Mysql 如何使用中间表在两个表上应用外部联接?
所需产出:Mysql 如何使用中间表在两个表上应用外部联接?,mysql,join,Mysql,Join,所需产出: table1 | id | value -------------------- 1 | john 2 | frank 3 | patel 4 | jim table2 | id | value -------------------- 6 | steve 7 | tim 8 | sunny 9 | bhaskar mer
table1 | id | value
--------------------
1 | john
2 | frank
3 | patel
4 | jim
table2 | id | value
--------------------
6 | steve
7 | tim
8 | sunny
9 | bhaskar
merged | tabid1 | tabid2
------------------------------
3 | 7
4 | 8
我试过:
output | tabid1 | tabval1 | tabid2 | tabval2
1 | john | NULL | NULL
2 | frank | NULL | NULL
3 | patel | 7 | tim
4 | jim | 8 | sunny
NULL | NULL | 6 | steve
NULL | NULL | 9 | bhaskar
但是它给出了16行而不是6行
SELECT *
FROM table1
LEFT JOIN merged m1 ON table1.id = m1.tabid1, merged m2
RIGHT JOIN table2 ON table2.id = m2.tabid2
我不知道你为什么觉得有必要在那里加入两次merged
SELECT table1.id AS tabid1, table1.value AS tabval1,
table2.id AS tabid2, table2.value AS tabval2
FROM table1
LEFT JOIN merged m1 ON table1.id = m1.tabid1
RIGHT OUTER JOIN table2 ON table2.id = m1.tabid2
或使用右连接:
select table1.id,table1.value,table2.id,table2.value
from table1 left join merged on table1.id=merged.tabid1
left join table2 on merged.tabid2=table2.id
union
select table1.id,table1.value,table2.id,table2.value
from table2 left join merged on table2.id=merged.tabid2
left join table1 on merged.tabid1=table1.id ;
这只返回四行。这不是jerrymouse想要的答案。mysql+1中没有完整的外部联接,但是使用缩写的表名有点难读懂。这很有效,我提交了一个编辑来使用@jerrymouse的表名
select table1.id,table1.value,table2.id,table2.value
from table1 left join merged on table1.id=merged.tabid1
left join table2 on merged.tabid2=table2.id
union
select table1.id,table1.value,table2.id,table2.value
from table2 left join merged on table2.id=merged.tabid2
left join table1 on merged.tabid1=table1.id ;
SELECT * FROM
table1 LEFT JOIN merged on table1.id=merged.tabid1
LEFT JOIN table2 ON merged.tabid2=table2.id
UNION
SELECT * FROM
table1 RIGHT JOIN merged on table1.id=merged.tabid1
RIGHT JOIN table2 ON merged.tabid2=table2.id WHERE table1.id IS NULL;