MySQL#查询_2

#去重
SELECT DISTINCT department_id FROM employees;

#案例:查询员工名和姓连接成一个字段，并显示为 姓名
SELECT CONCAT('a','b','c') AS 结果;
SELECT
      CONCAT(last_name,first_name) AS 姓名
FROM
	employees;

#显示表department的结构
DESC departments;

#显示employees表中的全部job_id（不能重复）
SELECT DISTINCT job_id FROM employees;

#显示employees的全部列，用“，”逗号链接，列头为out_put
SELECT 
	CONCAT(first_name,',',last_name,',',job_id, ',',IFNULL(commission_pct,0)) AS out_put
FROM 
	employees;

#ifnull判断是否为空，为空的话返回0
SELECT IFNULL(commission_pct,0) AS 奖金率,
	commission_pct
	FROM
	employees;