学习笔记：MYSQL查询

前言:之前花费两天晚上看了一遍Mysql必知必会，没想到后面效果太差。不如跟着网课视频敲一遍和完成练习题目（练习题没写注释就不记录了），再记下笔记。

一、基本的查询select语句
语法： select 查询列表 from 表名；
查询列表可以是表中的字段、常量值、表达式、函数，查询结果是一个虚拟的表格。

SELECT last_name FROM employees;

#2.查询表中的多个字段
SELECT first_name,last_name,phone_number,email FROM employees;

#3.查询表中的所有字段
SELECT * FROM employees;

#4.查询常量
SELECT 10;

#5.查询表达式
SELECT 1010*2;

#6.查询函数
SELECT VERSioN( );

#7.起别名
SELECT salary AS `收入` FROM employees;
SELECT first_name `名` FROM employees;

#8.去重
SELECT DISTINCT salary AS `工资样本` FROM employees;

#9.‘+’加号：仅有运算符的作用，没有拼接字符的作用
SELECT 1010 + 2023;

#10.使用concat连接字段
SELECT CONCAT(first_name,last_name) FROM employees;

二、条件查询
语法：select 查询列表 from 表名
where 筛选条件；

分类：
1.按条件表达式筛选
运算符： > < = != <> >= <=

按逻辑表达式筛选
运算符： || && ！
也可以使用 or and not

模糊查询
like
between and
in
is null

#按条件表达式查询
#1. 查询工资>12000的员工信息
SELECT * FROM employees WHERE salary >12000;

#2. 查询部门编号不等于90号的员工名和部门编号
SELECT last_name,department_id FROM employees WHERE department_id !=90;

#按逻辑表达式查询
#3.查询工资在10000~20000的员工名、工资、奖金
SELECT last_name, salary,commission_pct FROM employees WHERE salary >10000 AND salary <20000;

#4.查询部门编号不是在90~100,或者工资高于15000的员工信息
SELECT * FROM employees WHERE department_id <90 OR department_id >110 OR salary >15000;
SELECT * FROM employees WHERE NOT(department_id BETWEEN 90 AND 110) OR salary >15000;

#like查询
#5.查询员工名中有a字符的员工信息
SELECT * FROM employees WHERE last_name LIKE "%a%";

#6.查询员工名中第三个字符为n,第五个字符为l的员工名和工资
SELECT last_name,salary FROM employees WHERE last_name LIKE "__n_l%";

#7.查询员工名第二个字符为_的员工信息
SELECT * FROM employees WHERE last_name LIKE "_\_%";
SELECT * FROM employees WHERE last_name LIKE "_$_%" ESCAPE "$";

#between and 两个临界值不能调换顺序
#8.查询员工编号在100~120的员工信息
SELECT * FROM employees WHERE employee_id BETWEEN 100 AND 120;

#in
#9.查询员工工种是 IT_PROG,AD_VP,AD_PRSE中的一个的员工名和工种
SELECT last_name, job_id FROM employees WHERE job_id IN("IT_PROG","AD_VP","AD_PRSE");

#is null
#10. 查询没有奖金的员工名和奖金率
SELECT last_name,commission_pct FROM employees WHERE commission_pct IS NULL;

#安全等于
SELECT last_name, commission_pct FROM employees WHERE commission_pct <=> NULL;

#11. 查询有奖金的员工名和奖金率
SELECT last_name,commission_pct FROM employees WHERE commission_pct IS NOT NULL;

三、排序查询
语法：
select 查询列表 from 表名
where 筛选条件
order by 排序列表 ase | desc;

order by子句一般放在查询语句最后面，limit子句除外
order by子句支持单个字段、多个字段、表达式、别名、函数

#1.查询员工信息，要求按工资从低到高排序
SELECT * FROM employees 
ORDER BY salary ASC;

#2.查询部门编号>90的员工信息，按入职时间的先后排序
SELECT * FROM employees
WHERE department_id >90
ORDER BY hiredate ASC;

#3.按年薪的高低显示员工的信息和年薪
SELECT * ,salary*12 AS "年薪" FROM employees
ORDER BY salary*12 ASC;

#4.按姓名的长度显示员工姓名和薪水
SELECT CONCAT(first_name,last_name) AS "姓名",salary FROM employees
ORDER BY LENGTH(CONCAT(first_name,last_name)) ASC;

#5.查询员工信息，先按工资升序排序，再按员工编号降序排序
SELECT * FROM employees
ORDER BY salary ASC,employee_id DESC;

