当咱们须要在一条Sql语句中进行不一样条件的统计时,单独的COUNT()没法知足咱们的需求,这时能够经过SUM()来完成。sql
SELECT CourseID, TotalCount=COUNT(*), UnreadCount=SUM(CASE WHEN ReadStatus=0 THEN 1 ELSE 0 END) FROM CourseComments GROUP BY CourseID
COUNT() 函数返回匹配指定条件的行数。
函数
SUM 函数返回数值列的总数(总额)。code
在SUM中的CASE能够理解为先为查询到的结果集增长了外部行ReadCount,知足条件的计数1,不知足条件计数0,而后再用SUM函数求和获得统计的总数。例:class
SELECT CourseID, TotalCount=COUNT(*), UnreadCount=SUM(ReadCount) FROM (SELECT CourseID, ReadCount=(CASE WHEN ReadStatus=0 THEN 1 ELSE 0 END) FROM CourseComments) A GROUP BY A.CourseID