Question

I have the following query within inside a select if in my code and I can’t remember what the line containing @lastComment := time actually does?

SELECT 
    IF(
        (SELECT @lastComment := `time` 
         FROM usermessages 
         WHERE userId = $userId 
         ORDER BY id DESC 
         LIMIT 1)
    IS NOT NULL,
    DATE_SUB(NOW(), INTERVAL 30 SECOND) >= @lastComment, 1
    )

It looks like @lastComment is just a temporary variable holding time but I don’t understand why it’s needed, is it just to pass it to the other query?

Answer 1

Obviously you didn’t want to execute the query twice and that’s correct but you can easily omit the user-defined variable and thus make it clearer:

SELECT 
  IFNULL(
     (
       SELECT DATE_SUB(NOW(), INTERVAL 30 SECOND) >= `time` 
       FROM usermessages 
       WHERE userId = $userId 
       ORDER BY id DESC 
       LIMIT 1
     )
     , 1
)

which translates to: check if the given expression is NULL, if so return 1, else return the expression value.