DQL：用来查询数据库表中的记录

关键字：SELECT

语法：

DQL-基本查询

示例：

select name, entrydate from tb_emp;

select *
from tb_emp;

select name as 姓名, entrydate as 入职日期 from tb_emp;

select distinct job from tb_emp;

DQL-条件查询

        条件查询：select 字段列表 from where 条件列表；

示例：

select *
from tb_emp where name='杨逍';

select *
from tb_emp where id<=5;

select *
from tb_emp where job is null;

select *
from tb_emp where job is not null;

select *from tb_emp where password!=123456;

select *
from tb_emp
where entrydate between '2001-01-01' and '2010-01-01 '
  and gender = 2;

select *
from tb_emp
where job in (2, 3, 4);

select *
from tb_emp
where name like '__';

select *
from tb_emp
where name like '张%';

 DQL-分组查询

介绍：将一列数据作为一个整体，进行纵向计算

语法：select聚合函数（字段列表）from 表名；

示例：

select count(id)from tb_emp;
select count(0)from tb_emp;
select count(*)from tb_emp;

select min(entrydate)from tb_emp;

select avg(id)from tb_emp;

 注意：null值不参与所有聚合函数运算

            统计数量可以用：count（*）   count（字段）  count（常量）  推荐：count（*）

DQL-分组查询

        select 字段列表 from 表名 [where 条件] group by 分组字段名 [having 分组后过滤条件]

select gender,count(*) from tb_emp group by gender;

select job ,count(*) from tb_emp where entrydate<='2015-01-01' group by job having count(*)>=2;

 where和having区别

        1.执行时机不同：where是分组之前进行过滤，不满足where条件，不参与where条件，不参与分组；而having是分组之后对结果进行过滤

        2.判断条件不同：where不能对聚合函数进行判断，而having可以

 DQL-排序查询

示例：

select *
from tb_emp order by entrydate;

select *
from tb_emp order by entrydate ,update_time desc ;

DQL-分页查询

        select 字段列表 from 表名 limit 起始索引，查询记录数； 

示例：

select *
from tb_emp limit 0,5;

 注意：

案例：

select *
from tb_emp
where name like '张%'
  and gender = 1
  and entrydate between '2000-01-01' and '2015-01-01'
order by update_time desc
limit 0,10;

select (case job when 1 then '班主任' when 2 then '讲师' when 3 then '学工主管' when 4 then '教研主管'else'未分配'end)职位,
count(*)
from tb_emp group by job;