15-5 单表查询
一 查询语法介绍（单表查询）
1 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
2 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从上到下
from
where
group by
having
select
distinct
order by
limit
说明：
1.找到表:from
2.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select
6.去重
7.将结果按条件排序：order by
8.限制结果的显示条数
例子：
表结构： company.employee 员工id id int 姓名 emp_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 #创建表 create table employee( 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 ); #查看表结构 mysql> desc employee; +--------------+-----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +--------------+-----------------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | 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 | | | salary | double(15,2) | YES | | NULL | | | office | int(11) | YES | | NULL | | | depart_id | int(11) | YES | | NULL | | +--------------+-----------------------+------+-----+---------+----------------+ #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), ('成龙','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) ;
三种简单的查询：
#简单查寻： select * from employee; select name,salary from employee; #避免重复 mysql> select distinct post from employee; #通过四则运算查询 SELECT name, salary*12 AS Annual_salary FROM employee; #查询到的薪资乘以12，as是重命名 +------------+---------------+ | name | Annual_salary | +------------+---------------+ | egon | 87603.96 | | alex | 12000003.72 | | wupeiqi | 99600.00 | | yuanhao | 42000.00 | | liwenzhou | 25200.00 | | jingliyang | 108000.00 | | jinxin | 360000.00 | | 成龙 | 120000.00 | | 歪歪 | 36001.56 | | 丫丫 | 24004.20 | | 丁丁 | 12004.44 | | 星星 | 36003.48 | | 格格 | 48003.96 | | 张野 | 120001.56 | | 程咬金 | 240000.00 | | 程咬银 | 228000.00 | | 程咬铜 | 216000.00 | | 程咬铁 | 204000.00 | +------------+---------------+
#查询的时候哈可以定义显示格式
CONCAT() 函数用于连接字符串
例子：
mysql> SELECT CONCAT('姓名: ',name,' 年薪: ', salary*12) AS Annual_salary -> FROM employee; +---------------------------------------+ | Annual_salary | +---------------------------------------+ | 姓名: egon 年薪: 87603.96 | | 姓名: alex 年薪: 12000003.72 | | 姓名: wupeiqi 年薪: 99600.00 | | 姓名: yuanhao 年薪: 42000.00 | | 姓名: liwenzhou 年薪: 25200.00 | | 姓名: jingliyang 年薪: 108000.00 | | 姓名: jinxin 年薪: 360000.00 | | 姓名: 成龙 年薪: 120000.00 | | 姓名: 歪歪 年薪: 36001.56 | | 姓名: 丫丫 年薪: 24004.20 | | 姓名: 丁丁 年薪: 12004.44 | | 姓名: 星星 年薪: 36003.48 | | 姓名: 格格 年薪: 48003.96 | | 姓名: 张野 年薪: 120001.56 | | 姓名: 程咬金 年薪: 240000.00 | | 姓名: 程咬银 年薪: 228000.00 | | 姓名: 程咬铜 年薪: 216000.00 | | 姓名: 程咬铁 年薪: 204000.00 | +---------------------------------------+ 18 rows in set (0.32 sec)
二 where条件约束
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 'egon%'
pattern可以是%或_，
%表示任意多字符
_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
#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;
#4:关键字IS NULL(判断某个字段是否为NULL不能用等号，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，不是null 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
SELECT name,salary FROM employee WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ; SELECT name,salary FROM employee WHERE salary IN (3000,3500,4000,9000) ; SELECT name,salary FROM employee WHERE salary NOT IN (3000,3500,4000,9000) ;
#6:关键字LIKE模糊查询
通配符’%’
SELECT * FROM employee
WHERE name LIKE 'eg%';
通配符’_’#双下划线一个_代表一个字符，两个代表两个字符
例子：
mysql> select * from emp where name like '__'; +----+--------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+--------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ | 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 | | 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 | +----+--------+--------+-----+------------+-----------+--------------+----------+--------+-----------+ 7 rows in set (0.00 sec)
