1.SELECT O.ID AS ID,O.NAME AS NAME,COUNT(C.ID) AS CLASS_NUM FROM T_BASE_ORGANIZATION O
2.LEFT JOIN T_XSXX_CLASSINFORMATION C ON O.ID = C.FACULTY_ID
3.LEFT JOIN T_SZDW_CLASSMEETING_PLAN P ON C.ID = P.CLASS_ID
4.WHERE C.GRADE = 2010
5.GROUP BY O.ID,O.NAME,O.ORDERS
6.ORDER BY O.ORDERS
这表结构是：
T_BASE_ORGANIZATION 学院表
T_XSXX_CLASSINFORMATION 班级表
T_SZDW_CLASSMEETING_PLAN 班会表
一个学院 多个班级
一个班级 多个班会
总之引发查询出重复数据就是因为，表之间关联的关系不是一对一的，导致数据重复。
解决方式
方法一：在 GROUP BY 加上（多表的外键） C.ID
SELECT O.ID AS ID,O.NAME AS NAME,COUNT(C.ID) AS CLASS_NUM FROM T_BASE_ORGANIZATION O
LEFT JOIN T_XSXX_CLASSINFORMATION C ON O.ID = C.FACULTY_ID
LEFT JOIN T_SZDW_CLASSMEETING_PLAN P ON C.ID = P.CLASS_ID
WHERE C.GRADE = 2010
GROUP BY O.ID,O.NAME,O.ORDERS,C.ID
ORDER BY O.ORDERS
方法二：在返回结果数据的 地方加上DISTINCT函数, COUNT(DISTINCT(C.ID))
SELECT O.ID AS ID,O.NAME AS NAME,COUNT(DISTINCT(C.ID)) AS CLASS_NUM FROM T_BASE_ORGANIZATION O
LEFT JOIN T_XSXX_CLASSINFORMATION C ON O.ID = C.FACULTY_ID
LEFT JOIN T_SZDW_CLASSMEETING_PLAN P ON C.ID = P.CLASS_ID
WHERE C.GRADE = 2010
GROUP BY O.ID,O.NAME,O.ORDERS
ORDER BY O.ORDERS
方法三：在做left join、right join、inner join的时候将 关联的表先DISTINCT(外键) 子查询一遍
SELECT O.ID AS ID,O.NAME AS NAME,COUNT(C.ID) AS CLASS_NUM FROM T_BASE_ORGANIZATION O
LEFT JOIN T_XSXX_CLASSINFORMATION C ON O.ID = C.FACULTY_ID
LEFT JOIN (SELECT DISTINCT(Z.CLASS_ID) FROM T_SZDW_CLASSMEETING_PLAN Z ) P ON C.ID = P.CLASS_ID
WHERE C.GRADE = 2010
GROUP BY O.ID,O.NAME,O.ORDERS
ORDER BY O.ORDERS
作者：Java大忙人
来源：CSDN
原文：https://blog.csdn.net/weixin_42289383/article/details/89095485
版权声明：本文为博主原创文章，转载请附上博文链接！