MySQL query where JOIN depends on CASE

Question

As I am now aware, CASE can be used only in WHERE context. Though, I need to use different table depending on column value. What I've tried looks like this:

SELECT
    `ft1`.`task`,
    COUNT(`ft1`.`id`) `count`
FROM
    `feed_tasks` `ft1`
CASE
    `ft1`.`type`
WHEN
    1
THEN
    (INNER JOIN `pages` `p1` ON `p1`.`id` = `ft1`.`reference_id`)
WHEN
    2
THEN
    (INNER JOIN `urls` `u1` ON `u1`.`id` = `ft1`.`reference_id`)
WHERE
    `ft1`.`account_id` IS NOT NULL AND
    `a1`.`user_id` = {$db->quote($user['id'])}

Now that I know this is invalid syntax, what's the closest alternative?

Answer 1

It probably needs tweaking to return the correct results but I hope you get the idea:

SELECT ft1.task, COUNT(ft1.id) AS count
FROM feed_tasks ft1
LEFT JOIN pages p1 ON ft1.type=1 AND p1.id = ft1.reference_id
LEFT JOIN urls u1 ON ft1.type=2 AND u1.id = ft1.reference_id
WHERE COALESCE(p1.id, u1.id) IS NOT NULL
AND ft1.account_id IS NOT NULL
AND a1.user_id = :user_id

Edit:

A little note about CASE...END. Your original code does not run because, unlike PHP or JavaScript, the SQL CASE is not a flow control structure that allows to choose which part of the code will run. Instead, it returns an expression. So you can do this:

SELECT CASE
    WHEN foo<0 THEN 'Yes'
    ELSE 'No'
END AS is_negative
FROM bar

... but not this:

-- Invalid
CASE 
    WHEN foo<0 THEN SELECT 'Yes' AS is_negative
    ELSE SELECT 'No' AS is_negative
END
FROM bar