mysql视频教程栏目介绍查询语句的操作
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一.查询语句的基本操作
1.查询语句的基本操作
- select
- from
- where:约束条件
- group by:分组
- having：过滤
- distinct：去重
- order by：排序
- limit：限制查询记录的数量
- 聚合函数: count（计数）
max（最大值）
min（最小值）
avg（平均值）
sum(求和)
二.单表查询
1、前期表与数据准备
# 创建一张部门表
create table emp(
id int not null unique auto_increment,
name varchar(20) not null,
sex enum('male','female') not null default 'male', #大部分是男的
age int(3) unsigned not null default 28,
hire_date date not null,
post varchar(50),
post_comment varchar(100),
salary double(15,2),
office int, # 一个部门一个屋子
depart_id int
);
# 插入记录
# 三个部门：教学，销售，运营
insert into emp(name,sex,age,hire_date,post,salary,office,depart_id) values
('tank','male',17,'20170301','张江第一帅形象代言部门',7300.33,401,1), # 以下是教学部
('egon','male',78,'20150302','teacher',1000000.31,401,1),
('kevin','male',81,'20130305','teacher',8300,401,1),
('jason','male',73,'20140701','teacher',3500,401,1),
('owen','male',28,'20121101','teacher',2100,401,1),
('jerry','female',18,'20110211','teacher',9000,401,1),
('大饼','male',18,'19000301','teacher',30000,401,1),
('sean','male',48,'20101111','teacher',10000,401,1),
('歪歪','female',48,'20150311','sale',3000.13,402,2),# 以下是销售部门
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),
('张野','male',28,'20160311','operation',10000.13,403,3), # 以下是运营部门
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬银','female',18,'20130311','operation',19000,403,3),
('程咬铜','male',18,'20150411','operation',18000,403,3),
('程咬铁','female',18,'20140512','operation',17000,403,3);
# PS：如果在windows系统中，插入中文字符，select的结果为空白，可以将所有字符编码统一设置成gbk
- select * from emp; # 若数据比较多，比较凌乱，可以在表后面+ \G
- select * from emp\G
- select * from emp; # 若数据比较多，比较凌乱，可以在表后面+ \G - select * from emp\G
重点：
写SQL语句必须遵循两点:
- 书写顺序:
# 获取id为 4、5的两条记录
select * from emp where id > 3 and id < 6;
- select
- from
- where
- 执行顺序:
比如: 图书管理员，得先找到是哪一个图书馆(哪张表),
再找这本书在图书馆的哪个位置(哪一条记录), 最后查找这个本书中某一页(哪些字段值);
select * from emp where id > 3 and id < 6;
- from ---> 找到图书馆
- where ---> 找到书的位置
- select ---> 找到书本中的某一页
注意: 必须记住SQL语句的 书写顺序 与 执行顺序(*******);
1.where(约束条件)
# PS: 根据执行顺序来书写 SQL语句，一步一步来写;
# 1.查询id大于等于3小于等于6的数据(and:与)
select * from emp where id >= 3 and id <= 6;
mysql> SELECT * FROM EMP WHERE ID > 3 AND ID <= 6;
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
| 4 | jason | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 5 | owen | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
| 6 | jerry | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
3 rows in set (0.00 sec)
#可以使用between()and() :两者之间
mysql> select * from emp where id between 3 and 6;
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
| 3 | kevin | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 |
| 4 | jason | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 5 | owen | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
| 6 | jerry | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
4 rows in set (0.00 sec)
# 2.查询薪资是20000或者18000或者17000的数据
# or: 或者
select * from emp where salary=20000 or salary=18000 or salary=17000;
# in: 在什么里
select * from emp where salary in (20000, 18000, 17000);
mysql> select * from emp where salary = 20000 or salary = 18000 or salary = 17000;
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 |
| 17 | 程咬铜 | male | 18 | 2015-04-11 | operation | NULL | 18000.00 | 403 | 3 |
| 18 | 程咬铁 | female | 18 | 2014-05-12 | operation | NULL | 17000.00 | 403 | 3 |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
3 rows in set (0.00 sec)
# 也可以使用in(3,4,5,6)
select * from emp where id in(3,4,5,6);
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
| 3 | kevin | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 |
| 4 | jason | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 5 | owen | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |
| 6 | jerry | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | <b style="color:transparent">来&源gao@dai!ma.com搞$代^码%网</b> 1 |
+----+-------+--------+-----+------------+---------+--------------+---------+--------+-----------+
4 rows in set (0.00 sec)
# 3.查询员工姓名中包含o字母 的 员工姓名和薪资
# like: 模糊匹配
# %: 匹配0个或多个任意字符(若前后都有%(%o%),查所有带有索引字符的记录,若头有%而尾没有(%o),则查所有尾带有索引字符的记录,若尾有%而头没有(o%),则查所有头带有索引字符的记录,若前后都没有%(o),则只会查询只有索引字符的字符)
# _: 匹配一个任意字符
select name, salary from emp where name like "%o%";
+-------+------------+
| name | salary |
+-------+------------+
| egon | 1000000.31 |
