mysql操作进阶「建议收藏」

# ### part1 单表查询
# sql 查询语句的完整语法
“”” select .. from .. where .. group by .. having .. order by .. limit .. “””

# 一.where 条件的使用
“””功能:对表中的数据进行筛选过滤”””

“””
语法:
1.判断的符号:
= > < >= <= != <> 不等于
2.拼接条件的关键字
and or not
3.查询的区间范围值 between
between 小值 and 大值 [小值,大值] 查询两者之间这个范围的所有数据
4.查询具体某个值的范围 in
in(1,-9,-10,”a”) 指定范围
5.模糊查询 like “%” 通配符
like “%a” 匹配以a结尾的任意长度的字符串
like “a%” 匹配以a开头的任意长度的字符串
like “%a%” 匹配含有a字母的任意长度字符串
like “_a” 个数一共2个字符,必须以a结尾,前面这个字符随意
like “a__” 个数一共3个字符,必须以a开头,后面这个两字符随意
“””
# (1) 单条件的查询
# 查询部门是sale的所有员工姓名:
select emp_name from employee where post = “sale”;

# (2) 多条件的查询
# 部门是teacher,收入大于10000的所有数据
select * from employee where post = “teacher” and salary > 10000;

# (3) 关键字between .. and
# 收入在1万到2万之间的所有员工姓名和收入
select emp_name,salary from employee where salary between 10000 and 20000;
# 收入不在1万到2万之间的所有员工姓名和收入
select emp_name,salary from employee where salary not between 10000 and 20000;

# (4) null关键字 在搜索的时候,要用is进行判定,不能用=
# 查询 post_comment 是空的NULL 所有数据
select * from employee where post_comment = NULL 数据是空,搜索不到
select * from employee where post_comment is NULL
select * from employee where post_comment is not NULL

update employee set post_comment = “” where id = 1
select * from employee where post_comment = “”;

# (5) 关键字 in 的查询
# 查询收入是 3000 或 5000 或者 4000 或者 8000 所有员工姓名和收入
select emp_name,salary from employee where salary=3500 or salary=5000 or salary=8300 or salary=4000;
# 用in优化,在小括号里面写上可能的值
select emp_name,salary from employee where salary in (3500,5000,8300,4000);
# 不在括号中的值,搜索出来
select emp_name,salary from employee where salary not in (3500,5000,8300,4000);

# (6) 关键字 like 模糊查询
# (1) % 通配符
select emp_name,age,post from employee where emp_name like “%on”;
# (2) _ 通配符
select emp_name,age,post from employee where emp_name like “a_e_”;

# (7) concat
select concat(“姓名:”,emp_name,”薪资:”,salary) as aaa from employee;
# concat_ws(拼接的符号,参数1,参数2,参数3 … )
select concat_ws(” : “,emp_name,salary) as bbb from employee;
# 可以在sql中使用四则运算(+ – * /)
select concat_ws(” : “,emp_name, salary * 12 ) as bbb from employee;

# 二.group by 子句 分组,分类
“””group by 对数据进行分类, by 后面接的字段,就是select要搜索的字段”””
select sex from employee group by sex;
select post from employee group by post;
# group_concat 按照分组形式进行字段的拼接
select group_concat(emp_name),post from employee where id>1 group by post;

# 聚合函数
# 统计总数 count *所有
select count(*) from employee
# 统计最大值 max
select max(salary) from employee
# 统计最小值 min
select min(salary) from employee
# 统计平均值 avg
select avg(salary) from employee
# 统计总和 sum
select sum(salary) from employee

# 一般来说 使用时 分组 + 聚合函数 配合使用
# 1. 查询部门名以及各部门的平均薪资
select post , avg(salary) from employee group by post;
# 2. 查询部门名以及各部门的最高薪资
select post , max(salary) from employee group by post;
# 3. 查询部门名以及各部门的最低薪资
select post , min(salary) from employee group by post;
# 4. 查询公司内男员工和女员工的个数
select sex,count(*) from employee group by sex
# 5. 查询部门名以及部门包含的所有员工名字
select group_concat(emp_name) , post from employee group by post
select emp_name,post from employee group by post,emp_name

