How Can I Correctly Use Variables Within a MySQL SELECT Statement?

Using Variables within SELECT Statements

When working with MySQL, you may encounter scenarios where you want to define and use variables within the same SELECT statement. However, it's important to be aware of the limitations involved.

Variable Assignment and Readability

As per the MySQL documentation, it's generally not recommended to assign a value to a user variable and read it within the same statement. The order of evaluation for expressions involving user variables is undefined and may vary based on the statement's elements.

This means that the following query may not behave as expected:

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

In this case, you will likely get NULL for the second column.

Why the Difference with Procedures?

You may wonder why the following query, which uses a stored procedure instead of a user variable, works as expected:

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

The reason for this is that stored procedures are evaluated differently than user variables. The evaluation order for stored procedures is defined and consistent.

Using a Subquery

To achieve the desired functionality, you can use a subquery:

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

This approach uses a subquery to initialize the user variable (@z) and then references it in the main query.

The above is the detailed content of How Can I Correctly Use Variables Within a MySQL SELECT Statement?. For more information, please follow other related articles on the PHP Chinese website!