一、单表查询的语法
SELECT 字段1,字段2... FROM 表名 WHERE 条件 GROUP BY field HAVING 筛选 ORDER BY field LIMIT 限制条数
二、关键字的执行优先级(重点)
重点中的重点：关键字的执行优先级fromwheregroup byhavingselectdistinctorder bylimit1.找到表:from2.拿着where指定的约束条件，去文件/表中取出一条条记录3.将取出的一条条记录进行分组group by，如果没有group by，则整体作为一组4.将分组的结果进行having过滤5.执行select6.去重7.将结果按条件排序：order by8.限制结果的显示条数
三、单表查询示例
创建公司员工表，表的字段和数据类型:
company.employee员工id id int 姓名 name varchar 性别 sex enum 年龄 age int入职日期 hire_date date岗位 post varchar职位描述 post_comment varchar薪水 salary double办公室 office int部门编号 depart_id int
创建员工表，并插入记录：
#创建表,设置字段的约束条件create table employee( id int primary key 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 depart_id int);# 查看表结构mysql> desc employee;+--------------+-----------------------+------+-----+---------+----------------+| Field | Type | Null | Key | Default | Extra |+--------------+-----------------------+------+-----+---------+----------------+| id | int(11) | NO | PRI | NULL | auto_increment || emp_name | varchar(20) | NO | | NULL | || sex | enum('male','female') | NO | | male | || age | int(3) unsigned | NO | | 28 | || hire_date | date | NO | | NULL | || post | varchar(50) | YES | | NULL | || post_comment | varchar(100) | YES | | NULL | || salart | double(15,2) | YES | | NULL | || office | int(11) | YES | | NULL | || depart_id | int(11) | YES | | NULL | |+--------------+-----------------------+------+-----+---------+----------------+10 rows in set (0.08 sec)#插入记录#三个部门：教学，销售，运营insert into employee(name ,sex,age,hire_date,post,salary,office,depart_id) values('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部('alex','male',78,'20150302','teacher',1000000.31,401,1),('wupeiqi','male',81,'20130305','teacher',8300,401,1),('yuanhao','male',73,'20140701','teacher',3500,401,1),('liwenzhou','male',28,'20121101','teacher',2100,401,1),('jingliyang','female',18,'20110211','teacher',9000,401,1),('jinxin','male',18,'19000301','teacher',30000,401,1),('xiaomage','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); 1.where约束
where子句中可以使用：1.比较运算符：>、<、>=、<=、<>、!=2.between 80 and 100 ：值在80到100之间3.in(80,90,100)值是10或20或304.like 'xiaomagepattern': pattern可以是%或者_。%小时任意多字符，_表示一个字符5.逻辑运算符：在多个条件直接可以使用逻辑运算符 and or not
where约束验证：
#1 :单条件查询mysql> select id,emp_name from employee where id > 5;+----+------------+| id | emp_name |+----+------------+| 6 | jingliyang || 7 | jinxin || 8 | xiaomage || 9 | 歪歪 || 10 | 丫丫 || 11 | 丁丁 || 12 | 星星 || 13 | 格格 || 14 | 张野 || 15 | 程咬金 || 16 | 程咬银 || 17 | 程咬铜 || 18 | 程咬铁 |#2 多条件查询mysql> select emp_name from employee where post='teacher' and salary>10000;+----------+| emp_name |+----------+| alex || jinxin |+----------+#3.关键字BETWEEN AND SELECT name,salary FROM employee WHERE salary BETWEEN 10000 AND 20000; SELECT name,salary FROM employee WHERE salary NOT BETWEEN 10000 AND 20000;#注意''是空字符串，不是null SELECT name,post_comment FROM employee WHERE post_comment=''; ps： 执行 update employee set post_comment='' where id=2; 再用上条查看，就会有结果了#5:关键字IN集合查询mysql> SELECT name,salary FROM employee WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;+------------+---------+| name | salary |+------------+---------+| yuanhao | 3500.00 || jingliyang | 9000.00 |+------------+---------+2 rows in set (0.00 sec)mysql> SELECT name,salary FROM employee WHERE salary IN (3000,3500,4000,9000) ;+------------+---------+| name | salary |+------------+---------+| yuanhao | 3500.00 || jingliyang | 9000.00 |+------------+---------+mysql> SELECT name,salary FROM employee WHERE salary NOT IN (3000,3500,4000,9000) ;+-----------+------------+| name | salary |+-----------+------------+| egon | 7300.33 || alex | 1000000.31 || wupeiqi | 8300.00 || liwenzhou | 2100.00 || jinxin | 30000.00 || xiaomage | 10000.00 || 歪歪 | 3000.13 || 丫丫 | 2000.35 || 丁丁 | 1000.37 || 星星 | 3000.29 || 格格 | 4000.33 || 张野 | 10000.13 || 程咬金 | 20000.00 || 程咬银 | 19000.00 || 程咬铜 | 18000.00 || 程咬铁 | 17000.00 |+-----------+------------+16 rows in set (0.00 sec)#6:关键字LIKE模糊查询通配符’%’mysql> SELECT * FROM employee WHERE name LIKE 'jin%';+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+| 6 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 || 7 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 |+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+2 rows in set (0.00 sec)通配符'_'mysql> SELECT age FROM employee WHERE name LIKE 'ale_';+-----+| age |+-----+| 78 |+-----+1 row in set (0.00 sec)
练习：
1. 查看岗位是teacher的员工姓名、年龄2. 查看岗位是teacher且年龄大于30岁的员工姓名、年龄3. 查看岗位是teacher且薪资在9000-1000范围内的员工姓名、年龄、薪资4. 查看岗位描述不为NULL的员工信息5. 查看岗位是teacher且薪资是10000或9000或30000的员工姓名、年龄、薪资6. 查看岗位是teacher且薪资不是10000或9000或30000的员工姓名、年龄、薪资7. 查看岗位是teacher且名字是jin开头的员工姓名、年薪#对应的sql语句select name,age from employee where post = 'teacher';select name,age from employee where post='teacher' and age > 30; select name,age,salary from employee where post='teacher' and salary between 9000 and 10000;select * from employee where post_comment is not null;select name,age,salary from employee where post='teacher' and salary in (10000,9000,30000);select name,age,salary from employee where post='teacher' and salary not in (10000,9000,30000);select name,salary*12 from employee where post='teacher' and name like 'jin%';
2.group by分组查询
#1、首先明确一点：分组发生在where之后，即分组是基于where之后得到的记录而进行的#2、分组指的是：将所有记录按照某个相同字段进行归类，比如针对员工信息表的职位分组，或者按照性别进行分组等#3、为何要分组呢？ 取每个部门的最高工资 取每个部门的员工数 取男人数和女人数小窍门：‘每’这个字后面的字段，就是我们分组的依据#4、大前提： 可以按照任意字段分组，但是分组完毕后，比如group by post，只能查看post字段，如果想查看组内信息，需要借助于聚合函数
当执行以下sql语句的时候，是以post字段查询了组中的第一条数据，没有任何意义，因为我们现在想查出当前组的多条记录：
mysql> select * from employee group by post;+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+| 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 || 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 || 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 || 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 |+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+4 rows in set (0.00 sec)#由于没有设置ONLY_FULL_GROUP_BY,于是也可以有结果，默认都是组内的第一条记录，但其实这是没有意义的如果想分组，则必须要设置全局的sql的模式为ONLY_FULL_GROUP_BYmysql> set global sql_mode='ONLY_FULL_GROUP_BY';Query OK, 0 rows affected (0.00 sec)#查看MySQL 5.7默认的sql_mode如下：mysql> select @@global.sql_mode;+--------------------+| @@global.sql_mode |+--------------------+| ONLY_FULL_GROUP_BY |+--------------------+1 row in set (0.00 sec)mysql> exit;#设置成功后，一定要退出，然后重新登录方可生效Bye
继续验证通过group by分组之后，只能查看当前字段,如果想查看组内信息，需要借助于聚合函数：
mysql> select * from emp group by post;# 报错ERROR 1054 (42S22): Unknown column 'post' in 'group statement'mysql> select post from employee group by post;+-----------------------------------------+| post |+-----------------------------------------+| operation || sale || teacher || 老男孩驻沙河办事处外交大使 |+-----------------------------------------+4 rows in set (0.00 sec)
3.聚合函数
max()求最大值min()求最小值avg()求平均值sum() 求和count() 求总个数#强调：聚合函数聚合的是组的内容，若是没有分组，则默认一组# 每个部门有多少个员工select post,count(id) from employee group by post;# 每个部门的最高薪水select post,max(salary) from employee group by post;# 每个部门的最低薪水select post,min(salary) from employee group by post;# 每个部门的平均薪水select post,avg(salary) from employee group by post;# 每个部门的所有薪水select post,sum(age) from employee group by post;
