Mysql学习笔记（4）分组函数、分组查询、连接查询

分组函数

SELECT SUM(salary) AS 求和,AVG(salary) AS 平均值,MAX(salary) AS 最大值,MIN(salary) AS 最小值,COUNT(*) AS 总行数
FROM employees

分组查询 group by

#GROUP BY HAVING 分组后筛选，查询那个部门的员工数>2
SELECT COUNT(*) AS co,`department_id` FROM employees   GROUP BY `department_id` HAVING COUNT(*) >2#多字段分组
SELECT TRUNCATE(AVG(salary),2),`department_id`,`job_id` FROM employees GROUP BY `department_id`,`job_id`

多表连接查询

等值连接

#为表起别名，相同字段用表别名限定（已定义表别名，将不可再用表名做限定）
SELECT last_name,e.job_id,job_title FROM employees AS e,jobs AS j
WHERE e.`job_id` = j.`job_id`#加筛选条件
SELECT last_name,`department_name`,commission_pct FROM employees AS e,departments AS d
WHERE e.`department_id` = d.`department_id` AND e.`commission_pct` IS NOT NULL#加分组查询 group by
SELECT COUNT(*),city FROM departments AS d,`locations` AS l
WHERE d.`location_id` = l.`location_id`
GROUP BY city#加排序 order by
SELECT job_title,COUNT(*) AS co FROM jobs AS j,employees AS e
WHERE j.`job_id` = e.`job_id`
GROUP BY job_title ORDER BY co DESC#三表连接
SELECT last_name,`department_name`,city FROM employees AS e,departments AS d,`locations` AS l
WHERE e.`department_id` = d.`department_id` AND d.`location_id` = l.`location_id`AND city LIKE '%s%' ORDER BY `department_name` DESC

不等值连接

#不等值连接
SELECT last_name,salary,`grade_level` FROM employees AS e,`job_grades` AS j
WHERE e.`salary` BETWEEN j.`lowest_sal` AND j.`highest_sal` ORDER BY `grade_level` DESC,salary

自连接

#自连接
SELECT e.`employee_id`,e.`last_name`,m.`employee_id`,m.`last_name`
FROM employees AS e,employees AS m
WHERE e.`manager_id` = m.`employee_id`

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Mysql学习笔记（4）分组函数、分组查询、连接查询

分组函数

分组查询 group by

多表连接查询

等值连接

不等值连接

自连接

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