Mysql连接和子查询
作者：互联网
Sql92语法：仅仅支持内连接
①等值连接
②非等值连接
③自连接
Sql99语法(推荐使用)：支持内连接+外连接+交叉连接
按功能分类：
内连接：
1.等值连接
2.非等值连接
3.自连接
外连接：
1.左外连接
2.右外连接
3.全外连接(Mysql不支持)
交叉连接：
Sql92标准：
语法： select 字段1,字段2 from 表1 t1,表2 t2 where 条件;
1.等值连接
案例1：查询员工名和对应的部门名
SELECT last_name,department_name from employees e,departments d WHERE e.department_id=d.department_id;
案例2：查询员工名，工种号，工种名
SELECT last_name,job_title,j.job_id FROM employees e,jobs j where e.job_id=j.job_id;
employees表中也有job_id而jobs表中也有job_id，会有歧义，因此需要指明是哪个表中的job_id
案例3：查询有奖金的员工名、部门名
SELECT last_name,department_name FROM employees e,departments d WHERE e.department_id=d.department_id AND commission_pct is not null;
可以跟筛选：
案例4：查询城市名中第二个字母为o的部门名和城市名
SELECT department_name,city from departments d,locations l WHERE d.location_id=l.location_id AND city LIKE '_o%';
可以跟分组：
案例5：查询每个城市的部门个数
SELECT count(*),city from departments d,locations l WHERE d.location_id=l.location_id GROUP BY city;
可以跟排序：
案例6：查询每个工种的工种名和员工的个数，并且按员工个数降序
SELECT job_title,COUNT(*) FROM employees e,jobs j WHERE e.job_id=j.job_id GROUP BY job_title ORDER BY COUNT(*) desc;
可以实现多表的连接：
案例7：查询员工名、部门名和所在的城市
SELECT last_name,department_name,city FROM employees e,departments d,locations l where e.department_id=d.department_id AND d.location_id = l.location_id;
2.非等值连接
案例1：查询员工的工资和工资级别
SELECT salary,grade_level FROM employees e,job_grades jg WHERE e.salary BETWEEN lowest_sal AND highest_sal;
3.自连接(顾明思议自己连接自己)
案例：查询员工的名称和上级的名称
SELECT e.last_name,m.last_name FROM employees e,employees m WHERE e.manager_id=m.employee_id;
1. 显示所有员工的姓名，部门号和部门名称
SELECT last_name,d.department_id,department_name FROM employees e,departments d where e.department_id=d.department_id;
2.查询90号部门员工的job_id和90号部门的location_id
SELECT job_id,location_id FROM employees e,departments d WHERE e.department_id=d.department_id AND d.department_id='90';
3.查询所有有奖金的员工的last_name,depart_name,location_id,city
SELECT last_name,department_name,l.location_id,city FROM employees e,departments d,locations l WHERE e.department_id=d.department_id AND d.location_id=l.location_id and commission_pct is not null;
4.选择city在Toronto工作的员工的last_name,job_id,department_id,department_name
SELECT last_name,job_id,d.department_id,department_name,city FROM employees e,departments d,locations l WHERE e.department_id=d.department_id AND d.location_id=l.location_id AND city='Toronto';
5.查询每个工种、每个部门的部门名、工种名和最低工资
SELECT job_title,department_name,MIN(salary) FROM employees e,departments d,jobs j WHERE e.department_id=d.department_id AND e.job_id=j.job_id GROUP BY department_name,job_title;
6. 查询每个国家下的部门个数大于2的国家编号
SELECT country_id,COUNT(*) FROM departments d,locations l WHERE d.location_id=l.location_id GROUP BY country_id HAVING COUNT(*)>2;
7. 选择指定员工的姓名，员工号，以及他的管理者的姓名和员工号
SELECT e.last_name employees,e.employee_id 'Emp#',m.last_name 'Manager',m.employee_id 'Mgr#' FROM employees e,employees m where e.manager_id=m.employee_id;
总结：
1.多表连接的结果是它们之间的交集
2.n个表连接需要n-1个条件
3.一般要为表起别名
4.可以跟分组函数、分组查询、排序等组合使用
Sql99语法：
语法：
select 查询列表 from 表1 别名 [ 连接类型] join 表2 别名 on 连接条件
[ where 筛选条件] [ group by 分组] [having 筛选条件] [order by 排序列表]
分类：
1.内连接：inner join
2.外连接：
①左外连接:left [outer] join
②右外连接:right [outer] join
③全外连接:full [outer] join
3.交叉连接:cross join
内连接：
1.等值连接
案例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;
案例2：查询名字中包含e的员工名和工种名(刷选)
SELECT last_name,job_title FROM employees e INNER JOIN jobs j on e.job_id=j.job_id WHERE job_title LIKE '%e%';
案例3：查询部门个数>3的城市名和部门个数
SELECT city,COUNT(*) FROM departments d LEFT JOIN locations l ON d.location_id=l.location_id GROUP BY city HAVING COUNT(*)>3;
案例4：查询那个部门的部门员工个数>3的部门名和员工个数，并按个数降序(排序)
SELECT COUNT(*),department_name FROM employees e LEFT JOIN departments d on e.department_id=d.department_id GROUP BY department_name ORDER BY COUNT(*) desc;
案例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 LEFT JOIN departments d on e.department_id=d.department_id LEFT JOIN jobs j on e.job_id=j.job_id ORDER BY department_name desc;
2.非等值连接
案例：查询工资级别个数>2的个数，并且按工资级别排序
SELECT COUNT(*),grade_level FROM employees e INNER JOIN job_grades jg on salary BETWEEN lowest_sal AND highest_sal GROUP BY grade_level;
3.自连接
案例：查询员工的名字、上级的名字
SELECT e.last_name employees,m.last_name manager FROM employees e INNER JOIN employees m on e.manager_id=m.employee_id;
外连接：
1.查询主表中的所有记录，如果从表有和它匹配的则显示，如果没有和它匹配的则显示null。
2.查询结果=内连接结果+主表中有从表中没有的记录
左外连接：left左边的是主表
案例1：查询那个部门没有员工
SELECT department_name FROM departments d LEFT JOIN employees e on d.department_id=e.department_id where e.employee_id is null;
右外连接：right右边的是主表
全外连接：mysql暂时不支持，查询结果=内联结果+表1中有表2中没有+表2中有表1中没有的
交叉连接：笛卡尔积
1.SELECT beauty.*,boys.boyName FROM beauty LEFT JOIN boys on beauty.boyfriend_id = boys.id WHERE beauty.id>3;
2.SELECT DISTINCT city,department_id FROM locations l LEFT JOIN departments d on d.location_id=l.location_id WHERE department_id is NULL;
3.SELECT e.*,department_name FROM employees e LEFT JOIN departments d on e.department_id=d.department_id WHERE department_name in ('SAL','IT');
子查询：
含义：出现在其他语句中的select语句，被称为一个子查询，外围的查询语句被称为主查询或者外查询
按子查询出现的位置进行一个分类：
1.放在select后面，只能是标量子查询(一行一列)
2.放在from后面，作为数据源
支持表子查询
3.放在where/having后面
标量子查询
行子查询
列子查询
4.放在exists后面(相关子查询)
表子查询
按结果集行列数不同进行一个分类：
1.标量子查询(一行一列)
2.列子查询(一列多行)
3.行子查询(一行多列)
4.表子查询(结果集)
一、where/having后面的子查询
特点：
①使用一对()包含
②一般放在条件的右侧
③标量子查询一般搭配单行操作符(<,>,<=,>=,<>)使用
④列子查询一般搭配多行操作符(in,any/some,all)使用
#标量子查询
案例1：谁的工资比Abel高
SELECT last_name FROM employees where salary>(SELECT salary FROM employees WHERE last_name='Abel');
案例2： 返回job_id与141员工相同，salary比143号员工多的员工姓名，job_id和工资
SELECT 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');
案例3：返回公司工资最少的员工的last_name,job_id和salary
SELECT last_name,job_id,salary FROM employees WHERE 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
