一 、单表查询的语法
基本语法格式：
SELECT 字段1,字段2... FROM 表名 WHERE 条件 GROUP BY field 分租 HAVING 筛选 ORDER BY field 排序 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 from -->where--> group by--> having -->select -->distinct -->order by--> limit
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
三、 简单查询
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('duoduo','male',18,'20170301','天王盖地虎办事处外交大使',7300.33,401,1), #以下是教学部('黑魔导','male',78,'20150302','teacher',1000000.31,401,1),('青眼白龙','male',81,'20130305','teacher',8300,401,1),('真红眼黑龙','male',73,'20140701','teacher',3500,401,1),('黑暗大法师','male',28,'20121101','teacher',2100,401,1),('黑暗骑士','female',18,'20110211','teacher',9000,401,1),('圣天使','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);#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 #简单查询 SELECT id,name,sex,age,hire_date,post,post_comment,salary,office,depart_id FROM employee; SELECT * FROM employee; SELECT name,salary FROM employee;#避免重复DISTINCT SELECT DISTINCT post FROM employee; #通过四则运算查询 SELECT name, salary*12 FROM employee; SELECT name, salary*12 AS Annual_salary FROM employee; SELECT name, salary*12 Annual_salary FROM employee;#定义显示格式 CONCAT() 函数用于连接字符串 SELECT CONCAT('姓名: ',name,' 年薪: ', salary*12) AS Annual_salary FROM employee; CONCAT_WS() 第一个参数为分隔符 SELECT CONCAT_WS(':',name,salary*12) AS Annual_salary FROM employee; 结合CASE语句： SELECT ( CASE WHEN NAME = 'duoduo' THEN NAME WHEN NAME = '黑魔导' THEN CONCAT(name,'_BIGSB') ELSE concat(NAME, 'SB') END ) as new_name FROM emp; 四 、WHERE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304. 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 '黑%'; 通配符’_’ SELECT * FROM employee WHERE name LIKE '黑__';
五 、分组查询:GROUP BY
一 、什么是分组？为什么要分组？
#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
二 、ONLY_FULL_GROUP_BY
#查看MySQL 5.7默认的sql_mode如下：mysql> select @@global.sql_mode;ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION#！！！注意ONLY_FULL_GROUP_BY的语义就是确定select target list中的所有列的值都是明确语义，简单的说来，在ONLY_FULL_GROUP_BY模式下，target list中的值要么是来自于聚集函数的结果，要么是来自于group by list中的表达式的值。#设置sql_mole如下操作(我们可以去掉ONLY_FULL_GROUP_BY模式)：mysql> set global sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';
三 、GROUP BY
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，想要获取组内的其他相关信息，需要借助函数GROUP BY关键字和GROUP_CONCAT()函数一起使用 SELECT post,GROUP_CONCAT(name) FROM employee GROUP BY post;#按照岗位分组，并查看组内成员名 SELECT post,GROUP_CONCAT(name) as emp_members FROM employee GROUP BY post;GROUP BY与聚合函数一起使用 select post,count(id) as count from employee group by post;#按照岗位分组，并查看每个组有多少人
强调：
如果我们用unique的字段作为分组的依据，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
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;
六、 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
mysql> select * from emp where salary > 100000;mysql> select * from emp having salary > 100000;ERROR 1463 (42000): Non-grouping field 'salary' is used in HAVING clausemysql> 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 'salary' in 'having clause'mysql> select post,group_concat(name) from emp group by post having avg(salary) > 10000;
七 、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;
八、 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
九 、使用正则表达式查询
SELECT * FROM employee WHERE name REGEXP '^d';
SELECT * FROM employee WHERE name REGEXP '导$';
小结：对字符串匹配的方式
WHERE name = 'duoduo';
WHERE name REGEXP '黑魔导';