基本的SELLECT语句

列的别名

as:全称：alias(别名),可以省略

列的别名可以使用一对""引起来，不要使用''。

SELECT employee_id emp_id,last_name AS lname,department_id "部门id",salary * 12 AS "annual sal"
FROM employees;

去除重复行

查询员工表中一共有哪些部门 id 呢？

SELECT DISTINCT department_id
FROM employees;
SELECT COUNT(DISTINCT department_id)
FROM employees;

空值参与运算

空值：null

null 不等同于 0，''，'null'

SELECT \* FROM employees; #空值参与运算：结果一定也为空。

SELECT employee_id,salary "月工资",salary _ (1 + commission_pct) _ 12 "年工资",commission_pct
FROM employees; #实际问题的解决方案：引入 IFNULL

SELECT employee_id,salary "月工资",salary _ (1 + IFNULL(commission_pct,0)) _ 12 "年工资",commission_pct
FROM `employees`;

查询常数

SELECT '尚硅谷',123,employee_id,last_name
FROM employees;

5.显示表结构

DESCRIBE employees;
DESC employees;
DESC departments;

6.过滤数据

练习：查询 90 号部门的员工信息

SELECT \* FROM employees WHERE department_id=90; #练习：查询 last_name 为'King'的员工信息

SELECT \* FROM employees WHERE last_name='King';

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