mysql sql99语法 内连接等值连接

#案例：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：查询名字中包含e的员工名和工种名(筛选)

SELECT
last_name,job_title
FROM
employees e
INNER JOIN
jobs s
ON
e.`job_id`=s.job_id
WHERE
e.`last_name` LIKE '%a%' ;

#案例：查询部门个数大于3的城市名和部门个数

SELECT
city,COUNT(*)
FROM
departments d
INNER JOIN
locations l
ON
d.`location_id`=l.`location_id`
GROUP BY
city
HAVING
COUNT(*)>3;

#查询：员工名、部门名、工种名，并按部门名排序(三表连接)

SELECT
last_name,department_name,job_title
FROM
employees e
INNER JOIN
departments d    #内连接的表得和from的表有连接条件
ON
e.`department_id`=d.department_id
INNER JOIN

jobs j
ON
j.job_id=e.job_id
ORDER BY
department_name DESC;

#案例：查询哪个部门的员工个数>3的部门名和员工个数，并按个数排序(添加排序)

SELECT
COUNT(*),department_name
FROM
departments d
INNER JOIN
employees e
ON
d.`department_id`=e.`department_id`
GROUP BY
department_name
HAVING
COUNT(*)>3
ORDER BY
COUNT(*) DESC;