时间：2021-07-01 10:21:17 帮助过：11人阅读
强调： where是一种约束条件，mysql会拿着where指定的条件取表中取数据，而having则是在取出数据后进行过滤。
where字句中可以使用：
1.比较运算符： >< >= <= <> !=
2.between 80 and 100 值在 10 到 20 之间。
3.in （80,90,100)值是10 或 20 或 30.
4.like ‘aray%‘
patterm 可以是 %或_,
%表示任意字符， _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
1.单条件查询
select name from employee where post = ‘sale‘;
2.多条件查询
select name,salary from employee where post=‘teacher‘ and salary>10000;
3.关键字查询：between and
select name,salary from employee where salary between 10000 and 20000;
select name,salary from employee where sal‘‘ary not between 10000 and 20000;
4.关键字查询：is null（判断某个字段是否为null 不能用等号们需要用is）
select name,post_comment from employee where post_comment is null;
select name,post_comment from employee where post_comment is not null;
select name,post_comment from employee where post_comment =‘‘; #注意：‘‘是空字符串。
ps:
执行
update employee set post_comment=‘‘ where id=2;
再用上条查看，就会有结果了。
5.关键字查询： in
select name,salary from employee where salary=3000 or salary=3500 or salary=4000;
select name,salary from employee where salary in (3000,3500,4000);
select name,salary from employee where salary not in (3000,3500,4000);
6.关键字查询：like
通配符 ‘%‘
select * from employee where name like ‘ar%‘;
通配符： ‘_‘
select * from employee where name like ‘ar__‘;
小练习：
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
1. select name,age from employee where post =‘teacher‘; 2. select name,age from employee where post=‘teacher‘ and age>30; ‘‘ 3. select name,age,salary from employee where post=‘teacher‘ and salary between 9000 adn 10000; 4. select * from employee where post_comment is not null; 5. select name,age,salary from employee where post=‘teacher‘ and salary in(10000,9000,30000); 6. select name,age,salary from employee where post=‘teacher‘ and salary not in(10000,9000,30000); 7. select name,salary from empolyee where post=‘teacher‘ and name like ‘jin%‘;View Code
大前提： 可以按照任意字段分组， 但分完组后，只能查看分组的那个字段，要想取的组内的其他字段信息，需要借助函数。
单独使用 group by 关键字分组：
select post from employee group by post;
注意： 我们按照post字段分组，那么select 查询的字段只能是post，要获取组内的其他信息，要借助函数。
group by 关键字和 group_concat()函数一起使用。
select post,group concat(name) from employee group by post; #按岗位分组，并查看成员名。
select post,group concat(name) as emp_members from employee group by post;
group ty 与聚合函数一起使用
select post,count(id) as count from employee group by post; #按岗位分组，并查看每个组有多少人
强调：
如果我们用unique 的字段做为 分组的依据，则每一条记录自成一组，这种分组没有意义。 多条记录之间的某个字段值相同，该字段通常用来作为分组的依据。
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
1. select post,group_concat(name) from employee group by post; 2. select post,count(id) from employee group by post; 3. select sex,count(id) from employee group by sex; 4. select post,avg(salary) from employee group by post; 5. select post,max(salary) from employee group by post; 6. select post,min(salary) from employee group by post; 7. select sex,avg(salary) from employee group by sex;View Code
先from找到表
再用where的条件约束去表中取出记录
然后进行分组group by，没有分组则默认一组
然后进行聚合
最后select出结果
示例：
select count(*) from employee;
select count(*) from empoloyee where depart_id=1;
select max(salary) from emploee;
select min(salary) from employee;
select avg(salary) from employee;
select sum(salary) from employee;
select sum(salary) from employee where depart_id=3;
having 与 where 在语法上是一样的。
select * from employee where salary > 10000; select * from employee having salary > 10000;
不同点：
1.执行优先级：从高到低。
where > group by > 聚合函数 > having
2. where 是一个约束条件，是在结果返回之前起作用的。（先找到表，按where的约束条件，取出数据）
having 是一个过滤条件，是在结果返回以后起作用的。（取出数据后，在group by 分组，如果没有group by 则所有的记录为一组，在执行聚合函数，然后使用having 对聚合的结果进行过滤），在having中可以用聚合函数，where则不能。
3.having 可以放到 group by 之后，而 where 只能放到group by 之前。
4.在查询过程中聚合语句有：
(sum,min,max,avg,count) 要比having 子句优先执行，
而where 子句在查询过程中执行优先级高于聚合语句。
验证不同之处：
#验证之前再次强调：执行优先级从高到低：where > group by > 聚合函数 > having select count(id) from employee where salary > 10000; #正确，分析：where先执行，后执行聚合count(id),然后select出结果 select count(id) from employee having salary > 10000;#错误，分析：先执行聚合count(id)，后执行having过滤，无法对id进行salary>10000的过滤 #以上两条sql的顺序是 1:找到表employee--->用where过滤---->没有分组则默认一组执行聚合count(id)--->select执行查看组内id数目 2:找到表employee--->没有分组则默认一组执行聚合count(id)---->having 基于上一步聚合的结果（此时只有count(id)字段了）进行salary>10000的过滤，很明显，根本无法获取到salary字段
其他需要注意的问题：
select post,group_concat(name) from employee group by post having salary > 10000;#错误，分组后无法直接取到salary字段 select post,group_concat(name) from employee group by post having avg(salary) > 10000;
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 3. 查询各岗位平均薪资大于10000的岗位名、平均工资 4. 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资
1. select post,group_concat(name),count(id) from empoloyee group by post having count(id) < 2; 2. select post,avg(salary) from employee group by post having avg(salary) > 10000; 3. select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) < 20000;View Code
按单列排序：
select * from employee oeder by salary;
select * from employee order by asc;
select * from employee order by desc;
按多列排序：先按age排序，如果年纪相同，则按照薪资排序
select * from employee order by age,salary desc;
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
1. select * from employee order by age asc,hire_data desc; 2. select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) asc; 3. select post,avg(salary) from employee group by post having avg(salary) >10000 order bu avg(salary) desc;View Code
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：
1. 分页显示，每页5条
#题目1 mysql> select * from employee ORDER BY age asc,hire_date desc; #题目2 mysql> 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 | +-----------+---------------+ #题目3 mysql> 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 | +-----------+---------------+ 回到顶部 十 限制查询的记录数:LIMIT 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： 1. 分页显示，每页5条 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) 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 | 成龙 | 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) 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)View Code
select * from employee where name regexp ‘^ale‘;
select * from employee where name regexp ‘on$‘;
select * from employee where name regexp ‘m{2}‘;
小结：对字符串匹配的方式
WHERE name = ‘egon‘;
WHERE name LIKE ‘yua%‘;
WHERE name REGEXP ‘on$‘;
小练习：
查看所有员工中名字是jin开头，n或者g结果的员工信息
select * from employee where name regexp ‘^jin.*[gn]$‘;
Mysql ==》 单表查询
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