从同一个表中选择列

Question

我有一张表，有4列：id, name, to_id, to_id2

SELECT id, name, (name WHERE to_id=id) as to1, (name WHERE to_id2=id) as to2 FROM table

我想以某种方式显示名称而不是to_id和to_id2。此代码：SELECT id, name, to_id, to_id2 FROM TABLE将to_id，to_id2作为数字，因为它们是id-s。

Answer 1

使用CASE WHEN ... THEN ... END：

SELECT id, name, 
(CASE WHEN to_id = id THEN name ELSE NULL END) as "t1",
(CASE WHEN to_id2 = id THEN name ELSE NULL END) as "t2"
FROM `your_table`

您可以详细了解case声明here。

Answer 2

你必须使用JOIN，试试这个：

SELECT t1.id, t1.name, t2.name as t2Name, t3.name as t3Name
FROM tbl t1
LEFT JOIN tbl t2 ON t1.to_id = t2.id
LEFT JOIN tbl t3 ON t1.to_id2 = t3.id

DEMO