小练习：
1. 查询岗位名以及岗位包含的所有员工名字2. 查询岗位名以及各岗位内包含的员工个数3. 查询公司内男员工和女员工的个数4. 查询岗位名以及各岗位的平均薪资5. 查询岗位名以及各岗位的最高薪资6. 查询岗位名以及各岗位的最低薪资7. 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
4.HAVING过滤
HAVING与WHERE不一样的地方在于#！！！执行优先级从高到低：where > group by > having #1. Where 发生在分组group by之前，因而Where中可以有任意字段，但是绝对不能使用聚合函数。#2. Having发生在分组group by之后，因而Having中可以使用分组的字段，无法直接取到其他字段,可以使用聚合函数
验证：
验证：mysql> select * from employee where salary>1000000;+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+| 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 |+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+1 row in set (0.00 sec)mysql> select * from employee having salary>1000000;ERROR 1463 (42000): Non-grouping field 'salary' is used in HAVING clause# 必须使用group by才能使用group_concat()函数，将所有的name值连接mysql> select post,group_concat(name) from emp group by post having salary > 10000; ##错误，分组后无法直接取到salary字段ERROR 1054 (42S22): Unknown column 'post' in 'field list'
小练习：
1. 查询各岗位内包含的员工个数小于2的岗位名、岗位内包含员工名字、个数2. 查询各岗位平均薪资大于10000的岗位名、平均工资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
小练习答案：
# 题1：mysql> select post,group_concat(name),count(id) from employee group by post;+-----------------------------------------+-----------------------------------------------------------+-----------+| post | group_concat(name) | count(id) |+-----------------------------------------+-----------------------------------------------------------+-----------+| operation | 程咬铁,程咬铜,程咬银,程咬金,张野 | 5 || sale | 格格,星星,丁丁,丫丫,歪歪 | 5 || teacher | xiaomage,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex | 7 || 老男孩驻沙河办事处外交大使 | egon | 1 |+-----------------------------------------+-----------------------------------------------------------+-----------+4 rows in set (0.00 sec)mysql> select post,group_concat(name),count(id) from employee group by post having count(id)<2;+-----------------------------------------+--------------------+-----------+| post | group_concat(name) | count(id) |+-----------------------------------------+--------------------+-----------+| 老男孩驻沙河办事处外交大使 | egon | 1 |+-----------------------------------------+--------------------+-----------+1 row in set (0.00 sec)#题2：mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000;+-----------+---------------+| post | avg(salary) |+-----------+---------------+| operation | 16800.026000 || teacher | 151842.901429 |+-----------+---------------+2 rows in set (0.00 sec)#题3：mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) <20000;+-----------+--------------+| post | avg(salary) |+-----------+--------------+| operation | 16800.026000 |+-----------+--------------+1 row in set (0.00 sec)
5.order by查询排序
按单列排序： SELECT * FROM employee ORDER BY age; SELECT * FROM employee ORDER BY age ASC; SELECT * FROM employee ORDER BY age DESC;按多列排序: 先按照age升序排序，如果年纪相同，则按照id降序 SELECT * from employee ORDER BY age ASC, id DESC;
验证多列排序：
SELECT * from employee ORDER BY age ASC,id DESC;mysql> SELECT * from employee ORDER BY age ASC,id DESC;+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+| 18 | 程咬铁 | female | 18 | 2014-05-12 | operation | NULL | 17000.00 | 403 | 3 || 17 | 程咬铜 | male | 18 | 2015-04-11 | operation | NULL | 18000.00 | 403 | 3 || 16 | 程咬银 | female | 18 | 2013-03-11 | operation | NULL | 19000.00 | 403 | 3 || 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 || 12 | 星星 | female | 18 | 2016-05-13 | sale | NULL | 3000.29 | 402 | 2 || 11 | 丁丁 | female | 18 | 2011-03-12 | sale | NULL | 1000.37 | 402 | 2 || 7 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 || 6 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 || 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 || 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 || 13 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 || 5 | liwenzhou | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 || 10 | 丫丫 | female | 38 | 2010-11-01 | sale | NULL | 2000.35 | 402 | 2 || 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 || 8 | xiaomage | male | 48 | 2010-11-11 | teacher | NULL | 10000.00 | 401 | 1 || 4 | yuanhao | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 || 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 || 3 | wupeiqi | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 |+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+18 rows in set (0.01 sec)
小练习：
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,结果按平均薪资升序排列3. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资降序排列 # 答案：# 题目1select * from employee ORDER BY age asc,hire_date desc;# 题目2mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) asc;+-----------+---------------+| post | avg(salary) |+-----------+---------------+| operation | 16800.026000 || teacher | 151842.901429 |+-----------+---------------+2 rows in set (0.00 sec)# 题目3mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) desc;+-----------+---------------+| post | avg(salary) |+-----------+---------------+| teacher | 151842.901429 || operation | 16800.026000 |+-----------+---------------+2 rows in set (0.00 sec)
6.limit 限制查询的记录数
示例： SELECT * FROM employee ORDER BY salary DESC LIMIT 3; #默认初始位置为0 SELECT * FROM employee ORDER BY salary DESC LIMIT 0,5; #从第0开始，即先查询出第一条，然后包含这一条在内往后查5条 SELECT * FROM employee ORDER BY salary DESC LIMIT 5,5; #从第5开始，即先查询出第6条，然后包含这一条在内往后查5条
小练习：
分页显示，每页5条
# 第1页数据 mysql> select * from employee limit 0,5;+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+| 1 | egon | male | 18 | 2017-03-01 | 老男孩驻沙河办事处外交大使 | NULL | 7300.33 | 401 | 1 || 2 | alex | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 || 3 | wupeiqi | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 || 4 | yuanhao | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 || 5 | liwenzhou | male | 28 | 2012-11-01 | teacher | NULL | 2100.00 | 401 | 1 |+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+5 rows in set (0.00 sec)# 第2页数据mysql> select * from employee limit 5,5;+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+| 6 | jingliyang | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 || 7 | jinxin | male | 18 | 1900-03-01 | teacher | NULL | 30000.00 | 401 | 1 || 8 | xiaomage | male | 48 | 2010-11-11 | teacher | NULL | 10000.00 | 401 | 1 || 9 | 歪歪 | female | 48 | 2015-03-11 | sale | NULL | 3000.13 | 402 | 2 || 10 | 丫丫 | female | 38 | 2010-11-01 | sale | NULL | 2000.35 | 402 | 2 |+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+5 rows in set (0.00 sec)# 第3页数据mysql> select * from employee limit 10,5;+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+| id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id |+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+| 11 | 丁丁 | female | 18 | 2011-03-12 | sale | NULL | 1000.37 | 402 | 2 || 12 | 星星 | female | 18 | 2016-05-13 | sale | NULL | 3000.29 | 402 | 2 || 13 | 格格 | female | 28 | 2017-01-27 | sale | NULL | 4000.33 | 402 | 2 || 14 | 张野 | male | 28 | 2016-03-11 | operation | NULL | 10000.13 | 403 | 3 || 15 | 程咬金 | male | 18 | 1997-03-12 | operation | NULL | 20000.00 | 403 | 3 |+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+5 rows in set (0.00 sec)