Question

我尝试建立条件＆＃39; on条款＆＃39;在mysql连接中。

如果field1不为空，则应在＆＃39; on claus＆＃39;中使用但如果为空，则应使用field2。

SELECT * FROM table1
JOIN table2

IF (field1!='') THEN (
    ON  table1.field1 = table2.field1
    AND table1.field3 = table2.field3
    )
ELSE (
    ON  table1.field2 = table2.field2
    AND table1.field3 = table2.field3
    )
END IF

有任何想法是否可以做到这一点？

编辑：我忘了解释两个表都包含空字段，我试图阻止mysql使用这些字段进行连接，因为它提供了（非常）很多连接，所以我的想法应该更像这样：

SELECT * FROM table1
JOIN table2

IF (table1.field1!='' AND table2.field1!='') THEN (
    ON  table1.field1 = table2.field1
    AND table1.field3 = table2.field3
    )
ELSE (
    ON  table1.field2 = table2.field2
    AND table1.field3 = table2.field3
    )
END IF

Answer 1

请改为尝试：

SELECT * FROM table1 t1
LEFT JOIN table2 t2
LEFT JOIN table2 t3
on t1.Field1=t2.Field1 and t1.Field3=t2.Field3
on t1.Field2=t3.Field2 and t1.Field3=t3.Field3

然后使用条件从table2或table3中选择字段。

希望这会对你有所帮助。

修改

要选择正确的字段，请使用：

SELECT t1.*, IF(t2.field1 is null, t3.field1, t2.Field1) as Field1

Answer 2

您应该考虑将INNER JOIN转换为LEFT JOIN，如其他答案所示，如

SELECT * FROM table1 t1
     LEFT JOIN table2 t2
     ON t1.field1 = t2.field1 AND t1.field3 = t2.field3
     LEFT JOIN table2 t22
     ON t1.field2 = t22.field2 AND t1.field3 = t22.field3

您也可以尝试下面的内容

SELECT t1.*
FROM table1 t1
  JOIN table2 t2
    ON t1.field1 = t2.field1 AND t1.field3 = t2.field3 
       OR t1.field2 = t2.field2 AND t1.field3 = t2.field3

Answer 3

我认为你可以这样做：

SELECT * FROM table1 
JOIN table2 ON 
     (
        (table1.field1 = '' AND table1.field2 = table2.field2 AND table1.field3 = table2.field3) 
        OR 
        (table1.field1 != '' AND table1.field1 = table2.field1 AND table1.field3 = table2.field3)
     )

（注意：我不知道这种方法的效率如何）

mysql - 条件＆＃39; on子句＆＃39;在mysql中加入

3 个答案: