mysql sql99语法 内连接等值连接

Posted 2022-10-28

#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;