# 三.having 查询数据之后在进行过滤,一般是配合group by使用, 主要用分组后过滤
# 找出各部门的平均薪资,并且大于10000以上的所有部门
select post,avg(salary) from employee group by post having avg(salary) > 10000;
# 1.查询各岗位内包含的员工个数小于2的岗位名,员工名,个数
select post,group_concat(emp_name),count(*) from employee group by post having count(*) < 2
# 2.查询各岗位平均薪资小于10000的岗位名、平均工资
select post,avg(salary) from employee group by post having avg(salary) < 10000
# 3.查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资
select post,avg(salary) from employee group by post having avg(salary) between 10000 and 20000
select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) < 20000

# 四.order by 排序 , 按照什么字段进行排序
# 默认值asc 升序排序
# 按照desc 降序排序
select * from employee order by age (默认升序)
select * from employee order by age desc (降序)

# 1. 查询所有员工信息，先按照age升序排序，如果age相同则按照hire_date降序排序
select emp_name,sex,age,hire_date,post from employee order by age,hire_date desc
# 2. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资降序排列
select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) desc
# 3. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资升序排列
select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) asc

# 五.limit 限制查询的条数 (数据分页)
limit m,n m代表从第几条开始查询,n代表查询几条 m=0 代表的是第一条
select * from employee limit 0,5 从第一条开始查,查5条
select * from employee limit 5,5 从第六条开始查,查5条
# 只查询一条数据
select * from employee limit 1
# 想要瞬间得到数据表中,最后一条数据
select * from employee order by id desc limit 1
# 拿到最后三条数据
select * from employee order by id desc limit 3

# 六.(了解) 可以使用正则表达式查询数据 (不推荐使用,不好用效率不高)
select * from employee where emp_name regexp “.*on$” # .*? 的?号不识别
select * from employee where emp_name regexp “^程”;
select * from employee where emp_name regexp “^程.*金”;

# ### part2 多表查询
# 内连接:(内联查询 inner join ) : 两表或者多表满足条件的所有数据查询出来[两个表之间共同具有的数据]
“””
# 两表查询
select 字段 from 表1 inner join 表2 on 条件
# 多表查询
select 字段 from 表1 inner join 表2 on 条件 inner join 表3 on 条件

“””

# 基本语法 inner join on 接的表与表之间的必要连接条件
select * from employee inner join department on employee.dep_id = department.id
# 用as 起别名 (推荐)
select * from employee as e inner join department as d on e.dep_id = d.id
# 可以省略as
select * from employee e inner join department d on e.dep_id = d.id

# where 实现的就是内联查询
select * from employee,department where employee.dep_id = department.id
select * from employee as e,department as d where e.dep_id = d.id

# 外连接
# (1) 左连接 (左联查询 left join ) : 以左表为主,右表为辅,完整查询左表所有数据,右表没有的数据补NULL
“”” select 字段 from 表1 left join 表2 on 条件 “””
select * from employee left join department on employee.dep_id = department.id

# (2) 右连接 (右联查询 right join) : 以右表为主,左表为辅,完整查询右表所有数据,左表没有的数据补NULL
“”” select 字段 from 表1 right join 表2 on 条件 “””
select * from employee right join department on employee.dep_id = department.id
# (3) 全连接 (union) 所有数据全都合并起来
select * from employee left join department on employee.dep_id = department.id
union
select * from employee right join department on employee.dep_id = department.id

# ### part3 子查询
“””
子查询: 嵌套查询
(1) 子查询是查询的语句当中又嵌套的另外一条sql语句,用括号()抱起来,表达一个整体
(2) 一般应用在from 子句后面表达一张表,或者 where 子句后面表达一个条件
(3) 速度从快到慢 单表查询速度最快 -> 联表查询 -> 子查询
“””

# (1)找出平均年龄大于25岁以上的部门
# 普通的where 相当于内联查询
select
d.id,d.name
from
employee e,department d
where
e.dep_id = d.id
group by
d.id,d.name
having
avg(e.age) > 25;

# (2) inner join
select
d.id,d.name
from
employee e inner join department d on e.dep_id = d.id
group by
d.id,d.name
having
avg(e.age) > 25;

