MySQL

Posted 2021-05-15 秃头专业计算机

分组函数

用做统计使用，又称为聚合函数或统计函数。它的种类有sun、avg、max、min、count。

要求：

1、sum和avg一般用于处理数值型，max、min、count可以处理任何数据类型。

2、上述分组函数均忽略null值

3、都可以搭配distinct使用，实现去重的统计
select sum(distinct 字段) from 表;

4、count函数

count(字段)：统计该字段非空值的个数
count(*):统计结果集的行数

5、和分组函数一同查询的字段，要求是group by后出现的字段

#1.简单使用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;
SELECT SUM(salary) 和,ROUND(AVG(salary),2) 平均,MAX(salary) 最高,MIN(salary) 最低,COUNT(salary) 个数FROM employees;
#2.参数支持哪些数据类型SELECT SUM(last_name),AVG(last_name) FROM employees;SELECT SUM(hiredate),AVG(hiredate) FROM employees;SELECT MAX(last_name),MIN(last_name) FROM employees;SELECT MAX(hiredate),MIN(hiredate) FROM employees;SELECT COUNT(commission_pct) FROM employees;SELECT COUNT(last_name) FROM employees;
#3.忽略nullSELECT SUM(commission_pct),AVG(commission_pct) FROM employees;SELECT commission_pct FROM employees;SELECT SUM(commission_pct),AVG(commission_pct),SUM(commission_pct)/35,AVG(commission_pct)/107 FROM employees;SELECT MAX(commission_pct),MIN(commission_pct) FROM employees;SELECT COUNT(commission_pct) FROM employees;
#4.和distinct搭配SELECT SUM(DISTINCT salary),SUM(salary) FROM employees;SELECT COUNT(DISTINCT salary),COUNT(salary) FROM employees;
#5.count函数详解SELECT COUNT(salary) FROM employees;SELECT COUNT(*) FROM employees;SELECT COUNT(1) FROM employees;/*效率上：MyISAM存储引擎，count(*)最高InnoDB存储引擎，count(*)和count(1)效率>count(字段)*/
#6.和分组函数一同查询的字段有限制
SELECT AVG(salary),employee_id FROM employees;

分组查询

select 分组函数,分组后的字段

from 表

【where 分组前的筛选】

group by 分组的字段

【having 分组后的筛选】

【order by 排序列表】

          使用关键字  筛选的表   位置分组前筛选 where 原始表 group by的前面分组后筛选 having 分组后的结果 group by的后面1.分组函数做条件肯定是放在having子句中2.能用分组前筛选的，就优先考虑使用分组前筛选

#案例1:查询每个工种的最高工资SELECT MAX(salary),job_id FROM employees GROUP BY job_id;
#案例2:查询每个位置上的部门个数SELECT COUNT(*),location_idFROM departmentsGROUP BY location_id;
#添加筛选条件#案例1: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 employeesWHERE email LIKE '%a%' GROUP BY department_id;
#案例2: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 employeesWHERE commission_pct IS NOT NULLGROUP BY manager_id;
#添加复杂的筛选条件#案例1:查询哪个部门的员工个数>2#1.查询每个部门的员工个数SELECT COUNT(*),department_id FROM employeesGROUP BY department_id;
#2.根据1的结果进行筛选，查询哪个部门的员工个数大于2SELECT COUNT(*),department_id FROM employeesGROUP BY department_id HAVING COUNT(*)>2;

#案例2:查询每个工种有奖金的员工的最高工资>12000的工种编号和最高工资 #1.查询每个工种有奖金的员工的最高工资 SELECT MAX(salary),job_id FROM employees WHERE commission_pct IS NOT NULL GROUP BY job_id; 
#2.根据结果继续筛选，最高工资>12000 
SELECT MAX(salary), job_id FROM employees WHERE commission_pct IS NOT NULL GROUP BY job_id HAVING MAX(salary)>12000; 
#按表达式或函数分组
#案例:按员工姓名的长度分组,查询每一组的员工个数,筛选员工个数>5
#1.查询每个长度的员工个数 SELECT COUNT(*),LENGTH(last_name) len_name FROM employees GROUP BY LENGTH(last_name); 
#2.添加筛选条件SELECT COUNT(*) c,LENGTH(last_name) len_name FROM employees GROUP BY len_name HAVING c>5;#按多个字段查询，没有顺序要求#案例:查询每个部门每个工种的员工的平均工资
SELECT AVG(salary),department_id,job_idFROM employees GROUP BY department_id,job_id;
#添加排序#案例:查询每个部门每个工种的员工的平均工资,按平均工资的高低查询
SELECT AVG(salary),department_id,job_idFROM employees GROUP BY department_id,job_idORDER BY AVG(salary) DESC;

连接查询 

又称为多表查询，当查询的数据来自多个表时，就会用到连接查询

按功能分类分为内连接、外连接、交叉连接。而内连接包括等值连接、非等值连接、自连接。外连接包括左外连接、右外连接、全外连接。

#一、内连接/*语法：select 查询列表from 表1 别名inner join 表2 别名on 连接条件;inner可以省略*/#1、等值连接#案例1.查询员工名、部门名SELECT last_name,department_name FROM departments dINNER JOIN employees eON e.`department_id` = d.`department_id`;
#案例2.查询名字中包含e的员工名和工种名（添加筛选）SELECT last_name,job_title FROM employees eINNER JOIN jobs j ON e.`job_id`= j.`job_id`WHERE e.`last_name` LIKE '%e%';
#案例3.查询部门个数>3的城市名和部门个数，（添加分组+筛选）
#1.查询每个城市的部门个数#2.在1结果上筛选满足条件的SELECT city,COUNT(*) 部门个数FROM departments dINNER JOIN locations lON d.`location_id`=l.`location_id`GROUP BY cityHAVING COUNT(*)>3;
#案例4.查询哪个部门的员工个数>3的部门名和员工个数，并按个数降序（添加排序）
#1.查询每个部门的员工个数SELECT COUNT(*),department_nameFROM employees eINNER JOIN departments dON e.`department_id`=d.`department_id`GROUP BY department_name;
#2.在1结果上筛选员工个数>3的记录，并排序
SELECT COUNT(*) 个数,department_nameFROM employees eINNER JOIN departments dON e.`department_id`=d.`department_id`GROUP BY department_nameHAVING COUNT(*)>3ORDER BY COUNT(*) DESC;
#案例5.查询员工名、部门名、工种名，并按部门名降序（添加三表连接）
SELECT last_name,department_name,job_titleFROM employees eINNER 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;
#二、非等值连接
#查询员工的工资级别
SELECT salary,grade_levelFROM employees eJOIN job_grades gON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal`; #查询工资级别的个数>20的个数，并且按工资级别降序SELECT COUNT(*),grade_levelFROM employees eJOIN job_grades gON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal`GROUP BY grade_levelHAVING COUNT(*)>20 ORDER BY grade_level DESC;
#三、自连接 #查询员工的名字、上级的名字SELECT e.last_name,m.last_nameFROM employees eJOIN employees mON e.`manager_id`= m.`employee_id`; #查询姓名中包含字符k的员工的名字、上级的名字SELECT e.last_name,m.last_nameFROM employees eJOIN employees mON e.`manager_id`= m.`employee_id`WHERE e.`last_name` LIKE '%k%';
#二、外连接/*应用场景：用于查询一个表中有，另一个表没有的记录特点：1、外连接的查询结果为主表中的所有记录 如果从表中有和它匹配的，则显示匹配的值 如果从表中没有和它匹配的，则显示null 外连接查询结果=内连接结果+主表中有而从表没有的记录2、左外连接，left join左边的是主表 右外连接，right join右边的是主表3、左外和右外交换两个表的顺序，可以实现同样的效果 4、全外连接=内连接的结果+表1中有但表2没有的+表2中有但表1没有的。mysql不支持*/#左外连接,查询男朋友 不在男神表的的女神名 SELECT b.name FROM beauty bLEFT OUTER JOIN boys boON b.`boyfriend_id` = bo.`id`WHERE bo.`id` IS NULL; #案例1：查询哪个部门没有员工#左外SELECT d.*,e.employee_id FROM departments dLEFT OUTER JOIN employees eON d.`department_id` = e.`department_id`WHERE e.`employee_id` IS NULL;
#右外SELECT d.*,e.employee_idFROM employees eRIGHT OUTER JOIN departments dON d.`department_id` = e.`department_id`WHERE e.`employee_id` IS NULL;
#交叉连接，笛卡儿积SELECT b.*,bo.* FROM beauty bCROSS JOIN boys bo;

子查询

出现在其他语句中的select语句，成为子查询或内查询。外部的查询语句，成为主查询或外查询。 

按子查询出现的位置分为select后面、from后面、where或having后面、exists后面

按结果集的行列数不同分为标量子查询(结果集只有一行一列)、列子查询(结果集只有一列多行)、行子查询(结果集只有一行多列)、表子查询(结果集多行多列)

#一、where或having后面/*1、标量子查询（单行子查询）2、列子查询（多行子查询）3、行子查询（多列多行）
特点：①子查询放在小括号内②子查询一般放在条件的右侧③标量子查询，一般搭配着单行操作符使用> < >= <= = <>
列子查询，一般搭配着多行操作符使用in、any/some、all
④子查询的执行优先于主查询执行，主查询的条件用到了子查询的结果*/#1.标量子查询#案例1：谁的工资比 Abel 高?
#①查询Abel的工资SELECT salaryFROM employeesWHERE last_name = 'Abel';
#②查询员工的信息，满足 salary>①结果SELECT *FROM employeesWHERE salary>(
 SELECT salary FROM employees WHERE last_name = 'Abel' );
#案例2：返回job_id与141号员工相同，salary比143号员工多的员工 姓名，job_id 和工资#①查询141号员工的job_idSELECT job_id FROM employeesWHERE employee_id = 141;
#②查询143号员工的salarySELECT salary FROM employeesWHERE employee_id = 143;
#③查询员工的姓名，job_id 和工资，要求job_id=①并且salary>②
SELECT last_name,job_id,salaryFROM employeesWHERE job_id = ( SELECT job_id FROM employees  WHERE employee_id = 141) AND salary>( SELECT salary FROM employees WHERE employee_id = 143
);
#案例3：返回公司工资最少的员工的last_name,job_id和salary#①查询公司的最低工资SELECT MIN(salary) FROM employees;
#②查询last_name,job_id和salary，要求salary=①SELECT last_name,job_id,salaryFROM employeesWHERE salary=( SELECT MIN(salary) FROM employees);
#案例4：查询最低工资大于50号部门最低工资的部门id和其最低工资#①查询50号部门的最低工资SELECT MIN(salary)FROM employeesWHERE department_id = 50;
#②查询每个部门的最低工资
SELECT MIN(salary),department_idFROM employeesGROUP BY department_id;
#③ 在②基础上筛选，满足min(salary)>①SELECT MIN(salary),department_idFROM employeesGROUP BY department_idHAVING MIN(salary)>( SELECT MIN(salary) FROM employees WHERE department_id = 50);
#2.列子查询（多行子查询）#IN/NOT IN：等于列表中的任意一个#ANY/SOME：和子查询返回的某一个值比较#ALL：和子查询返回的所有值比较#案例1：返回location_id是1400或1700的部门中的所有员工姓名 
#①查询location_id是1400或1700的部门编号SELECT DISTINCT department_idFROM departmentsWHERE location_id IN(1400,1700);
#②查询员工姓名，要求部门号是①列表中的某一个
SELECT last_nameFROM employeesWHERE department_id <>ALL( SELECT DISTINCT department_id FROM departments WHERE location_id IN(1400,1700));

#案例2：返回其它工种中比job_id为‘IT_PROG’工种任一工资低的员工#的员工号、姓名、job_id 以及salary
#①查询job_id为‘IT_PROG’部门任一工资
SELECT DISTINCT salary FROM employeesWHERE job_id = 'IT_PROG';
#②查询员工号、姓名、job_id 以及salary，salary<(①)的任意一个SELECT last_name,employee_id,job_id,salaryFROM employeesWHERE salary<ANY( SELECT DISTINCT salary FROM employees WHERE job_id = 'IT_PROG'
) AND job_id<>'IT_PROG';
#或SELECT last_name,employee_id,job_id,salaryFROM employeesWHERE salary<( SELECT MAX(salary) FROM employees WHERE job_id = 'IT_PROG'
) AND job_id<>'IT_PROG';

#案例3：返回其它部门中比job_id为‘IT_PROG’部门所有工资都低的员工的员工号、姓名、job_id 以及salarySELECT last_name,employee_id,job_id,salaryFROM employeesWHERE salary<ALL( SELECT DISTINCT salary FROM employees WHERE job_id = 'IT_PROG'
) AND job_id<>'IT_PROG';#或SELECT last_name,employee_id,job_id,salaryFROM employeesWHERE salary<( SELECT MIN( salary) FROM employees WHERE job_id = 'IT_PROG'
) AND job_id<>'IT_PROG';
#3、行子查询（结果集一行多列或多行多列）#案例：查询员工编号最小并且工资最高的员工信息SELECT * FROM employeesWHERE (employee_id,salary)=( SELECT MIN(employee_id),MAX(salary) FROM employees);
#①查询最小的员工编号SELECT MIN(employee_id) FROM employees;
#②查询最高工资SELECT MAX(salary) FROM employees;
#③查询员工信息SELECT * FROM employeesWHERE employee_id=( SELECT MIN(employee_id) FROM employees)AND salary=( SELECT MAX(salary) FROM employees);

#二、select后面/*仅仅支持标量子查询*/
#案例：查询每个部门的员工个数SELECT d.*,( SELECT COUNT(*) FROM employees e WHERE e.department_id = d.`department_id` ) 个数 FROM departments d;  #案例2：查询员工号=102的部门名SELECT (  SELECT department_name,e.department_id FROM departments d INNER JOIN employees e ON d.department_id=e.department_id WHERE e.employee_id=102 ) 部门名;
#三、from后面/*将子查询结果充当一张表，要求必须起别名*/
#案例：查询每个部门的平均工资的工资等级 #①查询每个部门的平均工资SELECT AVG(salary),department_idFROM employees GROUP BY department_id;# ②SELECT * FROM job_grades;
#②连接①的结果集和job_grades表，筛选条件平均工资 between lowest_sal and highest_salSELECT ag_dep.*,g.`grade_level`FROM ( SELECT AVG(salary) ag,department_id FROM employees GROUP BY department_id) ag_depINNER JOIN job_grades gON ag_dep.ag BETWEEN lowest_sal AND highest_sal;
#四、exists后面（相关子查询）#语法：exists(完整的查询语句) 结果：1或0SELECT EXISTS(SELECT employee_id FROM employees WHERE salary=300000);
#案例1：查询有员工的部门名#inSELECT department_nameFROM departments dWHERE d.`department_id` IN( SELECT department_id FROM employees);
#existsSELECT department_nameFROM departments dWHERE EXISTS( SELECT * FROM employees e WHERE d.`department_id`=e.`department_id`);
#案例2：查询没有女朋友的男神信息
#inSELECT bo.*FROM boys boWHERE bo.id NOT IN( SELECT boyfriend_id FROM beauty); #existsSELECT bo.*FROM boys boWHERE NOT EXISTS( SELECT boyfriend_id FROM beauty b WHERE bo.`id`=b.`boyfriend_id`);