Question

表1

id，userid，eventid，name

表2

eventid，zoneid，userid

表3

eventid，userid，status

如果所有三个表都具有eventid意味着我不想选择该记录（我的意思是如果表3具有eventid），否则我需要选择记录

我尝试了我的查询

SELECT 
    * 
FROM 
    `table1` c1 
    INNER JOIN `table2` c2 ON c2.eventid = c1.eventid  
    LEFT JOIN table3 c3 ON c3.eventid = c1.eventid 
WHERE 
    c2.zoneid=2 
    AND c1.active='1' 
GROUP BY 
    c1.eventid

Answer 1

添加一个where子句，其中没有c3：

SELECT * 
FROM `table1` c1 
INNER JOIN `table2` c2 ON c2.eventid = c1.eventid  
LEFT JOIN table3 c3 ON c3.eventid = c1.eventid 
WHERE c2.zoneid=2 AND c1.active='1' 
AND c3.id IS NULL
group by c1.eventid

Answer 2

SELECT 
    * 
FROM 
    `table1` c1 
    INNER JOIN `table2` c2 ON c2.eventid = c1.eventid  
WHERE 
    c2.zoneid=2 
    AND c1.active='1' 
    AND NOT EXISTS (SELECT * FROM table3 c3 WHERE c3.eventid = c1.eventid)
GROUP BY 
    c1.eventid

在已通过LEFT / RIGHT OUTER JOIN连接的表上应用WHERE条件（如其他一些建议的建议）实际上会使其成为常规连接。已发布的其他示例要求c1.eventid等于c3.eventid，并且c3.eventid为NULL - 很有可能结果不符合您的预期，具体取决于数据库如何处理c1.eventid = c3.eventid两者是NULL（我必须阅读）。

Answer 3

第3个表上的左连接，条件WHERE C.eventid IS NULL应该可以正常工作。

SELECT * 
FROM   table1 A 
       INNER JOIN table2 B 
                    ON A.eventid = B.eventid 
       LEFT OUTER JOIN table3 C 
                    ON A.eventid = C.eventid 
WHERE  C.eventid IS NULL

如果第3个表没有该值，则连接3个表

3 个答案: