Question

我不记得如何将表连接到自己..我的表是：

| id | proc | value | kind |
| 1  | 1    | foo   | a    |
| 2  | 1    | bar   | b    |
| 3  | 2    | some  | a    |

我需要检索值为col的地方，其中proc是$ proc，kind是'a'和'b'..好吧，我需要做那个（寻找proc = 1）：

| v_a | v_b |
| foo | bar |

所以我写了这个查询：

SELECT
    a.value AS v_a,
    b.value AS v_b
FROM
    (SELECT value FROM table WHERE proc = '1' AND kind = 'a') AS a,
    (SELECT value FROM table WHERE proc = '1' AND kind = 'b') AS b

并且有效，但只有在表中我有两行for kind = a和kind = b。

但我需要，如果错过一行，我将有一个空值：如果我寻找proc = 2我必须得到：

| v_a | v_b |
| foo | NULL|

相反，根据我的查询，如果b或行丢失，我就不会得到任何东西。

我正在使用mysql ......怎么做？

修改我可以使用UNION子句，但是当一行丢失时，这不允许我有一个NULL值（kind = a或kind = b）

Answer 1

我错过了检索NULL的必要性。这很复杂，但似乎有效：

SELECT
    a.value AS a_val,
    (SELECT value FROM t b2 WHERE b2.proc = a.proc AND b2.id != a.id) AS b_val
FROM `t` a
WHERE a.kind = 'a'

UNION DISTINCT

SELECT
    (SELECT value FROM t a2 WHERE a2.proc = b.proc AND a2.id != b.id) AS a_val,
    b.value AS b_val
FROM `t` b
WHERE b.kind = 'b'

Answer 2

你必须做一个完整的连接，以便存在'a'而不是'b'的情况，反之亦然：

SELECT
    a.value v_a, 
    b.value v_b
FROM (SELECT proc, value FROM tab WHERE kind = 'a') a LEFT JOIN 
(SELECT proc, value FROM tab WHERE kind = 'b') b
  ON a.proc = b.proc
WHERE a.proc = '1'
UNION
SELECT
    a.value v_a, 
    b.value v_b
FROM (SELECT proc, value FROM tab WHERE kind = 'b') b LEFT JOIN 
(SELECT proc, value FROM tab WHERE kind = 'a') a
  ON a.proc = b.proc
WHERE b.proc = '1'

编辑：第一个给了MS SQL Server查询（FULL JOIN），但显然它不受MYSQL的支持，所以我把它改成了2 LEFT JOIN的UNION

Answer 3

select * from t1 full outer join t2 on some_id = some_other_id？

Answer 4

SELECT
    a.value AS v_a,
    b.value AS v_b
FROM
    (SELECT proc, value FROM table WHERE proc = '1' AND kind = 'a') AS a
LEFT OUTER JOIN
    (SELECT proc, value FROM table WHERE proc = '1' AND kind = 'b') AS b
ON a.proc=b.proc

左连接在同一个表上

4 个答案: