SQL第二天

Posted 2021-01-25 lsls

筛选条件

筛选：

功能：
mysql> SELECT * FROM students WHERE age>18;
数据库   选择 所有 从  表名称 其中 年龄大于18的 = 选择表中年龄大于18的
运行结果如下：
+-----------+----------+------+------------+
| number    | name     | age  | birth      |
+-----------+----------+------+------------+
| 201804004 | Gollum4  |   19 | 2001-01-04 |
| 201804005 | Gollum5  |   20 | 2000-01-05 |
| 201804006 | Gollum6  |   21 | 1999-01-06 |
| 201804007 | Gollum7  |   22 | 1999-01-07 |
| 201804008 | Gollum8  |   23 | 1999-01-08 |
| 201804009 | Gollum9  |   24 | 1999-01-09 |
| 201804010 | Gollum10 |   25 | 1999-01-10 |
+-----------+----------+------+------------+
7 rows in set (0.00 sec)

mysql> SELECT * FROM students WHERE age<>18;
不等于18的数据

+-----------+----------+------+------------+
| number | name | age | birth |
+-----------+----------+------+------------+
| 201804001 | Gollum1 | 16 | 2002-01-01 |
| 201804002 | Gollum2 | 17 | 2001-01-02 |
| 201804004 | Gollum4 | 19 | 2001-01-04 |
| 201804005 | Gollum5 | 20 | 2000-01-05 |
| 201804006 | Gollum6 | 21 | 1999-01-06 |
| 201804007 | Gollum7 | 22 | 1999-01-07 |
| 201804008 | Gollum8 | 23 | 1999-01-08 |
| 201804009 | Gollum9 | 24 | 1999-01-09 |
| 201804010 | Gollum10 | 25 | 1999-01-10 |
| 201804012 | Gollum12 | 12 | 1999-01-10 |
+-----------+----------+------+------------+

重点：

age BETWEEN 12 AND 18; 查询12-18之间的数据

name = ‘666‘ OR name = ‘777‘;  输出参数等于666或777的数据

age in(12,18); 输出 12和 18

排序（order by）

SELECT * FROM students ORDER BY age;: 正序从小到大排列

SELECT * FROM students ORDER BY age desc;逆序从大到小

限制 linmit

SELECT * FROM students LIMIT 3; 输出前三的数据

SELECT * FROM students LIMIT 3,4; 不取前三条返回前三的后四条

去重 distinct

SELECT DISTINCT grade FROM grades; 查询科目有几种 类似筛选

SELECT * FROM students WHERE name LIKE‘Gollum%‘;
匹配大概是Gollum的数据%是任意值 
SELECT * FROM students WHERE name LIKE‘Gollum_‘;
_是任意一个字符

常用的聚合函数

SELECT COUNT(*)FROM students;
查询这个表的个数

SELECT MAX(age)FROM students;查询这个字段的最大值