mysql-mysql连接中的条件“on子句”

我尝试在mysql连接中建立一个条件“on子句”

如果字段1不为空，则应在“on claus”中使用， 但如果为空，则应使用字段2

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

你知道如何做到这一点吗

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，因为这会产生很多连接，所以我的想法应该是这样的：

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

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)
     )

请尝试以下方法：

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

然后使用条件从表2或表3中选择字段

希望这对你有帮助

编辑

要选择正确的字段，请使用以下命令：

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

请尝试以下方法：

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

然后使用条件从表2或表3中选择字段

希望这对你有帮助

编辑

要选择正确的字段，请使用以下命令：

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

请尝试以下方法：

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

然后使用条件从表2或表3中选择字段

希望这对你有帮助

编辑

要选择正确的字段，请使用以下命令：

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

请尝试以下方法：

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

然后使用条件从表2或表3中选择字段

希望这对你有帮助

编辑

要选择正确的字段，请使用以下命令：

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

---

你应该考虑把你的内部连接转换成左连接，如在其他的答案中，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

你也可以试试下面的方法

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

---

你应该考虑把你的内部连接转换成左连接，如在其他的答案中，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

你也可以试试下面的方法

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

---

你应该考虑把你的内部连接转换成左连接，如在其他的答案中，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

你也可以试试下面的方法

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

---

你应该考虑把你的内部连接转换成左连接，如在其他的答案中，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

你也可以试试下面的方法

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

---

我想你可以这样做：

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

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有多高

我想你可以这样做：

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

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有多高

我想你可以这样做：

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

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有多高

我想你可以这样做：

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

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有多高

条件应该放在where子句中，对吗？不，在select中。我编辑我的回答目前我在“on子句”Try t1.Field1中得到未知列“t2.Field1”。可能与大写字母有关。条件应放在where子句中，对吗？否。在select中。我编辑我的回答目前我在“on子句”Try t1.Field1中得到未知列“t2.Field1”。可能与大写字母有关。条件应放在where子句中，对吗？否。在select中。我编辑我的回答目前我在“on子句”Try t1.Field1中得到未知列“t2.Field1”。可能与大写字母有关。条件应放在where子句中，对吗？否。在select中。我编辑我的回答目前我在“on子句”Try t1.Field1中得到未知列“t2.Field1”。可能与大写字母有关。