| jason | 3500.00 |
| owen | 2100.00 |
+-------+------------+
3 rows in set (0.00 sec)
mysql> select name, salary from emp where name like "o";
+------+--------+
| name | salary |
+------+--------+
| o | NULL |
+------+--------+
1 row in set (0.00 sec)
# 4.查找名字个数为3个的员工 名字 与 薪资
select name, salary from emp where name like "___";
+-----------+----------+
| name | salary |
+-----------+----------+
| 程咬金 | 20000.00 |
| 程咬银 | 19000.00 |
| 程咬铜 | 18000.00 |
| 程咬铁 | 17000.00 |
+-----------+----------+
4 rows in set (0.00 sec)
# 或者 # char_length(name): 计算名字字符的长度
select name, salary from emp where char_length(name) = 4;
mysql> select name, salary from emp where char_length(name) = 3;
+-----------+----------+
| name | salary |
+-----------+----------+
| 程咬金 | 20000.00 |
| 程咬银 | 19000.00 |
| 程咬铜 | 18000.00 |
| 程咬铁 | 17000.00 |
+-----------+----------+
4 rows in set (0.00 sec)
# 5.查询id小于3或者大于6的数据
# not in: 不再什么什么中
select * from emp where id not in (3, 4, 5, 6);
select * from emp where id not between 3 and 6;
# 6.查询薪资不在20000，18000，17000范围的数据
select * from emp where salary not in (20000, 18000, 17000);
# 7.查询岗位描述为空的 员工名 与 岗位名 post_comment
# 用等于号无法判定空字段
select name, post from emp where post_comment = null;
select * from emp where post_comment = null;
Empty set (0.00 sec)
# 注意: 针对null的值 需要使用 is
select name, post from emp where post_comment is null;
select name, post_comment from emp where post_comment is null;
+-----------+--------------+
| name | post_comment |
+-----------+--------------+
| tank | NULL |
| egon | NULL |
| kevin | NULL |
| jason | NULL |
| owen | NULL |
| jerry | NULL |
| 大饼 | NULL |
| sean | NULL |
| 歪歪 | NULL |
| 丫丫 | NULL |
| 丁丁 | NULL |
| 星星 | NULL |
| 格格 | NULL |
| 张野 | NULL |
| 程咬金 | NULL |
| 程咬银 | NULL |
| 程咬铜 | NULL |
| 程咬铁 | NULL |
| o | NULL |
+-----------+--------------+
19 rows in set (0.00 sec)
2.group by(分组)
- 书写顺序:
- select
- from
- where
- group by
- 执行顺序:
- from
- where
- group by
- select
# 什么叫分组
# 比如: 一张员工表中有性别字段，可以根据性别分组，一组是男性，一组是女性，或者是根据部门分组，有教学部、销售部等...
# 1.根据部门分组
# 非严格模式下可以获取 分组条件post 以外的字段数据
select post, salary from emp group by post;
# 设置严格模式:
show variables like "%mode%";
# 全局设置: 永久有效
set global sql_mode="strict_trans_tables,only_full_group_by";
"""
设置sql_mode为only_full_group_by，意味着以后但凡分组，只能取到分组的依据，
不应该在去取组里面的单个元素的值，那样的话分组就没有意义了，并且会报错,因为不分组就是对单个元素信息的随意获取
"""
mysql> select post, salary from emp group by post;
ERROR 1046 (3D000): No database selected
# 可以同聚合函数，间接获取其他字段数据
聚合函数:
count: 计数
max: 最大值
min: 最小值
avg: 平均值
sum: 求和
group_concat(name): 可以将分组后的 所有名字获取并进行拼接
# 指定以:拼接
select post, group_concat(name) from emp group by post;
select post, group_concat(name, ':') from emp group by post;
2.获取每个 部门 的最高工资
select post,max(salary) from emp group by post;
mysql> select post,max(salary) from emp group by post;
+-----------------------------------+-------------+
| post | max(salary) |
+-----------------------------------+-------------+
| NULL | NULL |
| operation | 20000.00 |
| sale | 4000.33 |
| teacher | 1000000.31 |
| 张江第一帅形象代言部门 | 7300.33 |
+-----------------------------------+-------------+
5 rows in set (0.00 sec)
补充: as 别名: 可以给字段 加一个 别名
select post as '部门', max(salary) as '薪资'
from emp group by post;
# 也可以简写,但是不推荐
select post '部门', max(salary) '薪资'
from emp group by post;
mysql> select post as '部门', max(salary) as'薪资'
-> from emp group by post;
+-----------------------------------+------------+
| 部门 | 薪资 |
+-----------------------------------+------------+
| NULL | NULL |
| operation | 20000.00 |
| sale | 4000.33 |
| teacher | 1000000.31 |
| 张江第一帅形象代言部门 | 7300.33 |
+-----------------------------------+------------+
5 rows in set (0.00 sec)
3.每个部门的最低工资
select post, min(salary) from emp group by post;
4.每个部门的平均工资
select post, avg(salary) from emp group by post;
5.每个部门的工资总和
select post, sum(salary) from emp group by post;
6.每个部门的员工个数
# count(): 括号中可以填任意非空值
select post, count(salary) from emp group by post;
select post, count(post_comment) from emp group by post;
查询岗位名以及各岗位内包含的员工个数
select post, count(id) from emp group by post;
查询公司内男员工和女员工的个数
select sex, count(*) from emp group by sex;
查询男员工与男员工的平均薪资，女员工与女员工的平均薪资
select sex, avg(salary) from emp group by sex;
.统计各部门年龄在30岁以上的员工平均工资:
# 步骤: 先找到表，再找年龄30岁以上，再根据部门分组，最后求平均薪资;
select post, avg(salary) from emp where age > 30 group by post;
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转载请注明原文链接：掌握MySQL之查询语句的基本操作
转载请注明原文链接：掌握MySQL之查询语句的基本操作