四、常见函数
语法：
select 函数名（实参列表）from 表名；
分类：

1. 单行函数
   比如：concat、length、ifnull
2. 分组函数

#（一）字符函数
#1.length 获取参数值的字节个数
SELECT LENGTH("中文");
SELECT LENGTH("English");

#2.concat 拼接字符串
SELECT CONCAT(first_name," ",last_name) FROM employees;

#3.upper、lower
#将姓大写、名小写，再拼接
SELECT CONCAT(UPPER(first_name)," ",LOWER(last_name)) AS "姓名" FROM employees;

#4.substr、substring 索引从一开始
SELECT SUBSTR("一二三四五",1,4);

#将名中的首字符大写，其余字符小写，再拼接起来
SELECT CONCAT(UPPER(SUBSTR(last_name,1,1)),"_",LOWER(SUBSTR(last_name,2))) FROM employees;

#5.instr 返回子串第一次出现的索引，没有则返回0
SELECT INSTR("五岳泰山","泰山");

#6.trim
SELECT TRIM("    23232  ");
SELECT TRIM("o" FROM "oooo中文oooo");

#7. lpad 用指定的字符左填充到指定的长度
SELECT LPAD("中文",9,"*");

#8. Rpad 用指定的字符右填充到指定的长度
SELECT RPAD("英文",10,"o");

#9. replace 替换
SELECT REPLACE("中文****","*","o");

##（二）数学函数
#1.round 四舍五入
SELECT ROUND(1.4);

#2.ceil 向上取整，返回>=该参数的最小整数
SELECT CEIL(1.4);

#3.floor 向下取整，返回<=该参数的最大整数
SELECT FLOOR(1.5);

#4.truncate 截断,保留几位小数
SELECT TRUNCATE(1.8888,0);

#5.mod 取余
SELECT MOD(34,8);

##（三）日期函数
#1.now 返回当前系统日期和时间
SELECT NOW();

#2.curdate 返回当前系统日期，不包括时间
SELECT CURDATE();

#3.curtime 返回时间，不包括日期
SELECT CURTIME()

#4.获取指定部分，年、月、日、小时、分钟、秒
SELECT YEAR(NOW());

SELECT MONTH(NOW());

SELECT MONTHNAME(NOW());

SELECT MINUTE(NOW());

#5.str_to_date 将字符通过指定的格式转换为日期
SELECT STR_TO_DATE("2021-4-13","%Y-%c-%d") out_put;

#查询入职日期是1992-4-3的员工
SELECT * FROM employees WHERE hiredate = STR_TO_DATE("4-3 1992","%c-%d %Y");

#date_format 将日期转换成字符
SELECT DATE_FORMAT(NOW(),"%Y年%c月%d日");

#查询有奖金的员工名和入职日期（xx年xx月xx日）
SELECT CONCAT(first_name," ",last_name),DATE_FORMAT(hiredate,"%Y年%c月%d日") 入职日期 FROM employees
WHERE commission_pct IS NOT NULL;

##（四）其它函数
#1.显示mysql的版本号
SELECT VERSION();

#2.显示当前打开的数据库
SELECT DATABASE();

#3.显示当前登录的用户
SELECT USER();

##（五)流程控制函数
#if if else的效果
SELECT IF(35>10,"大","小");

SELECT CONCAT(first_name," ",last_name),IF(commission_pct,"有奖金","没奖金") FROM employees;

#case 要判断的字符或表达式
#when 常量 then 要显示的值或语句
#when 常量 then 要显示的值或语句
#...
#else 要显示的值或语句
#end

#查询员工的工资，要求：
#部门编号为30，显示的工资为1.1倍
#部门编号为40，显示的工资为1.2倍
#部门编号为50，显示的工资为1.3倍
#其它部门，显示的工资为1倍

SELECT salary 原始工资,department_id 部门编号,
CASE department_id
WHEN 30 THEN salary*1.1
WHEN 40 THEN salary*1.2
WHEN 50 THEN salary*1.3
ELSE salary
END AS 新工资
FROM employees;

#case 
#when 条件1 then 要显示的值或表达式
#when 条件2 then 要显示的值或表达式
#when 条件3 then 要显示的值或表达式
#else 要显示的值或表达式
#end

#查询员工的工资情况

SELECT salary,
CASE salary
WHEN salary>20000 THEN "等级A"
WHEN salary>15000 THEN "等级B"
WHEN salary>10000 THEN "等级C"
ELSE "等级D"
END AS 工资等级
FROM employees;



