MySQL : Using alias inside a join condition

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

我正在尝试在连接条件中使用别名 (profile_id)。它失败。有办法做到这一点吗？

Answer 1

SELECT 子句中定义的列别名不能在连接条件中使用。这是因为 FROM 子句在 SELECT 子句之前进行计算。

如果我正确地理解了你，你可能想要:

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