SET a variable in SELECT statement - MySQL

Question

I'm using this code which has an error:

SET @rejects = '';

SELECT *
FROM list
WHERE maker = 1
    AND by_ids IN ('10','11')
    AND country LIKE '%I%'
    AND (
        src IS NULL
        || src NOT IN (@rejects)
        AND checkSrc(src) = 'yes'
        AND SET @rejects = CONCAT(@rejects,',',src)
    );

What's causing the issue?

Answer 1

The issue is that you cannot mix select and set in one statement, there'll surely be syntax error:

select*from t where 1 and set@a=1;

^{ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'set@a=1' at line 1}

If you want to do set within select , use the colon equals syntax. Change this:

select*from t where 1 and set@a=1;

into:

select*,@a:=1 from t where 1;

Here's how you update the variable upon each row:

create table t(id int); insert t values(1),(2),(3);
set@a=0;
select@a:=id from t;

 +--------+ | @a:=id | +--------+ | 1 | | 2 | | 3 | +--------+ 

And you can even do concat :

set@a='0';
select @a:=concat(@a,',',id)from t;

 +-----------------------+ | @a:=concat(@a,',',id) | +-----------------------+ | 0,1 | | 0,1,2 | | 0,1,2,3 | +-----------------------+ 

Or concat without the leading 0 :

set@a='';
select @a:=concat(@a,if(@a='','',','),id)from t;

 +------------------------------------+ | @a:=concat(@a,if(@a='','',','),id) | +------------------------------------+ | 1 | | 1,2 | | 1,2,3 | +------------------------------------+ 

However, the manual explicitly states that this is dangerous:

...you should never assign a value to a user variable and read the value within the same statement...

...you might get the results you expect, but this is not guaranteed .

...the order of evaluation for expressions involving user variables is undefined .

This has also been mentioned on Xaprb .

Lastly, if you're doing quirky things like assigning differing value types to the variable and etc, checkout the manual to be sure you understand the intricate mechanisms.

Answer 2

Then you might write your query like this.

SET @rejects = '';
SELECT @rejects = CONCAT(@rejects,',',src) FROM list WHERE maker = 1 AND by_ids IN ('10','11') AND country LIKE '%I%' AND 
(src IS NULL OR src NOT IN (@rejects) AND checkSrc(src) = 'yes');
SELECT @rejects;