mysql打卡学习2

进阶一：基础查询

语法

select 查询列表
from 表名 from 表名;
特点
1.查询列表可以是：表中字段，常量值，表达式，函数
2.查询结构是虚拟表格
USE myemployees;

#1.查询单个字段
SELECT last_name FROM employees;

#2.查询多个字段
SELECT last_name,salary,email FROM employees;

#3.查询全部
SELECT	`first_name`,`last_name`,`email`,`commission_pct`
FROM employees;

SELECT * FROM employees;

SELECT 'name' FROM employees;

#4 查询常量值
SELECT 100;
SELECT 'john';

#5 查询表达式
SELECT 100*98;

#6 查询函数
SELECT VERSION();

#7 起别名
SELECT 100&98 AS 结果;
SELECT last_name AS 姓名,first_name AS 名 FROM employees;
SELECT last_name 姓名,first_name 名 FROM employees;
SELECT salary AS 'out put' FROM employees;

#8.去重
# 查询所有员工的部门编号
SELECT department_id FROM employees;
SELECT DISTINCT department_id FROM employees;

#9.+号的作用 运算符
/*
select 100+90;190
select '123'+90; 213
select 'john'+90; 0+90
select null+100; null
*/

#10. 拼接  有null的情况
SELECT CONCAT('a','b','c') AS 结果;

SELECT CONCAT(last_name,'_',first_name) AS 姓名
FROM employees;

SELECT CONCAT(last_name,'_',first_name,'_',`job_id`) AS 姓名
FROM employees;

SELECT IFNULL(commission_pct,0) AS 奖金率,commission_pct
FROM employees;

SELECT CONCAT(last_name,'_',first_name,'_',`job_id`,'_',IFNULL(commission_pct,0)) AS out_put
FROM employees;