#分组函数


#1.简单的使用
SELECT SUM(salary) FROM employees;
SELECT AVG(salary) FROM employees;
SELECT MAX(salary) FROM employees;
SELECT MIN(salary) FROM employees;
SELECT COUNT(salary) FROM employees;

#2.支持哪些参数类型 sum、 avg一般用于处理数值型 max、min、count可以处理任何类型
SELECT SUM(last_name) FROM employees;
SELECT MAX(last_name) FROM employees;
SELECT COUNT(last_name) FROM employees;

#3.是否忽略null值 下面的分组函数都忽略null值
SELECT MAX(commission_pct),SUM(commission_pct),AVG(commission_pct) FROM employees;
SELECT COUNT(employee_id),COUNT(commission_pct) FROM employees;

#4.和distinct搭配使用
SELECT SUM(DISTINCT(salary)) FROM employees;
SELECT SUM(salary) FROM employees;

#5.count 函数的详细介绍

SELECT COUNT(salary) FROM employees;
SELECT COUNT(*) FROM employees;
SELECT COUNT(1) FROM employees;

#6.和分组函数一同查询的字段的限制：要求是group by后面的字段
SELECT SUM(salary),employee_id FROM employees;

五、分组查询
语法：
select 分组函数、列（要求是出现在group by后面的字段）
from 表名
where 筛选条件
group by 分组的列表
order by 子句

注意：查询列表必须特殊，要求是分组函数和group by后面的字段

分组查询中的筛选条件分为两类：
分组前筛选 数据源是原始表 位置在group by前面 关键字是where
分组后筛选 分组后的结果集 在group by后面 having
分组函数作为筛选条件的，肯定放在having子句中。

group by子句支持单个字段分组、多个字段分组、表达式或函数，也可以添加排序

#分组查询
#1.查询每个工种的最高工资
SELECT MAX(salary),job_id FROM employees
GROUP BY job_id;

#2.查询每个位置上的部门个数
SELECT COUNT(*),location_id FROM departments
GROUP BY location_id;

#3.添加筛选条件：查询邮箱中包含a字符的，每个部门的平均工资
SELECT AVG(salary),department_id FROM employees
WHERE email LIKE "%a%"
GROUP BY department_id;

#4.查询有奖金的每个领导手下员工的最高工资
SELECT MAX(salary),manager_id FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY manager_id;

#添加复杂的筛选条件
#5.查询哪个部门的员工个数>2
#查询每个部门的员工个数
SELECT COUNT(*),department_id FROM employees GROUP BY department_id;
#根据上面的结果筛选，查询哪个部门的员工个数>2
SELECT COUNT(*),department_id FROM employees GROUP BY department_id
HAVING COUNT(*) >2;

#6.查询每个工种有奖金员工的最高工资>12000的工种编号和最高工资
SELECT MAX(salary),job_id FROM employees
GROUP BY job_id
HAVING MAX(salary) >12000;

#7.查询领导编号>102的每个领导手下的最低工资>5000的领导编号是哪个，以及其最低工资
SELECT manager_id,MIN(salary) FROM employees
WHERE manager_id >102
GROUP BY manager_id
HAVING MIN(salary) >5000;

#按表达式或函数分组
#8.按员工名字的长度，查询每一组员工个数，筛选员工个数>5的有哪些
SELECT COUNT(*),LENGTH(last_name) FROM employees
GROUP BY LENGTH(last_name)
HAVING COUNT(*) >5;

#按多个字段分组
#9.查询每个部门每个工种的员工的平均工资
SELECT AVG(salary),department_id,job_id FROM employees
GROUP BY department_id,job_id;

#添加排序
#10.查询每个部门每个工种的员工的平均工资,按平均工资的高低排序
SELECT AVG(salary),department_id,job_id FROM employees
GROUP BY department_id,job_id
HAVING AVG(salary) >10000
ORDER BY AVG(salary) ASC;

六、多表查询
按标准分类：
sql92：仅支持内连接， sql99

按功能分类：
内连接 自连接 外连接

sql99语法：

select 查询列表
from 表名 连接类型
join 表名
on 连接条件
where 筛选条件
group by 分组条件
having 筛选条件
order by 筛选条件

分类：
内连接: inner
外连接： 左外连接 left outer, 右外连接 right outer， 全外 full outer
交叉连接：cross join

