pg14-sql基础（五）-case

条件表达

SELECT first_name, last_name,
  CASE
     WHEN salary <5000 THEN '低收入'
	  WHEN salary between 5000 and 100000 THEN '中等收入'
	  ELSE '高收入'
  END AS "收入等级"
FROM employees;

--查询部门编号为10、20、30有各多少人
SELECT COUNT(CASE department_id WHEN 10 THEN 1 END) dep_cnt_10,
-- 换种写法：COUNT(*) FILTER (WHERE department_id=10) dep_cnt_10,
       COUNT(CASE department_id WHEN 20 THEN 1 END) dep_cnt_20,
	   COUNT(CASE department_id WHEN 30 THEN 1 END) dep_cnt_30
FROM employees;