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:

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

Answer 2

Use outer joins on both tables and move the CASE inside your COUNT:

SELECT
    ft1.task,
    COUNT(case ft1.id when 1 then p1.id when 3 then u1.id end) as count
FROM feed_tasks ft1
LEFT JOIN pages p1 ON p1.id = ft1.reference_id
LEFT JOIN urls u1 ON u1.id = ft1.reference_id
WHERE ft1.account_id IS NOT NULL
AND a1.user_id = {$db->quote($user['id'])}

Non-hits for the CASE will give a null id and won't be counted.

Note: Table a1 is in your where clause, but doesn't seem to be a selected table