MySql01 select的使用

USE atguigudb;

#dual 空表
SELECT "iphone",112 FROM DUAL;

#1.全选
SELECT * FROM employees;
SELECT first_name AS NAME,email mail,phone_number "phone2",salary money FROM employees;

#2.去除重复行
SELECT DISTINCT department_id FROM employees;

#3.无意义去重
SELECT DISTINCT department_id,salary FROM employees;

#4.空值参与运算 空值null不等同于0 'null',''
SELECT * FROM employees;

#5.空值参与运算：结果也一定为空***
SELECT employee_id,salary "月工资",salary*(1+commission_pct)*12 "年工资",commission_pct
FROM employees;
#用0替换null，引入ifnull
SELECT employee_id ,salary "月工资",salary *(1+IFNULL(commission_pct,0))*12 "年工资",commission_pct
FROM employees;

#6.着重号 `` 关键字order与表名order重复，要加着重号修饰
SELECT * FROM `order`;

#7.查询常数
SELECT '尚硅谷',123,employee_id,last_name
FROM employees;

#8.显示表结构
DESCRIBE employees; #显示表中字段的详细信息
DESC employees;

#9.过滤数据 添加过滤条件
#查询90号部门员工的信息
SELECT *
FROM employees
WHERE department_id = 90;
#查询last_name为‘king’的全部员工信息
SELECT *
FROM employees
WHERE last_name = 'king';#MySQL不注意大小写，king与King