Why Does Using Variables in a Single SQL SELECT Statement Often Return Unexpected Results?

Declaring and Utilizing Variables Within a SELECT Statement

In SQL, assigning a value to a variable and retrieving that value within the same query can be challenging. Consider the following example:

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

This query attempts to calculate the sum of the "item" column and store it in a variable named "@z". It then multiplies the value of "@z" by 2. However, surprisingly, the second column always returns NULL.

This occurs because MySQL's documentation explicitly states that assigning a value to a user variable and reading it within the same statement should not be relied upon. The evaluation order of such expressions is undefined and subject to change in future releases.

To illustrate this further, consider the alternative query:

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

In this query, "@z" is assigned a value via a procedure call. In this scenario, the query works as expected because the procedure call is evaluated separately from the variable assignment.

To overcome the limitation with user variables, you can utilize a subquery to assign the value and retrieve it within the main query:

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

In this subquery, the value of "@z" is assigned using SELECT. The main query then selects "@z" and performs the necessary calculations.

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