(1)内连接：
语法：select 查询列表
from 表名
inner join 表名
on 连接条件

分类：等值连接 非等值连接 自连接

(2)外连接：
应用场景：用于查询一个表中有，另外一个表中没有的记录。

特点：
外连接的查询结果为主表中的所有记录，
如果从表中有与它匹配的，则显示匹配的值，
从表中没有与它匹配的，则显示null值

外连接查询结果=内连接结果+主表中有而从表中没有的记录
左外连接，left join左边的是主表
右外连接，right join右边的是主表
全外连接=内连接结果+主表中有而从表中没有的记录+从表中有而主表中没有的记录

##自连接

#一、内连接
#等值连接
#1.查询员工名、部门名
SELECT last_name,department_name
FROM employees e
INNER JOIN departments d
ON e.department_id = d.department_id;

#2.查询员工名包含e的员工名和工种名（添加筛选）
SELECT last_name,job_title 
FROM employees e
INNER JOIN jobs j
ON e.`job_id`= j.`job_id`
WHERE e.last_name LIKE "%e%";

#3.查询部门个数>3的城市名和部门个数（添加分组和筛选）
SELECT COUNT(*),city
FROM departments d
INNER JOIN locations l
ON d.location_id  = l.location_id
GROUP BY city
HAVING COUNT(*) >3;

#4.查询那个部门的员工个数>3的部门名和员工个数，并按个数降序（添加排序）
SELECT COUNT(*),department_name
FROM employees e
INNER JOIN departments d
ON e.`department_id` = d.`department_id`
GROUP BY department_name
HAVING COUNT(*) >3
ORDER BY COUNT(*) DESC;

#5.查询员工名、部门名、工种名，并按部门名降序
SELECT last_name,department_name,job_title
FROM employees e
INNER JOIN departments d ON e.department_id = d.department_id
INNER JOIN jobs j ON e.job_id = j.job_id
ORDER BY department_name DESC;

##非等值连接
#6.查询员工的工资级别
SELECT last_name,salary,grade_level FROM employees e
INNER JOIN job_grades j ON e.salary BETWEEN j.lowest_sal AND j.highest_sal;

#7.查询工资级别的个数>20的个数，并按工资级别降序
SELECT COUNT(*),grade_level FROM employees e
INNER JOIN job_grades j ON e.salary BETWEEN j.lowest_sal AND j.highest_sal
GROUP BY j.grade_level
HAVING COUNT(*) >20
ORDER BY  j.grade_level DESC;

##自连接
#8.查询员工的名字、上级的名字
SELECT e.last_name 员工名,m.last_name  上级 FROM employees e
INNER JOIN employees m ON e.manager_id = m.employee_id;

#9.查询员工的名字中包含字符k、上级的名字
SELECT e.last_name 员工名,m.last_name 上级 FROM employees e
INNER JOIN employees m ON e.manager_id = m.employee_id
WHERE e.last_name LIKE "%k%";

#二、外连接
#左外连接
#1.查询没有男朋友的女神
SELECT be.name,b.boyname FROM beauty be
LEFT OUTER JOIN boys b ON be.boyfriend_id = b.id
WHERE b.id IS NULL;

#2.查询哪个部门没有员工
SELECT department_name,employee_id FROM departments d
LEFT OUTER JOIN employees e ON d.department_id = e.department_id;
WHERE d.department_id IS NULL;

#右外连接
SELECT department_name FROM employees e
RIGHT OUTER JOIN departments d ON d.department_id = e.department_id;

#全外 mysql中不支持
SELECT be.*,bo.* FROM beauty be
FULL OUTER JOIN boys bo ON be.boyfriend_id = bo.id;

#交叉连接
SELECT be.*,bo.* FROM beauty be
CROSS  JOIN boys bo ;

七、子查询
含义：出现在其他语句中的select语句，称为子查询或内查询
外部的查询语句，称为主查询或外查询

分类：
按子查询出现的位置：
select 后面：仅支持标量子查询；
from后面： 支持表子查询；
where或having后面：标量子查询或列子查询、行子查询；
exist后面：相关子查询

按结果集的行列数不同划分：
标量子查询（结果集为一行一列）
行子查询（一行多列）
列子查询（一列多行）
表子查询（多行多列）

#一、where或having后面


##标量子查询
#1.谁的工资比Abel高？
SELECT salary FROM employees
WHERE last_name = "Abel";

