如何在MySQL中获得当前和缺勤学生的数量？
为此，您可以将其IF()与聚合函数一起使用SUM()。让我们首先创建一个表-
mysql> create table DemoTable1617
-> (
-> Attendance varchar(20),
-> CurrentYear int
-> );使用插入命令在表中插入一些记录-
mysql> insert into DemoTable1617 values('Present',2019);
mysql> insert into DemoTable1617 values('Absent',2019);
mysql> insert into DemoTable1617 values('Absent',2017);
mysql> insert into DemoTable1617 values('Present',2019);
mysql> insert into DemoTable1617 values('Present',2018);
mysql> insert into DemoTable1617 values('Present',2019);使用select语句显示表中的所有记录
mysql> select * from DemoTable1617;
这将产生以下输出-
+------------+-------------+ | Attendance | CurrentYear | +------------+-------------+ | Present | 2019 | | Absent | 2019 | | Absent | 2017 | | Present | 2019 | | Present | 2018 | | Present | 2019 | +------------+-------------+ 6 rows in set (0.00 sec)
以下是查询以获取当前和缺勤学生一年的数量的查询
mysql> select sum(if(Attendance='Present',1,0)) as Present ,
-> sum(if(Attendance='Absent',1,0)) as Absent
-> from DemoTable1617
-> where CurrentYear LIKE '2019%';这将产生以下输出-
+---------+--------+ | Present | Absent | +---------+--------+ | 3 | 1 | +---------+--------+ 1 row in set (0.00 sec)