SQL JOINS

关联查询

内连接 inner join

select 字段名 from 表1,表2 on 关联条件;

左外连接 left join

• where b. is null

• 返回左表中的所有行，如果左表中的行在右表中没有匹配行，则右表中的列返回空值

联合查询 union

• where a. is null, where b.is null

自连接

• table1，table2本质上是同一张表，只是用取别名的方式虚拟成两张表，然后两张表再进行连接查询

select的五个子句

where 条件查询

group by + 聚合函数

• avg() #返回平均值
• count()
• min()
• max()
• sum()

having 筛选

• having 和 where 类似，可以筛选数据

• having 只用于 group by 分组统计语句

order by 排序

limit 分页

#显示前3条
limit 0,3

子查询

where 型子查询

• in：等于任何一个

• all：和子查询返回的所有值比较，如 sal > all(1,2,3)

• any：和子查询返回的任意一个值比较，如 sal > any(1,2,3)

• exists：判断子查询是否有返回结果（不关心具体行数和内容）

  #查询比“孙红雷”的工资高的员工编号
  SELECT * FROM t_salary
  WHERE basic_salary > (SELECT basic_salary FROM t_employee INNER JOIN t_salary ON t_employee.eid=t_salary.eid WHERE t_employee.ename='孙红雷');
  
  #查询和孙红雷，李晨在同一个部门的员工
  SELECT * FROM t_employee
  WHERE dept_id IN(SELECT dept_id FROM t_employee WHERE ename='孙红雷' OR ename = '李晨');
  
  #查询全公司工资最高的员工编号，基本工资
  SELECT eid,basic_salary FROM t_salary
  WHERE basic_salary = (SELECT MAX(basic_salary) FROM t_salary);
  
  SELECT eid,basic_salary FROM t_salary
  WHERE basic_salary >= ALL(SELECT basic_salary FROM t_salary);

from 型子查询

#找出比部门平均工资高的员工编号，基本工资
SELECT t_employee.eid,basic_salary 
FROM t_salary INNER JOIN t_employee INNER JOIN (
    SELECT emp.dept_id AS did,AVG(s.basic_salary) AS avg_salary
    FROM t_employee AS emp,t_salary AS s
    WHERE emp.eid = s.eid
    GROUP BY emp.dept_id) AS temp
ON t_salary.eid = t_employee.eid AND t_employee.dept_id = temp.did
WHERE t_salary.basic_salary > temp.avg_salary;

exists 型子查询

#查询部门信息，该部门必须有员工
SELECT * FROM t_department 
WHERE EXISTS (SELECT * FROM t_employee WHERE t_employee.dept_id = t_department.did);