SELECT last_name,salary FROM employees
WHERE salary >(
	SELECT salary FROM employees
WHERE last_name = "Abel");
 
#2.查询job_id 与141号员工相同，salary比143号多的员工姓名、job_id和工资
SELECT job_id FROM employees
WHERE employee_id = 141;

SELECT salary FROM employees
WHERE employee_id = 143;

SELECT last_name,job_id,salary FROM employees
WHERE job_id = (
		SELECT job_id FROM employees
		WHERE employee_id = 141) 
AND salary > (
		SELECT salary FROM employees
		WHERE employee_id = 143);

#3.返回工资最少的员工的姓名、job_id和salary
SELECT MIN(salary) FROM employees;

SELECT last_name,job_id,salary FROM employees
WHERE salary = (SELECT MIN(salary) FROM employees);

#having
##4.查询最低工资大于50号部门的最低工资的部门id和其最低工资
SELECT MIN(salary),department_id FROM employees
GROUP BY department_id
HAVING department_id = 50;

SELECT MIN(salary),department_id FROM employees
GROUP BY department_id
HAVING MIN(salary) > (
	SELECT MIN(salary) FROM employees
	GROUP BY department_id
	HAVING department_id = 50);

#行子查询
#5.查询员工编号最小且工资最高的员工信息
SELECT MIN(employee_id) FROM employees;

SELECT MAX(salary) FROM employees;

SELECT * FROM employees
WHERE employee_id = (SELECT MIN(employee_id) FROM employees)
AND salary = (SELECT MAX(salary) FROM employees);

SELECT * FROM employees
WHERE (employee_id,salary) =(SELECT MIN(employee_id), MAX(salary) FROM employees);

#二、select后面:仅支持标量子查询）
#6.查询每个部门的员工个数
SELECT COUNT(*),department_name FROM departments d
LEFT OUTER JOIN employees e ON d.department_id = e.department_id
GROUP BY department_name;

SELECT d.*,(
	SELECT COUNT(*) FROM employees e
	WHERE e.department_id = d.department_id)
FROM departments d;

#7.查询员工号位为102的部门名
SELECT department_name FROM employees e
INNER JOIN departments d ON e.department_id = d.department_id
WHERE employee_id = 102;

#三、from后面:列子查询,将子查询的结果当做一张表，要求必须取别名
#8.查询每个部门的平均工资的工资等级

SELECT AVG(salary),department_id,grade_level FROM employees e
INNER JOIN job_grades ON salary BETWEEN lowest_sal AND highest_sal
GROUP BY department_id;

SELECT ag.*, grade_level 
FROM (
	SELECT AVG(salary) s,department_id FROM employees
	GROUP BY department_id) ag
INNER JOIN job_grades ON ag.s BETWEEN lowest_sal AND highest_sal;

#四、exist后面

#9. 查询有员工的部门名
SELECT department_name FROM departments d
WHERE EXISTS(
	SELECT * FROM employees e
	WHERE e.department_id = d.department_id);

SELECT department_name FROM departments d
INNER JOIN employees e ON d.department_id = e.department_id;

八、分页查询
要显示的数据，一页显示不全，需要分页提交sql请求
语法：
select 查询列表
from 表名
inner join 表名 on 连接条件
where 筛选条件
group by 分组条件
having 分组后的筛选
order by 排序的字段
limit offset,size

offset 要显示条目的起始索引（起始索引从零开始）
size 要显示的条目数

注意：limit放在查询语句的最后
公式：要显示的页数 limit (page-1)*size,size

#分页查询
#查询前五条员工信息
SELECT * FROM employees LIMIT 0,5;

#2.查询第11-25条员工信息
SELECT * FROM employees LIMIT 10,15;

#3.有奖金的员工信息，并且将工资较高的前10条显示出来
SELECT * FROM employees WHERE commission_pct IS NOT NULL 
ORDER BY salary
LIMIT 0,10;

九、联合查询
uNIOn 将多条查询语句的结果合并成一个结果
语法:
查询语句1
union
查询语句2
union
...
应用场景：查询的结果来自于多个表，且多个表没有直接的连接关系，但查询的信息一致时
特点： 要求多个查询语句的查询列表是一致的
多个查询语句的查询的每一列的类型和顺序最好是一致的
union关键字默认去重，如果使用union all可以包含重复项

#联合查询
#查询部门编号>90或邮箱包含a的员工信息
SELECT * FROM employees WHERE department_id > 90
UNION
SELECT * FROM employees WHERE email LIKE "%a%";