三 分组查询:GROUP BY
#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
如果想分组，则必须要设置全局的sql的模式为ONLY_FULL_GROUP_BY
1 mysql> set global sql_mode='ONLY_FULL_GROUP_BY'; 2 3 #查看MySQL 5.7默认的sql_mode如下： 4 mysql> select @@global.sql_mode; 5 +--------------------+ 6 | @@global.sql_mode | 7 +--------------------+ 8 | ONLY_FULL_GROUP_BY | 9 +--------------------+ 10 row in set (0.00 sec) 11 12 mysql> exit;#设置成功后，一定要退出，然后重新登录方可生效
然后查询：使用分组必须和聚合函数一起使用
mysql> select post,count(id) from employee group by post; +-----------------------------------------+-----------+ | post | count(id) | +-----------------------------------------+-----------+ | operation | 5 | | sale | 5 | | teacher | 7 | | 老男孩驻沙河办事处外交大使 | 1 | +-----------------------------------------+-----------+ 4 rows in set (0.34 sec) GROUP BY关键字和GROUP_CONCAT()函数一起使用 mysql> SELECT post,GROUP_CONCAT(name) FROM employee GROUP BY post; +-----------------------------------------+---------------------------------------------------------+ | post | GROUP_CONCAT(name) | +-----------------------------------------+---------------------------------------------------------+ | operation | 张野,程咬金,程咬银,程咬铜,程咬铁 | | sale | 歪歪,丫丫,丁丁,星星,格格 | | teacher | alex,wupeiqi,yuanhao,liwenzhou,jingliyang,jinxin,成龙 | | 老男孩驻沙河办事处外交大使 | egon | +-----------------------------------------+---------------------------------------------------------+ 4 rows in set (0.00 sec)
四 聚合函数
#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
示例：
SELECT COUNT(*) FROM employee;
SELECT COUNT(*) FROM employee WHERE depart_id=1;
SELECT MAX(salary) FROM employee;
SELECT MIN(salary) FROM employee;
SELECT AVG(salary) FROM employee;
SELECT SUM(salary) FROM employee;
SELECT SUM(salary) FROM employee WHERE depart_id=3;
五 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
例子：
mysql> select post,avg(salary) from emp group by post having avg(salary)> 10000; +-----------+---------------+ | post | avg(salary) | +-----------+---------------+ | operation | 16800.026000 | | teacher | 151842.901429 | +-----------+---------------+ 2 rows in set (0.27 sec) mysql> select post,avg(salary) from emp group by post having salary > 10000; ERROR 1054 (42S22): Unknown column 'salary' in 'having clause'
分组后无法直接取到salary字段
例子3：
mysql> select post,group_concat(name) from emp group by post having avg(salary) > 10000; +-----------+---------------------------------------------------------+ | post | group_concat(name) | +-----------+---------------------------------------------------------+ | operation | 张野,程咬金,程咬银,程咬铜,程咬铁 | | teacher | alex,wupeiqi,yuanhao,liwenzhou,jingliyang,jinxin,成龙 | +-----------+---------------------------------------------------------+ 2 rows in set (0.00 sec)
六 查询排序:ORDER BY
按单列排序
SELECT * FROM employee ORDER BY salary;
SELECT * FROM employee ORDER BY salary ASC;
SELECT * FROM employee ORDER BY salary DESC;
按多列排序:先按照age排序，如果年纪相同，则按照薪资排序
SELECT * from employee
ORDER BY age,
salary DESC;
七 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
八 使用正则表达式查询
SELECT * FROM employee WHERE name REGEXP '^ale';
SELECT * FROM employee WHERE name REGEXP 'on$';
SELECT * FROM employee WHERE name REGEXP 'm{2}';
小结：对字符串匹配的方式
WHERE name = 'egon';
WHERE name LIKE 'yua%';
WHERE name REGEXP 'on$';
例子：
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]$';
