MySQL 在更新查询中设置局部变量

Question

I have the following query inside a stored procedure:

SET @myVar = '';

UPDATE `my_table` t1
SET 
    t1.field1 = CASE
        WHEN
            -- set @myVar to 'one' when this exists is true
            EXISTS (
                SELECT 1 
                FROM `table2` t2
                WHERE t1.field1 = t2.field1
            ) OR
           -- set @myVar to 'two' when this exists is true 
            EXISTS (
                SELECT 1
                FROM `table3` t3
                WHERE t1.field1 = t3.field1
            )
            THEN FALSE
        ELSE
            TRUE
    END,
    t1.field3 = CASE
        WHEN
            @myVar = 'one'
            THEN 'okay'
        WHEN
            @myVar = 'two'
            THEN 'not okay'
        ELSE
            'netural'
    END
WHERE t1.field5 = 'value';

Is there a way I can set some flag value ('one', 'two', etc...) in a local variable when first or second EXISTS condition is true in the CASE clause for first set . Then I want to use that flag value to set another field.

Is something like this possible? I have searched for while but can't seem to get a sufficient solution to this issue.

EDIT : Added comments for where the session/local var should change.

Answer 1

This select query gives you an 1 as answer and sets @myvar to one

SELECT IF(IF(EXISTS (SELECT 1 FROM `df` t3 WHERE 1 = 1), @myVar :='one',0) = 'one',1,0);
SELECT @myVar;       

So try

UPDATE `my_table` t1
SET 
    t1.field1 = CASE
        WHEN
            IF(IF(            
            EXISTS (
                SELECT 1 
                FROM `table2` t2
                WHERE t1.field1 = t2.field1
            ), @myVar :='one',0) = 'one',1,0) OR
           IF(IF( 
            EXISTS (
                SELECT 1
                FROM `table3` t3
                WHERE t1.field1 = t3.field1
            ), @myVar :='two',0) = 'one',1,0)
            THEN FALSE
        ELSE
            TRUE
    END,
    t1.field3 = CASE
        WHEN
            @myVar = 'one'
            THEN 'okay'
        WHEN
            @myVar = 'two'
            THEN 'not okay'
        ELSE
            'netural'
    END
WHERE t1.field5 = 'value';

It doesn't give an error, but without data, you must test it yourself

And i am not sure if the session variable has an vqalue. like i said no data.

But still set the session varoable before hand, it doewsn't make sense doiung it in the update statement.

Answer 2

There's no way to be certain of the successful completion of this query using variables the way you want to as MySQL does not guarantee the ordering of SET evaluation in an UPDATE query ("generally" != "guaranteed"). The safest solution is to repeat your EXISTS clauses:

UPDATE `my_table` t1
SET 
    t1.field1 = CASE
        WHEN
            EXISTS (
                SELECT 1 
                FROM `table2` t2
                WHERE t1.field1 = t2.field1
            ) OR
            EXISTS (
                SELECT 1
                FROM `table3` t3
                WHERE t1.field1 = t3.field1
            )
            THEN FALSE
        ELSE
            TRUE
    END,
    t1.field3 = CASE
        WHEN
            EXISTS (
                SELECT 1 
                FROM `table2` t2
                WHERE t1.field1 = t2.field1
            )
            THEN 'okay'
        WHEN
            EXISTS (
                SELECT 1
                FROM `table3` t3
                WHERE t1.field1 = t3.field1
            )
            THEN 'not okay'
        ELSE
            'netural'
    END
WHERE t1.field5 = 'value';

The optimiser should be smart enough to realise that the subqueries are being used more than once and re-use the result.