How to re-assign two variables in a conditional UPDATE

Question

Consider a simple update as

SET @var1 = 20;
SET @var2 = 26;
UPDATE table1 SET
    col1=IF(@var1>50, @var1 := @var1-col4, @var1 := @var1+col5),
    col2=IF(@var2>50, @var2 := @var2-col6, @var2);

How we can change two variables in one condition. For example, consider that when @var1<50, we want to change not only @var1 := @var1+col5, but also re-assign @var2 := @var2 + 100. In fact, regardless of the condition 2, we want to increase @var2, if the first condition fails (the second part).

Is it possible to re-assign two user-variable in a IF STATEMENT or we need to add another IF STATEMENT?

This will be equivalent to

SET @var1 = 20;
SET @var2 = 26;
UPDATE table1 SET
    col1=IF(@var1>50, @var1 := @var1-col4, @var1 := @var1+col5),
    virtual_col=IF(@var1>50, NULL, @var2 := @var2+100),
    col2=IF(@var2>50, @var2 := @var2-col6, @var2);

Here we have created a useless column, not to collect the data, but to repeat the IF STATEMENT for the @var2. Obviously, it is not good from performance point of view, as we are running a useless UPDATE action.

Answer 1

It is possible to do two assignments in one IF statement.

There are several ways; I present one, using CASE statement:

SET @var1 = 20;
SET @var2 = 26;
UPDATE table1 SET
    col1=IF(
      @var1>50, 
      @var1 := @var1-col4, 
      CASE
        WHEN @var1 := @var1+col5 IS NULL THEN NULL
        WHEN @var2 := @var2 + 100 IS NULL THEN NULL
        ELSE @var1
      END
    ),
col2=IF(@var2>50, @var2 := @var2-col6, @var2);

The idea is to create a CASE statement where first WHEN clauses fail. They fail because we want them to, but along they way they get evaluated.

But in order for them to get evaluated, they must first computer the assignment expression, such as @var1 := @var1+col5 and @var2 := @var2 + 100.

CASE expressions work from first WHEN clause to the next one, and must work in this order.

Please consider reading more about it in my presentation of last year's Percona Live London (2011) conference. For referece, relevant slides are 13 - 15: Programmatic queries: things you can code with sql