count 用法

1

count：通过分组，进行统计，与group by 联合使用，当count()得到的数作为查询条件，需要与having联合使用，而不是where

use datebase_name

go 

create table text1

([number] [int] NOT NULL  primary key,

[indate] [datetime] NULL,

[name] [nvarchar](255) NULL,

[grade] [nvarchar](255) NULL,

[teacher] [nvarchar](255) NULL,

[money] [money] NULL,

[class] [nvarchar](50) NULL,

[course]  [nvarchar](50) NULL)

go 

insert into text1

values(1,'2020-10-27','张三','98','刘四','4000','一年级','英语'),
(2,'2020-10-27','张三','98','刘四','4000','一年级','语文'),
(3,'2020-10-27','张三','98','刘四','4000','一年级','数学'),
(4,'2020-10-27','李四','98','刘四','4000','一年级','英语'),
(5,'2020-10-27','李四','98','刘四','4000','一年级','语文'),
(6,'2020-10-27','李四','98','刘四','4000','一年级','数学');

select count(name),course
from text1
where name = '张三'
group by course
having count(name) is not null

2

count：count可单独使用，统计的是符合条件的所有记录数量，count（0）与count（字段名1 /字段名2）作用相同

select count(0)

from text1