一内容回顾
insert
insert into 表名 (字段名) values (值)
insert into 表名 values (有多少个字段写多少个值)
insert into 表名 values (有多少个字段写多少个值),
(有多少个字段写多少个值),
(有多少个字段写多少个值),
update
update 表名 set 字段名 = 新的值 where 条件
update 表名 set 字段名 = 新的值, 字段名 = 新的值 where 条件;
delete
delete from 表名 where 条件;
单表查询
select distinct 字段 from 表 where 条件
group by 分组
having 过滤组
order by 排序
limit m,n
select distinct 字段 from 表
concat('name : ',name,' ,age : ',age)
concat_ws('-',name,age,salary)
egon-18-10000
四则运算
select age+1 from tablename
select salary*12 from tablename
重命名 as
select name as n from 表
select name n from 表
case 语句
case
when 条件
then 显示的内容
when 条件
then 显示的内容
else
显示的内容
end
distinct 关键字 去重功能
where 条件
比较运算 > < = != <> >= <=
范围 between and/in
like
'%'
'_'
逻辑运算符 and or not
select * from 表 where a=1 and b=2 and c=3
and来说 有任意一个不成立就不需要继续判断了
select * from 表 where a=1 or b =2 or c =3
or来说,有任意一个不成立并不影响继续判断其他条件
select * from 表 where a not between and
group by
group_concat(字段) 可以显示一个分组内的所有字段内容
having
对分组之后的结果进行过滤
应该和group by连用
聚合函数
count
sum
avg
max
min
order by
默认从小到大 升序 asc
select * from 表 order by score
select * from 表 order by score asc
降序排列 从大到小
select * from 表 order by score desc
limit
根据前面的条件筛选出多行,取前n行
取前n名
order by 和 limit连用
limit n
分页
我要取所有的符合条件的项目
但是一次性显示到页面中显示所以先返回一部分,然后再返回剩余的
limit 0,25
limit 25,25
limit 50,25
怎么用python操作mysql
sql注入
今日内容
多表查询
连表查询
先把表连起来,再查询
子查询
先把一个结果从一张表中查不出来,再根据结果查询另一个表
能用子查询做的 就不用 连表
多表查询
# 一 \ 内连接 # select * from department # inner join employee # on department.id = employee.dep_id; # 只会把左表和右表对应的行显示出来 # 如果左表中的department.id在employee.dep_id中没有出现,那么会抛弃这个行 # 如果右表中的employee.dep_id在department.id中没有出现,那么也会抛弃这个行 # 内连接的另一种方式 用where # select * from department,employee # where department.id = employee.dep_id; # 从不同的表中取字段 # select employee.name,employee.sex,department.name # from department,employee # where department.id = employee.dep_id; # 给表起别名,让sql更简单 # select emp.name,emp.sex,dep.name # from department as dep,employee as emp # where dep.id = emp.dep_id; # 给字段起别名,让显示的效果更明确 # select emp.name,emp.sex,dep.name as dep_name # from department as dep,employee as emp # where dep.id = emp.dep_id; # 外连接 # 二 \左外连接 ： 显示左表中的所有项，和右表中所有满足拼接条件的项 # select * from department # left join employee # on department.id = employee.dep_id; # 三 \右外连接：显示右表中的所有项，和左表中所有满足拼接条件的项 # select * from department # right join employee # on department.id = employee.dep_id; # 四 \全外连接：左表和右表都完全显示出来了 # select * from department left join employee on department.id = employee.dep_id # union all # select * from department right join employee on department.id = employee.dep_id # 练习 # 示例一 : 找到年龄> 25的员工的姓名和部门 # select * from employee # inner join department # on employee.dep_id = department.id # where age>25; # 示例一变式 : 找到alex员工的年龄和部门 # select age,department.name from employee # inner join department # on employee.dep_id = department.id # where employee.name = 'alex'; # 给表重命名 # select age,dep.name from employee as emp # inner join department as dep # on emp.dep_id = dep.id # where emp.name = 'alex'; # 给字段重命名引起的错误 # select employee.name as emp_name,age,department.name from employee # inner join department # on employee.dep_id = department.id # where emp_name = 'alex'; # 报错,因为在select处重名名不能在where/group by/having中使用,由于mysql的词法分析顺序导致该问题 # 示例2：以内连接的方式查询employee和department表，并且以age字段的升序方式显 # select * from employee # inner join department # on employee.dep_id = department.id # order by age;
多表查询_子查询
# 示例一 : 查询平均年龄在25岁以上的部门名 # 涉及到年龄 员工表 # 部门名字 部门表 # 你的结果在哪个表,那个表一定不是子查询的表 # 1. 连表查询的结果 # 先内连接得到一张大表 # select * from department # inner join employee # on department.id = employee.dep_id # 再根据部门分组 # select department.name from department # inner join employee # on department.id = employee.dep_id # group by department.id # having avg(age) > 25; # 2. 子查询的结果 # 先完成一部分需求,求每一个部门的人的平均年龄 # select dep_id,avg(age) from employee group by dep_id # 再筛选出平均年龄大于25的部门 # select dep_id,avg(age) from employee group by dep_id having avg(age)>25 # 由于我们只需要部门名称,而和部门名称相关的项就只有部门id,所以我们只留下dep_id字段 # select dep_id from employee group by dep_id having avg(age)>25 # 查询部门表,找到id在上面这个查询结果内的内容 # select name from department where id in ( # select dep_id from employee group by dep_id having avg(age)>25 # ) pass # 示例2 : 查看"技术"部员工姓名 # 结果是 : 姓名 - 员工表 # 怎么知道技术部是谁? 怎么和员工表关联? # 如果我能知道技术部的id是多少,就可以查询了 # 1.查询技术部的id # select id from department where name = '技术' # 2.取id=200的所有人 # select * from employee where dep_id = ( # select id from department where name = '技术'); # 3.只取名字 # select name from employee where dep_id = ( # select id from department where name = '技术'); pass # 示例3 :查看不足1人的部门名 # 结果是 部门名 - 部门表 # 先操作员工表 # 1.找到员工表中所有人的部门id # select dep_id from employee group by dep_id; # select distinct dep_id from employee; # 2.操作部门表查看id not in 上面范围中的项目 # select name from department where id not in ( # select dep_id from employee group by dep_id); # select name from department where id not in ( # select distinct dep_id from employee);