Question

很抱歉，如果描述不正确，我不是SQL专家......

我有以下查询 -

SELECT t1.ids, t1.name, t2.ids t3.ids FROM tab1 t1
LEFT JOIN tab2 t2 on t2.ids = t1.ids
LEFT JOIN tab3 t3 on t3.ids = t1.ids

此查询的结果是返回tab1的所有内容，如果tab2或tab3中没有匹配，则会将字段显示为空...

| t1.ids | t1.name | t2.ids | t3.ids |
| 1      | n1      | null   | 1      |
| 2      | n2      | 2      | null   |

我需要的是，如果tab1与tab2或tab3中的某些内容不匹配，仍会返回tab2和tab3行，但{ {1}}字段为null，如下所示：

tab1

知道最有效的方法是什么？

Answer 1

你想要的是一个FULL OUTER JOIN，遗憾的是在MySQL中没有。在这种情况下，您可以使用UNION ALL：

SELECT t1.ids, t1.name, t2.ids t3.ids 
FROM tab1 t1
LEFT JOIN tab2 t2 on t2.ids = t1.ids
LEFT JOIN tab3 t3 on t3.ids = t1.ids

UNION ALL

-- Get all t2 rows having no match in t1 
SELECT t1.ids, t1.name, t2.ids t3.ids 
FROM tab2 t2
LEFT JOIN tab1 t1 on t2.ids = t1.ids
LEFT JOIN tab3 t3 on t3.ids = t2.ids
WHERE t1.id IS NULL

UNION ALL

-- Get all rows in t3 having no match in t1 AND no match
-- in t2. Rows of t3 having no match in t1 and a match in t2
-- are returned by th previous query.
SELECT t1.ids, t1.name, t2.ids t3.ids 
FROM tab3 t3
LEFT JOIN tab1 t1 on t3.ids = t1.ids
LEFT JOIN tab2 t2 on t2.ids = t3.ids
WHERE t1.id IS NULL AND t2.id IS NULL

Answer 2

您正在寻找MySQL不支持的完整外连接。最简单的方法是首先选择所有ID，然后选择外连接所有表：

SELECT t1.ids, t1.name, t2.ids t3.ids 
FROM (SELECT ids FROM tab1 UNION SELECT ids FROM tab2 UNION SELECT ids FROM tab3) x
LEFT JOIN tab1 t1 on t1.ids = x.ids
LEFT JOIN tab2 t2 on t2.ids = x.ids
LEFT JOIN tab3 t3 on t3.ids = x.ids;

如果左边存在某些内容，则返回左表中的所有结果以及右表

2 个答案: