03 基本的Select语句
作者：互联网
3.1 select .....
select 1; # 没有字句
/*
查询结果:
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.01 sec)
*/
3.2 select ... from ...
# 语法
select 标识选择哪些列(*表示全选) from 表示从那个表中选择;
# 例子
select * from departments;
/*
+---------------+----------------------+------------+-------------+
| department_id | department_name | manager_id | location_id |
+---------------+----------------------+------------+-------------+
| 10 | Administration | 200 | 1700 |
| 20 | Marketing | 201 | 1800 |
| 30 | Purchasing | 114 | 1700 |
| 40 | Human Resources | 203 | 2400 |
| 50 | Shipping | 121 | 1500 |
| 60 | IT | 103 | 1400 |
| 70 | Public Relations | 204 | 2700 |
| 80 | Sales | 145 | 2500 |
| 90 | Executive | 100 | 1700 |
| 100 | Finance | 108 | 1700 |
| 110 | Accounting | 205 | 1700 |
| 120 | Treasury | NULL | 1700 |
| 130 | Corporate Tax | NULL | 1700 |
| 140 | Control And Credit | NULL | 1700 |
| 150 | Shareholder Services | NULL | 1700 |
| 160 | Benefits | NULL | 1700 |
| 170 | Manufacturing | NULL | 1700 |
| 180 | Construction | NULL | 1700 |
| 190 | Contracting | NULL | 1700 |
| 200 | Operations | NULL | 1700 |
| 210 | IT Support | NULL | 1700 |
| 220 | NOC | NULL | 1700 |
| 230 | IT Helpdesk | NULL | 1700 |
| 240 | Government Sales | NULL | 1700 |
| 250 | Retail Sales | NULL | 1700 |
| 260 | Recruiting | NULL | 1700 |
| 270 | Payroll | NULL | 1700 |
+---------------+----------------------+------------+-------------+
27 rows in set (0.05 sec)
*/
# 选择特定的列
select manager_id, location_id from departments;
/*
+------------+-------------+
| manager_id | location_id |
+------------+-------------+
| 200 | 1700 |
| 201 | 1800 |
| 114 | 1700 |
| 203 | 2400 |
| 121 | 1500 |
| 103 | 1400 |
| 204 | 2700 |
| 145 | 2500 |
| 100 | 1700 |
| 108 | 1700 |
| 205 | 1700 |
| NULL | 1700 |
| NULL | 1700 |
| NULL | 1700 |
| NULL | 1700 |
| NULL | 1700 |
| NULL | 1700 |
| NULL | 1700 |
| NULL | 1700 |
| NULL | 1700 |
| NULL | 1700 |
| NULL | 1700 |
| NULL | 1700 |
| NULL | 1700 |
| NULL | 1700 |
| NULL | 1700 |
| NULL | 1700 |
+------------+-------------+
*/
3.3 列的别名
- 给列起别名之后查询出来的表列名用别名替代
- as 可以省略
- 别名中如果包含空格需要用双引号
select last_name as lname, commission_pct as comm from employees;
# 省略as也可以
select last_name lname, commission_pct comm from employees;
/*
+-------------+------+
| lname | comm |
+-------------+------+
| King | NULL |
| Kochhar | NULL |
| De Haan | NULL |
| Hunold | NULL |
| Ernst | NULL |
| Austin | NULL |
| Pataballa | NULL |
| Lorentz | NULL |
| Greenberg | NULL |
*/
3.4 去除重复行
通过DISTINCT关键词来去除重复行
- DISTINCT必须要放在所有查询列名之前
- DISTINCT针对后面所有的列名组合去重，简单的说就是去掉后面列名都一致的重复行
SELECT DISTINCT department_id FROM employees;
/*
+---------------+
| department_id |
+---------------+
| NULL |
| 10 |
| 20 |
| 30 |
| 40 |
| 50 |
| 60 |
| 70 |
| 80 |
| 90 |
| 100 |
| 110 |
+---------------+
12 rows in set (0.05 sec)
*/
# 这条语句会查询出所有列，因为employee_id是表的主键，唯一的，因此每一列都不是重复列
SELECT DISTINCT department_id, employee_id FROM employees;
3.5 空值参与运算
- 所有的运算符或者是列值遇到null值，运算的结果都为null
# 查询员工一年的工资
SELECT employee_id,salary,commission_pct,
12 * salary * (1 + commission_pct) "annual_sal"
FROM employees;
注：null不等于空字符串。一个空字符串的长度是0，空值的长度是空。
3.6 着重号
当表名或者是列名和关键词或者已有函数重名需要用着重号
select * from ORDER;
/*1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'ORDER' at line 1
*/
select * from `ORDER`;
/*
+----------+------------+
| order_id | order_name |
+----------+------------+
| 1 | shkstart |
| 2 | tomcat |
| 3 | dubbo |
+----------+------------+
3 rows in set (0.04 sec)
*/
3.7 查询常数
查询的时候添加长度可以增加一列字段，用于整合不同的数据源
select 'jiang' as corporation, last_name as lname from employees;
/*
+-------------+-------------+
| corporation | lname |
+-------------+-------------+
| jiang | King |
| jiang | Kochhar |
| jiang | De Haan |
| jiang | Hunold |
| jiang | Ernst |
| jiang | Austin |
| jiang | Pataballa |
| jiang | Lorentz |
| jiang | Greenberg |
| jiang | Faviet |
| jiang | Chen |
*/
3.8 显示表结构
使用DESCRIBE或者DESC命令，表示表结构。
DESCRIBE employees;
DESC employees;
/*
+-----------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------------+-------------+------+-----+---------+-------+
| department_id | int | NO | PRI | 0 | |
| department_name | varchar(30) | NO | | NULL | |
| manager_id | int | YES | MUL | NULL | |
| location_id | int | YES | MUL | NULL | |
+-----------------+-------------+------+-----+---------+-------+
4 rows in set (0.05 sec)
*/
- Filed表示字段名
- Type表示类型
- Null表示是否可以为null
- key表示是否存在索引，PRI是主键，UNI表示唯一索引，MUL表示不唯一
- Defalut表示是否存在默认值，默认值为多少
- Extra表示可以获取与给定列有关的附加信息，例如AUTO_INCREMENT等
3.10 过滤表数据
用where条件来过滤数据
SELECT 字段1,字段2
FROM 表名
WHERE 过滤条件;
# WHERE需要紧跟这FROM后面
# 查询部门id为90的员工号、姓名、工作id、部门id
SELECT employee_id, last_name, job_id, department_id
FROM employees
WHERE department_id = 90 ;
3.10 练习
-
查询员工12个月的工资总和，并起别名为ANNUAL SALARY
select employee_id, 12 * salary as "ANNUAL SALARY" from employees; # 如果要算上奖金系数的话，需要用到IFNULL函数，如果commission_pct为null取0，不为null取其值 select employee_id, 12 * salary * (1 + IFNULL(commission_pct, 0)) as "ANNUAL SALARY" from employees; -
查询employees表中去除重复的job_id以后的数据
select DISTINCT job_id from employees; -
查询工资大于12000的员工姓名和工资
select first_name fname, last_name lname, salary from employees WHERE salary > 12000; -
查询员工号为176的员工的姓名和部门号
select first_name fname, last_name lname, department_id from employees WHERE employee_id = 176; -
显示表 departments 的结构，并查询其中的全部数据
DESC departments; select 8 from departments;
标签：语句,03,jiang,1700,employees,select,NULL,id,Select 来源： https://www.cnblogs.com/jiangblog/p/16395370.html