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

我正在尝试在加入条件中使用别名（profile_id）。它失败。有办法吗？

Answer 1

在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子句是在SELECT子句之前求值的。

如果我正确地跟随了您，您可能想要：

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

MySQL：在连接条件内使用别名

1 个答案: