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''。

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

2. 去除重复行

#查询员工表中一共有哪些部门id呢？

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

3. 空值参与运算

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SELECT * FROM employees;

#空值参与运算：结果一定也为空。

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SELECT employee_id,salary "月工资",salary * (1 + commission_pct) * 12 "年工资",commission_pctFROM employees;

#实际问题的解决方案：引入IFNULL

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

4. 查询常数

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SELECT '尚硅谷',123,employee_id,last_nameFROM employees;

5.显示表结构

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DESCRIBE employees; DESC employees;DESC departments;

6.过滤数据

#练习：查询90号部门的员工信息

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SELECT * FROM employees WHERE department_id=90;

#练习：查询last_name为'King'的员工信息

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SELECT * FROM employees WHERE last_name='King';