MySQL Case with Insert and Update

Question

I need help writing a conditional query to copy data from one table to another, everything was pretty straight forward until I decided I wanted to include versioning in my application!

I have looked at some of the examples, but they are mostly oriented towards creating a mysql procedure, this is what I have tried:

        SELECT CASE 
            WHEN NOT EXISTS (
                SELECT `version`
                FROM `archive_courses`
                WHERE `original_course_id` = '$course_id'
                AND `version` = '$current_version'
            )
            THEN
                BEGIN
                    INSERT INTO `archive_course_users`      <------ syntax error
                    (`course_id`, `user_id`, `course_qty`)
                    SELECT @new_course_id, '$user_id', `course_qty`
                    FROM `current_course_users`
                    WHERE `course_id` = '$course_id'
                    AND `user_id` = '$user_id'
                END
            ELSE
                BEGIN
                    UPDATE `archive_course_users`
                    SET `course_qty` = (SELECT `course_qty` FROM `current_course_users` WHERE `user_id` = '$user_id')
                    WHERE `original_course_id` = '$course_id'
                    AND `user_id` = '$user_id'
                END
            END
        END

It seems the case is working /OK/ but throws a syntax error when it gets to my INSERT query. How should I be doing this?

Answer 1

It shouldn't work as well that way. Change your query to be like

               INSERT INTO `archive_course_users`     
                (`course_id`, `user_id`, `course_qty`)
                SELECT @new_course_id, '$user_id', `course_qty`
                FROM `current_course_users`
                WHERE `course_id` = '$course_id'
                AND `user_id` = '$user_id'
                AND NOT EXISTS (
            SELECT 1
            FROM `archive_courses`
            WHERE `original_course_id` = '$course_id'
            AND `version` = '$current_version');

Answer 2

have you tried "insert.. on duplicate key update" ?

Heres an example from the mysql manual:

INSERT INTO table (a,b,c) VALUES (1,2,3)
  ON DUPLICATE KEY UPDATE c=c+1;

UPDATE table SET c=c+1 WHERE a=1;

http://dev.mysql.com/doc/refman/5.7/en/insert-on-duplicate.html

Hope it helps

Answer 3

As per @Rahul's answer I created two queries:

    //INSERT IF NOT EXISTS
        INSERT INTO `archive_course_users`
        (`course_id`, `user_id`, `course_qty`)
        SELECT @new_course_id, '$user_id', `course_qty`
        FROM `current_course_users`
        WHERE NOT EXISTS (
            SELECT `version`
            FROM `archive_courses`
            WHERE `original_course_id` = '$course_id'
            AND `version` = '$current_version'
        )
        AND `course_id` = '$course_id'
        AND `user_id` = '$user_id'

    //UPDATE IF EXISTS
        UPDATE `archive_course_users`
        SET `course_qty` = (SELECT `course_qty` FROM `current_course_users` WHERE `user_id` = '$user_id')
        WHERE EXISTS (
            SELECT `version`
            FROM `archive_courses`
            WHERE `original_course_id` = '$course_id'
            AND `version` = '$current_version'
        )
        AND `original_course_id` = '$course_id'
        AND `user_id` = '$user_id'

Answer 4

Cannot have statements inside a SELECT . So either

Move the code out of the SELECT :

 IF (...) THEN   -- Note:  No "CASE"; not "WHEN"
     INSERT ...
 ELSE
     UPDATE
 END IF

Or SELECT some kind of flag, then use an IF statement outside.