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