mysql-distinct去除重复数据

select distinct a 
from ...
-- 查询字段a 并去除重复数据

select distinct a,b
from ...
-- ab组合 去除重复数据

1.部门编号

USE hr;
SELECT DISTINCT department_id
FROM employees
WHERE department_id IS NOT NULL;

2.工种代码

USE hr;
SELECT DISTINCT job_id
FROM employees ORDER BY job_id;

3.主管id

USE hr;
SELECT DISTINCT manager_id
FROM employees 
WHERE manager_id IS NOT NULL;

4.部门中的主管id

USE hr;
SELECT DISTINCT department_id,manager_id
FROM employees 
WHERE department_id IS NOT NULL
AND manager_id IS NOT NULL
ORDER BY department_id,manager_id;