如需用我的练习表,发消息qq:[email protected]
CREATE DATABASE test;
SELECT MAX(salary),MIN(salary),AVG(salary),SUM(salary),job_id
FROM employees
GROUP BY job_id
ORDER BY job_id;
SELECT MAX(salary)-MIN(salary) DIFFERENCE
FROM employees;
SELECT department_id,COUNT(*),AVG(salary) a
FROM employees
GROUP BY department_id
ORDER BY a DESC;
SELECT COUNT(*) ,job_id
FROM employees
GROUP BY job_id;
#进阶6:连接查询
/*
含义:又称多表查询,当查询的字段来自于多个表时,就会用到连接查询。
笛卡尔乘积现象:表1有n行,表2有m行,结果=m*n行
发生原因:没有有效的连接条件
如何避免:添加有效的连接条件
分类: 按年代分类
sql:92语法:仅仅支持连接
sql:99语法【推荐】:mysql中:支持内连接+外连接(左外和右外)+交叉连接
按功能分类:
内连接:
等值连接
非等值连接
自连接
外连接:
左外连接
右外连接
全外连接
交叉连接
*/
SELECT * FROM beauty;
SELECT * FROM boys;
SELECT NAME,boyName FROM boys,beauty
WHERE beauty.boyfriend_id=boys.id;
#一、sql92的标准
#1、等值连接
#查询1:查询女神名和对应的男神名
SELECT NAME,boyName
FROM boys,beauty
WHERE beauty.boyfriend_id=boys.id;
#案例2:查询员工名和对应的部门名
SELECT last_name,department_name
FROM employees,departments
WHERE employees.department_id=departments.department_id;
#2、为表起别名(可以起别名。但是必须要统一,
#要么都用原名,要么都用别名,
#对于同一个列名必须注意)
/*
1.提高语句的简洁度
2.区分多个重名字段
*/
#查询员工名、工种号、工种名
SELECT last_name,e.job_id,j.job_title
FROM employees e,jobs j
WHERE e.job_id=j.job_id;
#3.两个表的顺序是否可以调换
#可以
#4.可以加筛选吗?
#案例:查询有奖金的员工名、部门名
SELECT last_name,department_name,commission_pct
FROM employees e,departments d
WHERE e.department_id=d.department_id
AND e.commission_pct IS NOT NULL;
#案例2:查询城市名中第二个字符为o的部门名和城市名
#注意列名后面的s !!!记得加上,细心点
SELECT department_name,city
FROM departments d,locations l
WHERE d.location_id=l.location_id
AND city LIKE ‘_o%’;
#5.可以加分组?
#案例:查询每个城市的部门个数
SELECT COUNT(*) 个数,city
FROM departments d,locations l
WHERE d.location_id=l.location_id
GROUP BY city;
#案例2:查询有奖金的每个部门的部门名和部门的领导编号和该部门的最低工资
SELECT department_name,d.manager_id,MIN(salary)
FROM departments d,employees e
WHERE d.department_id=e.department_id
AND commission_pct IS NOT NULL
GROUP BY department_name,d.manager_id;
#7.可不可以加排序?
#案例: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
AND city LIKE ‘s%’
ORDER BY department_name DESC;
#8非等值连接
#9自连接(同一表查询不同列名信息)
SELECT e.employee_id,e.last_name,m.employee_id,m.last_name
FROM employees e,employees m
WHERE e.manager_id=m.employee_id;