条件下的mysql加入

Question

我有这张表 id_name：

+----+------+
| id | name |
+----+------+
| 1  |  one |
| 2  |  two |
| 3  | three|

...等，以及另一个表ID

+----+-----+-----+-----+----
| id | aid | bid | cid | ...
+----+-----+-----+-----+----
| 1  |  1  |  2  |  3  | ...
| 2  |  2  | null| null| ...
| 3  |  2  |  3  | null| ...

我想选择这样的东西：

+----+-----+-----+-----+-----+-----+-----+----
| id | aid |aname| bid |bname| cid |cname| ...
+----+-----+-----+-----+-----+-----+-----+----
| 1  |  1  | one |  2  | two |  3  |three| ...
| 2  |  2  | two | null| null| null| null| ...
| 3  |  2  | two |  3  |three| null| null| ...

基本上，如果id不为null，则取name到对应的列，否则保持name为null。

在我的项目中，我有 7 列 (a-g)。

我尝试了 7 级连接，基本上是这样的：

SELECT id,a.id AS aid,a.name AS aname,b.id AS bid,b.name AS bname, ...
  FROM ids AS t
JOIN id_name AS a ON a.id=t.aid
JOIN id_name AS b ON b.id=t.bid
... 

等等。但后来我注意到，如果 t.b 为空，那么 b.id=null 始终为假，所以我不会选择任何行。 我该怎么做 IF b.id IS NULL THEN JOIN id_name AS b ON b.id=t.bid ELSE "keep bname null"？

我知道我可以创建一个临时表并进行 7 次更新。但是有没有办法使用单个 SELECT 来完成此操作？

谢谢！

Answer 1

您必须使用外连接。例如

SELECT id,a.id AS aid,a.name AS aname,b.id AS bid,b.name AS bname, ...
  FROM ids AS t 
LEFT OUTER JOIN id_name AS a ON a.id=t.aid
LEFT OUTER JOIN id_name AS b ON b.id=t.bid
...

更多信息http://dev.mysql.com/doc/refman/5.0/en/join.html

Answer 2

您也可以使用cross join 获得结果，而无需多次加入同一个表

SQL Fiddle

select ids.id, 
       aid, 
       max(case when aid = id_name.id then id_name.name end)  as aname, 
       bid, 
       max(case when bid = id_name.id then id_name.name end ) as bname
from ids
cross join id_name 
group by ids.id, aid, bid

Answer 3

select
  i.id,
  a.id as aid,
  a.name as aname,
  b.id as bid,
  b.name as bname,
  c.id as cid,
  c.name as cname
from ids i
  left join id_name a
    on a.id = i.aid
  left join id_name b
    on b.id = i.bid
  left join id_name c
    on c.id = i.cid

SQL 小提琴：http://www.sqlfiddle.com/#!2/3ea82/1