# (3) 子查询
# 1.先选出平均年龄大于25岁的部门id
select dep_id from employee group by dep_id having avg(age) > 25;
# 2.通过部门id,找部门名字
select name from department where id in (201,202)
# 3.综合拼接:
select id,name from department where id in (select dep_id from employee group by dep_id having avg(age) > 25)

# (2)查看技术部门员工姓名
# 1.普通where查询

select
e.name
from
employee e ,department d
where
e.dep_id = d.id and d.name = “技术”

# 2.inner join 实现
select
e.name
from
employee e inner join department d on e.dep_id = d.id
where
d.name = “技术”

# 3.子查询
# 1.找技术部门对应id
select id from department where name = “技术”

# 2.通过id找员工姓名
select name from employee where employee.dep_id = ?

# 3.综合拼接
select name from employee where employee.dep_id = (select id from department where name = “技术”)

# (3)查看哪个部门没员工

# 联表写法
select
d.id,d.name
from
employee e right join department d on e.dep_id = d.id
where
e.dep_id is NULL

# 子查询
# 1.先查询,员工都在哪些部门
select dep_id from employee group by dep_id => (200,201,202,204)
# 2.把不在部门列表中的数据找出来
select from department where id not in (1)
# 3.综合拼接
select id,name from department where id not in (select dep_id from employee group by dep_id)

# (4)查询大于平均年龄的员工名与年龄
# 假设平均年龄是18岁
select name,age from employee where age > ?
# 找平均年龄
select avg(age) from employee
# 综合拼装
select name,age from employee where age > (select avg(age) from employee)

# (5)把大于其本部门平均年龄的员工名和姓名查出来
# employee
+—-+————+——–+——+——–+
| id | name | sex | age | dep_id || dep_id | avg(age) |
+—-+————+——–+——+——–+
| 1 | egon | male | 18 | 200 |
| 2 | alex | female | 48 | 201 |
| 3 | wupeiqi | male | 38 | 201 |
| 4 | yuanhao | female | 28 | 202 |
| 5 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | female | 18 | 204 |
+—-+————+——–+——+——–+
# department
+——+————–+
| id | name |
+——+————–+
| 200 | 技术 |
| 201 | 人力资源 |
| 202 | 销售 |
| 203 | 运营 |
+——+————–+
# 1.先计算平均年龄
select dep_id,avg(age) from employee group by dep_id
+——–+———-+
| dep_id | avg(age) |
+——–+———-+
| 200 | 18.0000 |
| 201 | 43.0000 |
| 202 | 28.0000 |
| 204 | 18.0000 |
+——–+———-+
# 2.把子查询查出来的数据和employee作拼接,联合成一张更大的表,做一次单表查询;
select
*
from
employee as t1 inner join (1) as t2 on t1.dep_id = t2.dep_id

# 3.综合拼接
select
*
from
employee as t1 inner join (select dep_id,avg(age) as avg_age from employee group by dep_id) as t2 on t1.dep_id = t2.dep_id

# 4.把额外的比较的条件加进去
select
*
from
employee as t1 inner join (select dep_id,avg(age) as avg_age from employee group by dep_id) as t2 on t1.dep_id = t2.dep_id
where
t1.age > t2.avg_age

# (6)查询每个部门最新入职的那位员工 # 利用上一套数据表进行查询;
# 1.找每个部门最大的入职时间
select post,max(hire_date) as max_date from employee group by post

# 2.把子查询查出来的数据和employee联合成一张更大的表,做一次单表查询
select
from
employee as t1 inner join (1) as t2 on t1.post = t2.post
where
t1.hire_date = t2.max_date

# 3.综合拼接
select
t1.emp_name,t1.hire_date
from
employee as t1 inner join (select post,max(hire_date) as max_date from employee group by post) as t2 on t1.post = t2.post
where
t1.hire_date = t2.max_date

# (7)带EXISTS关键字的子查询
“””
exists 关键字表达存在
如果内层sql 能够查到数据, 返回True , 外层sql执行查询语句
如果内层sql 不能查到数据, 返回False, 外层sql不执行查询语句
“””
select * from employee where exists (select * from employee where id = 1)

“””
子查询总结:
子查询可以单独作为一个子句,也可以作为一个表或者某个字段
一般用在from where select 子句后面
通过查询出来的临时表,可以跟任意的表重新拼接,组成更大的表,在通过筛选达成自己的目的
“””