mysql - 如何使用中间表在2个表上应用外部联接？

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

需要的输出:

    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

我试过了:

SELECT *
FROM table1
LEFT JOIN merged m1 ON table1.id = m1.tabid1, merged m2
RIGHT JOIN table2 ON table2.id = m2.tabid2

但是它给出了16行而不是需要的6行

最佳答案

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 ;

或具有正确的加入:

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;

关于mysql - 如何使用中间表在2个表上应用外部联接？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/9090804/