable Within a SQL SELECT Statement?

Defining and Using a Variable within a Select Statement

In SQL, it is generally not recommended to assign a value to a user variable and use it within the same select statement. MySQL's documentation explicitly states that the order of evaluation for such expressions is undefined and may vary based on statement elements and server releases.

To illustrate this point, consider the following query:

SELECT 
    @z:=SUM(item),
    2*@z
FROM
    TableA;

In this query, you would expect the second column to return the value of @z multiplied by two. However, MySQL may return NULL, as the evaluation order is not guaranteed. This behavior differs from using a stored procedure instead of a user variable, as shown below:

SELECT 
    @z:=someProcedure(item),
    2*@z
FROM
    TableA;

In this case, the expected results are obtained because the stored procedure call is evaluated before the variable assignment.

To resolve this issue and define a variable within a select statement, you can use a subquery:

select @z, @z*2
from (SELECT @z:=sum(item)
      FROM TableA
     ) t;

This approach ensures that the variable assignment occurs before it is used in the outer query.

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