MySQL：在连接条件内使用别名

Question

SELECT t1.*, IFNULL(t2.profile_id, t3.profile_id) AS `profile_id`
FROM table1 AS t1
LEFT JOIN table2 AS t2
    ON t1.id = t2.some_coulmn
LEFT JOIN table3 AS t3
    ON t1.id = t3.some_coulmn
LEFT JOIN table4 AS t4
    ON profile_id = t4.some_column

I'm trying to use an alias ( profile_id ) inside my join condition. It fails. Is there a way to do this?

Answer 1

Column aliases defined in th SELECT clause cannot be used in the join conditions. This is because the FROM clause is evaluated before the SELECT clause.

If I followed you correctly, you probably want:

SELECT t1.*, IFNULL(t2.profile_id, t3.profile_id) AS `profile_id`
FROM table1 AS t1
LEFT JOIN table2 AS t2
    ON t1.id = t2.some_coulm
LEFT JOIN table3 AS t3
    ON t1.id = t3.some_coulm
LEFT JOIN table4 AS t4
    ON IFNULL(t2.profile_id, t3.profile_id) = t4.some_column