SELECT department_id,MIN(salary) FROM employees GROUP BY department_id HAVING MIN(salary)>(SELECT MIN(salary) FROM employees WHERE department_id='50');
列子查询：
一般与in，any/some，all搭配使用
案例1：返回location_id是1400或1700的部门中的所有员工姓名
SELECT last_name FROM employees WHERE department_id in (SELECT department_id FROM departments WHERE location_id in ('1400','1700'));
案例2：返回工种比job_id为'IT_PROG'所有工资都低的员工的员工号、姓名、job_id、以及salary
SELECT employee_id,last_name,job_id,salary FROM employees WHERE salary <(SELECT MIN(salary) FROM employees WHERE job_id='IT_PROG');
行子查询：
案例1：查询员工编号最小并且工资最高的员工信息
SELECT e.* FROM employees e where (employee_id,salary)=(SELECT MIN(employee_id),MAX(salary) FROM employees);
二、放在select后面的标量子查询
案例1：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 ) FROM departments d;
案例2：查询员工号为102的部门名
SELECT (SELECT d.department_name FROM departments d LEFT JOIN employees e on e.department_id=d.department_id WHERE e.employee_id='102');
三、放在from后面
案例1：查询每个部门的平均工资的工资等级
SELECT grade_level,mm.d_id,mm.ag_sal FROM job_grades,(SELECT avg(salary) ag_sal,department_id d_id FROM employees GROUP BY department_id) mm WHERE mm.ag_sal BETWEEN lowest_sal AND highest_sal;
案例2： 查询员工号为102的部门名
SELECT d.department_name FROM departments d,(SELECT department_id d_id FROM employees WHERE employee_id='102') mm WHERE d.department_id=mm.d_id;
四、exists后面(相关子查询)
案例1：查询所有员工的部门名
SELECT DISTINCT department_name FROM departments d WHERE EXISTS (SELECT * FROM employees e WHERE e.department_id=d.department_id);
1.查询和Zlotkey相同部门的员工姓名和工资
SELECT last_name,salary FROM employees WHERE department_id = (SELECT department_id FROM employees WHERE last_name='Zlotkey');
2.查询工资比公司平均工资高的员工的员工号，姓名和工资
SELECT employee_id,last_name,salary FROM employees WHERE salary>(SELECT avg(salary) FROM employees);
3.查询各部门中工资比本部门平均工资高的员工的员工号，姓名和工资
SELECT e.employee_id,e.last_name,e.salary FROM employees e,(SELECT avg(salary) ag_sal,department_id FROM employees GROUP BY department_id) mm WHERE mm.department_id =e.department_id AND salary > mm.ag_sal;
4.查询和姓名中包含字母u的员工在相同部门的员工的员工号和姓名
SELECT employee_id,last_name FROM employees WHERE department_id in (SELECT DISTINCT department_id FROM employees WHERE last_name LIKE '%u%');
5.查询在部门的location_id为1700的部门工作的员工的员工号
SELECT employee_id from employees WHERE department_id in (SELECT DISTINCT d.department_id FROM departments d WHERE d.location_id='1700');
6.查询管理者是K_ing的员工姓名和工资
SELECT last_name,salary FROM employees WHERE manager_id in (SELECT employee_id FROM employees WHERE last_name='K_ing');
7.查询工资最高的员工的姓名，要求first_name和last_name显示为一列，列名为姓.名
SELECT CONCAT(last_name,first_name) FROM employees WHERE salary = (SELECT MAX(salary) FROM employees);
标签：name,employees,查询,和子,Mysql,department,id,SELECT 来源： https://blog.csdn.net/weixin_56674682/article/details/121300902