3 查询

1.查询的基本语法

select * from 表名;

• from关键字后面写表名，表示数据来源于是这张表
• select后面写表中的列名，如果是*表示在结果中显示表中所有列
• 在select后面的列名部分，可以使用as为列起别名，这个别名出现在结果集中
• 如果要查询多个列，之间使用逗号分隔

mysql> select id,name from students;
+----+--------+
| id | name   |
+----+--------+
|  1 | 腾旭   |
|  2 | 腾旭   |

2.消除重复行

• 在select后面列前使用distinct可以消除重复的行

mysql> select distinct gender from students ;
+--------+
| gender |
+--------+
|       |
|        |
+--------+

3.条件

• 使用where子句对表中的数据筛选，结果为true的行会出现在结果集中
• 语法如下：

select * from 表名 where 条件;

　　1）比较运算符

• 等于=
• 大于>
• 大于等于>=
• 小于<
• 小于等于<=
• 不等于!=或<>

• 查询编号大于3的学生

select * from students where id>3;

• 查询编号不大于4的科目

select * from subjects where id<=4;

• 查询姓名不是“黄蓉”的学生

select * from students where sname!='黄蓉';

• 查询没被删除的学生

select * from students where isdelete=0;

　　2）逻辑运算符

• and
• or
• not
• 查询编号大于3的女同学

select * from students where id>3 and gender=0;

• 查询编号小于4或没被删除的学生

select * from students where id<4 or isdelete=0;

+----+--------+--------+---------------------+----------+
| id | name   | gender | birthday            | isDelete |
+----+--------+--------+---------------------+----------+
|  1 | 腾旭   |       | 1999-09-09 00:00:00 |          |
|  2 | 腾旭   |       | 1990-02-02 00:00:00 |          |
|  3 | 网易   |       | NULL                |          |
|  4 | 小米   |       | NULL                |          |
|  7 | QQ     |       | NULL                |          |
+----+--------+--------+---------------------+----------+

　　3）模糊查询

• like
• %表示任意多个任意字符
• _表示一个任意字符
• 查询姓黄的学生

select * from students where sname like '黄%';

mysql> select * from students where name like '腾%';
+----+-----------+--------+---------------------+----------+
| id | name      | gender | birthday            | isDelete |
+----+-----------+--------+---------------------+----------+
|  1 | 腾旭      |       | 1999-09-09 00:00:00 |          |
|  2 | 腾旭      |       | 1990-02-02 00:00:00 |          |
|  8 | 腾讯云    |       | NULL                |          |
+----+-----------+--------+---------------------+----------+

• 查询姓黄并且名字是一个字的学生

select * from students where sname like '黄_';

mysql> select * from students where name like '腾%';
+----+-----------+--------+---------------------+----------+
| id | name      | gender | birthday            | isDelete |
+----+-----------+--------+---------------------+----------+
|  1 | 腾旭      |       | 1999-09-09 00:00:00 |          |
|  2 | 腾旭      |       | 1990-02-02 00:00:00 |          |
|  8 | 腾讯云    |       | NULL                |          |
+----+-----------+--------+---------------------+----------+

• 查询姓黄或叫靖的学生

select * from students where sname like '黄%' or sname like '%靖';

　　4）范围查询

• in表示在一个非连续的范围内
• 查询编号是1或3或8的学生

select * from students where id in(1,3,8);

+----+-----------+--------+---------------------+----------+
| id | name | gender | birthday | isDelete |
+----+-----------+--------+---------------------+----------+
| 1 | 腾旭 | | 1999-09-09 00:00:00 | |
| 3 | 网易 | | NULL | |
| 8 | 腾讯云 | | NULL | |
+----+-----------+--------+---------------------+----------+

• between ... and ...表示在一个连续的范围内
• 查询学生是3至8的学生

select * from students where id between 3 and 8;

+----+-----------+--------+---------------------+----------+
| id | name | gender | birthday | isDelete |
+----+-----------+--------+---------------------+----------+
| 3 | 网易 | | NULL | |
| 4 | 小米 | | NULL | |
| 6 | 酷狗 | | 2017-02-13 00:00:00 | |
| 7 | QQ | | NULL | |
| 8 | 腾讯云 | | NULL | |
+----+-----------+--------+---------------------+----------+

• 查询学生是3至8的男生

select * from students where id between 3 and 8 and gender=1;

　　5）空判断

• 注意：null与''是不同的
• 判空is null
• 查询没有填写地址的学生

select * from students where hometown is null; 

mysql> select * from students where birthday is null;
+----+-----------+--------+----------+----------+
| id | name      | gender | birthday | isDelete |
+----+-----------+--------+----------+----------+
|  3 | 网易      |       | NULL     |          |
|  4 | 小米      |       | NULL     |          |
|  7 | QQ        |       | NULL     |          |
|  8 | 腾讯云    |       | NULL     |          |
+----+-----------+--------+----------+----------+

• 判非空is not null
• 查询填写了地址的学生

select * from students where hometown is not null; 

mysql> select * from students where birthday is not null;
+----+--------+--------+---------------------+----------+
| id | name   | gender | birthday            | isDelete |
+----+--------+--------+---------------------+----------+
|  1 | 腾旭   |       | 1999-09-09 00:00:00 |          |
|  2 | 腾旭   |       | 1990-02-02 00:00:00 |          |
|  6 | 酷狗   |        | 2017-02-13 00:00:00 |         |
+----+--------+--------+---------------------+----------+

• 查询填写了地址的女生

select * from students where hometown is not null and gender=0; 

mysql> select * from students where birthday is not null and gender=0;
+----+--------+--------+---------------------+----------+
| id | name   | gender | birthday            | isDelete |
+----+--------+--------+---------------------+----------+
|  6 | 酷狗   |        | 2017-02-13 00:00:00 |         |
+----+--------+--------+---------------------+----------+

 

　　6）优先级

• 小括号，not，比较运算符，逻辑运算符
• and比or先运算，如果同时出现并希望先算or，需要结合()使用