MySQLDemo2

-- 查询所有数据库
show databases

-- 删除数据库
drop database a
-- use `数据库名称`; 表示使用此数据库
use mybatis
-- 查看表结构
show create table a1

-- 删除表
drop table a
-- 查询表总共有多少条数据
select count(1) from user

select count(id) from user

-- 添加一条数据
insert into user (name,age) values (‘蓝凤凰‘,19)
-- 添加多条数据
insert into user (name,age) values (‘蓝凤凰‘,19),(‘蓝孔雀‘,20)
-- 这两个一起运行
insert into user (name,age) values (‘曼联‘,21);
insert into user (name,age) values (‘张倩‘,19);
-- 更新一条数据
update user set name = ‘阿紫‘,age =18 where id = 8
-- 更新多条数据
-- 这两个一起运行
update user set name = ‘小张‘,age = 19 where id = 9;
update user set name = ‘张三‘,age = 20 where id = 10;
-- 删除一条数据
delete from user where id = 1
-- 删除多条数据
delete from user where id in (2,3)
-- 这两个一起运行
delete from user where id = 5;
delete from user where id = 6;
-- 查询年龄在18到20之间
select * from user where age between 18 and 20

-- 查询年龄>= 18并且年龄<=20数据
select * from user where age >=18 and age <=20

-- 从id=10开始算总条数
select count(8) from user where id > 9

-- 查询age的总数
select sum(age) from user

-- 这里查询名字没有总数就返回0
select ifnull(sum(name),0) from user

-- 这里是按照name分组去重,名字相同的就把age的值加起来
select name, sum(age) from user group by name;

-- 查询tx_delete判断是否删除
-- 第一种写法
select id,tx_delete,
case 
when tx_delete = 0 then ‘未删除‘
else ‘已删除‘
end as delete_sc
from user

select id,tx_delete,
case
when tx_delete = 0 then ‘未删除‘
when tx_delete = 1 then ‘已删除‘
end as delete_sc
from user

select id,tx_delete,
case
when tx_delete = 0 then ‘未删除‘
when tx_delete = 1 then ‘已删除‘
else ‘‘ end as delete_sc
from user

-- 第二种写法
-- 这个随便怎么样都是已删除
select id,tx_delete,
case
when 0 then ‘未删除‘
when 1 then ‘已删除‘
end as delete_sc
from user

-- 这个添加过数据的就是未删除
select id,tx_delete,
case tx_delete
when 0 then ‘未删除‘
else ‘已删除‘
end as delete_sc
from user

-- 第三种写法
select tx_delete, if(tx_delete = 0, ‘未删除‘, ‘已删除‘) as delete_sc from user

-- 左连接
select * from employee e left join dept d on e.dept_id = d.id

-- 右连接
select * from employee e right join dept d on e.dept_id = d.id

-- 内连接
select * from employee e join dept d on e.dept_id = d.id

select * from employee e inner join dept d on e.dept_id = d.id

-- where连接
select * from employee e, dept d where e.dept_id = d.id

-- 日期格式化

-- 拼接字符串
-- concat_ws用法
-- 这个拼接的 ‘-‘ 都是在中间
select name,age, concat_ws (‘-‘,name,age) from user

select name,age, concat_ws (‘-‘,name,age,‘%‘,‘=‘) from user

-- concat用法
-- 这个拼接的 ‘-‘ 这个符号放在哪个位置就在哪里        
select name,age, concat(name,‘-‘,age) from user

select name,age, concat(name,‘-‘,age,‘%‘,‘=‘) from user

-- 查询name相同的合并成一个name,id也查询出来
select name,group_concat(id) from user group by name

select name,group_concat(id order by id desc) from user group by name

-- 分割
select name,group_concat(id separator ‘-‘) from user group by name

-- 查询name,id,age,id和age用 ‘-‘ 分割
select name,group_concat(concat_ws(‘-‘,id,age)) as id_age from user group by name

-- 保留两位小数
select name,price,round(price,2) from user

select round(111.245,2)

select round(123.1243/0,2)

select round(0/1235.1545,2) 

-- 去重
select distinct name,id from user

-- 去重来查询总数
select count(distinct name) from user

-- 模糊查询
select id,name from user where name like ‘蓝%‘

select id,name from user where name like ‘%孔%‘

select id,name from user where name like ‘%雀‘

-- 不查询id为7和8的信息
select id,name from user where not id in (7,8)

-- 只查询id为7和8的信息
select id,name from user where id in (7,8)

--这几个的优先级: where > group by > having > order by > limit 
select e.dept_id,d.name from employee e left join dept d on e.dept_id = d.id

select count(1) c,d.name from employee e left join dept d on e.dept_id = d.id group by dept_id

select count(1) c,d.name from employee e left join 
dept d on e.dept_id = d.id 
group by dept_id having c > 1 order by c desc

select count(1) c,d.name from employee e left join 
dept d on e.dept_id = d.id where e.name!=‘天宇‘
group by dept_id having c > 1 order by c desc limit 1

-- 根据status和age来统计 成年人已删除，成年人未删除，未成年人已删除，未成年人未删除的总数
select 
    ifNull(sum(case when status = 0 and age >= 18 then 1 else 0 end), 0) as ‘成年人未删除总数‘
  , ifNull(sum(case when status = 0 and age < 18 then 1 else 0 end), 0) as ‘未成年人未删除总数‘
  , ifNull(sum(case when status = 1 and age >= 18 then 1 else 0 end), 0) as ‘成年人已删除总数‘
  , ifNull(sum(case when status = 1 and age < 18 then 1 else 0 end), 0) as ‘未成年人已删除总数‘
from user

-- 查询科目前二的分数
select u1.* from user1 u1 where
(select count(1) from user1 u2 where u1.course = u2.course and u1.score < u2.score) < 2
order by u1.course, u1.score desc;

-- 这句代码的意思是不会去重
select * from user where age > 18 union all
select * from user where status = 0;

-- 这句代码的意思是会去重
select * from user where age > 19 union
select * from user where status = 0;

-- 这句代码的意思是可以把u1表中的数据拷贝多次去u2表中
insert into u2(name,age) select name,age from u1
-- 这句代码的意思是不可以把t1表中的数据拷贝多次去t2表中,拷贝多次会报错,只能拷贝一次
insert into t2 select * from t1 
-- 这句代码的意思是把t1表的结构复制一份再创建一个表数据会过去,但是id自动递增不会设置
create table tt3 select * from t1;
-- 查询a1表设置进b1表中的数据
select * from a1
where exists (select 1 from b1 where a1.id = b1.aid)

-- 查询a1表没有设置进b1表中的数据
select * from a1
where not exists (select 1 from b1 where a1.id = b1.aid)

-- 查询a1表id为2的数据
select * from a1
where exists (select 1 from b1 where a1.id = 2)

-- 查询b1表连接a1表中的数据
select * from a1
where exists (select 1 from b1 where b1.aid = 1)