Question

我正在使用mysql，我有三个表T1，T2，T3 它们包含如下数据：

+-------+
| t1.id |
+-------+
| 1    |
| 2    |
| 3    |
| 4    |
| 5    |
+------+

+-------+
| t2.id |
+-------+
| 1    |
| 2    |
| 3    |
| 4    |
+-------+

+-------+
| t3.id |
+-------+
| 1    |
| 2    |
| 3    |
+-------+

    I want to be able to select data as follows:

id      t1.id   t2.id   t3.id
1       1       1       1
2       2       2       2
3       3       3       3
4       4       4       null
5       5       null    null

我该怎么做？

Answer 1

假设您打算使用T1作为基础id，一对LEFT JOIN将为您提供所需的结果：

SELECT 
  /* base id */
  T1.id AS id,
  /* Redundant t1.id */
  T1.id AS t1_id,
  T2.id AS t2_id,
  T3.id AS t3_id
FROM
  T1
  LEFT JOIN T2 ON T1.id = T2.id
  LEFT JOIN T3 ON T1.id = T3.id

请注意，这会大量使用T1.id，但这正是您在建议输出中所拥有的内容，因此我在此重复使用。

如果您真正想要的是从所有表中获取所有可能的ID，请使用UNION加入。这会将三个表中不同的id组合组合成一个子查询allids，用作LEFT JOIN每个其他三个表的基本id。如果T2中存在特定ID但不 T1，则它仍将显示在输出中。

SELECT 
  allids.id AS id,
  T1.id AS t1_id,
  T2.id AS t2_id,
  T3.id AS t3_id
FROM
  ( SELECT id FROM T1 UNION SELECT id FROM T2 UNION SELECT id FROM T3 ORDER BY id) allids
  LEFT JOIN T1 ON allids.id = T1.id
  LEFT JOIN T2 ON allids.id = T2.id
  LEFT JOIN T3 ON allids.id = T3.id

Answer 2

SELECT t1.id, t2.id, t3.id
FROM t1 
LEFT JOIN t2 ON t1.id=t2.id
LEFT JOIN t3 ON t1.id=t3.id

如果保证t1具有所有ID的严格超集，则

有效。

如果没有，您需要一个帮助表：

SELECT th.id,t1.id, t2.id, t3.id
FROM (
  SELECT id FROM t1
  UNION DISTINCT 
  SELECT id FROM t2
  UNION DISTINCT 
  SELECT id FROM t3
) AS th
LEFT JOIN t1 ON th.id=t1.id
LEFT JOIN t2 ON th.id=t2.id
LEFT JOIN t3 ON th.id=t3.id

Answer 3

我认为您想要的解决方案涉及完全外部联接：

SELECT coalesce(t1.id, t2.id, t3.id) as id,
       T1.id as t1_id, T2.id as t2_id, T3.id as t3_id
FROM T1 full outer JOIN
     T2 ON T2.id = T1.id full outer JOIN
     T3 ON T3.id = coalesce(t1.id, t2.id)

我建议使用完全外连接，以防其他两个表中的id不在T1中。您没有指定t1是驱动表。

从mysql中的不同表中选择不相等的列

3 个答案: