Mysql select查询语句单表操作指南

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单表查询

1 简介

在当前工作环境下，程序员在开发项目时一般需要保存数据到某种数据库中，而常用的数据库有Oracle，Mysql，SQL Server，DB2，MongoDB等数据库。MySQL数据库管理系统是由瑞典的MySQL AB公司开发的，但是几经辗转最后成为Oracle的产品。由于MySQL的开源、跨平台的特性，使得MySQL的使用非常普遍。本文档主要介绍的是在Mysql数据库中对单表进行的查询。

2 操作

2.1 简单查询

Mysql的查询语句为SELECT语句，基本的语法如下：

SELECT  [DISTINCT] *|{字段名1, 字段名2, 字段名3,……}
                       FROM 表名
             [WHERE 条件表达式1]
             [GROUP BY 字段名 [HAVING 条件表达式2]]
             [ORDER BY 字段名 [ASC|DESC]]
             [LIMIT [OFFSET] 记录数]

其中DISTINCT用于记录去重。GROUP BY用于分组。ORDER BY用于排序，LIMIT 用于限制查询到的记录数。

Create database test;
Use test;

然后建立表student

create table student(
    id INT(3) Primary key auto_increment,
    name VARCHAR(20) not null,
    grade FLOAT,
    gender CHAR(2)
);

使用insert语句向student表中插入多行记录：

mysql> INSERT INTO student (name, grade, gender)
    ->  values('songjiang', 40, 'M'),
    ->          ('wuyong', 100, 'M'),
    ->          ('qinming', 90, 'M'),
    ->          ('husanniang', 88, 'F'),
    ->          ('sunerniang', 66, 'F'),
    ->          ('wusong', 86, 'M'),
    ->          ('linchong', 92, 'M'),
    ->          ('yanqing', 90, NULL);
Query OK, 8 rows affected (0.09 sec)
Records: 8  Duplicates: 0  Warnings: 0

查询所有字段

mysql> select * from student;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
|  8 | yanqing    |    90 | NULL   |
+----+------------+-------+--------+
8 rows in set (0.00 sec)

查询指定字段

mysql> select name, grade, gender from student;
+------------+-------+--------+
| name       | grade | gender |
+------------+-------+--------+
| songjiang  |    40 | M      |
| wuyong     |   100 | M      |
| qinming    |    90 | M      |
| husanniang |    88 | F      |
| sunerniang |    66 | F      |
| wusong     |    86 | M      |
| linchong   |    92 | M      |
| yanqing    |    90 | NULL   |
+------------+-------+--------+
8 rows in set (0.00 sec)

需要注意的事项如下：
注意：字段之间要使用英文逗号分隔
由于student表中id为主键，但同时添加了auto_increment关键字，表示自动增长，因此添加纪录时不需要再传入记录。
注意下述的操作过程：

mysql> insert into student (id, name) values(7, 'a');
ERROR 1062 (23000): Duplicate entry '7' for key 'PRIMARY'
mysql> insert into student (id, name) values(9, 'a');
Query OK, 1 row affected (0.18 sec)

mysql> select * from student;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
|  8 | yanqing    |    90 | NULL   |
|  9 | a          |  NULL | NULL   |
+----+------------+-------+--------+
9 rows in set (0.00 sec)

mysql> insert into student (id, name) values(20, 'a');
Query OK, 1 row affected (0.04 sec)

mysql> select * from student;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
|  8 | yanqing    |    90 | NULL   |
|  9 | a          |  NULL | NULL   |
| 20 | a          |  NULL | NULL   |
+----+------------+-------+--------+
10 rows in set (0.00 sec)

mysql> insert into student (id, name) values(9, null);
ERROR 1048 (23000): Column 'name' cannot be null
mysql> insert into student(name) values('b')
    -> ;
Query OK, 1 row affected (0.05 sec)

mysql> select * from student;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
|  8 | yanqing    |    90 | NULL   |
|  9 | a          |  NULL | NULL   |
| 20 | a          |  NULL | NULL   |
| 21 | b          |  NULL | NULL   |
+----+------------+-------+--------+
11 rows in set (0.00 sec)

通过上述的操作过程来看
1. 虽然id被标记为auto_increment，但我们仍然可以传递不重复的自定义值。如在上述的操作中，在id自动增长为9之后插入了20，再次自动增长变成21.
2. 由于在建表时指定了name字段not null属性，因此必须传递非空的字符串给该字段。不然插入数据失败。在select语句中的查询字段中，字段的顺序是可以改变的，无须按照其表中定义的顺序进行排列。
3. 可以使用通配符*代替所有字段。
4. 查询指定字段的语法为：

select 字段名1， 字段名2， 字段名3，… from 表名

2.1.1 查询时指定字段名

2.1.1.1 使用as关键字

在下述的代码片段中使用as关键字指定查询字段的显示名，三个字段分别使用as指定显示名，未使用as关键字指定，以及使用默认的grade字段显示。

mysql> select name as 姓名, gender 性别, grade from student;
+------------+--------+-------+
| 姓名       | 性别   | grade |
+------------+--------+-------+
| songjiang  | M      |    40 |
| wuyong     | M      |   100 |
| qinming    | M      |    90 |
| husanniang | F      |    88 |
| sunerniang | F      |    66 |
| wusong     | M      |    86 |
| linchong   | M      |    92 |
| yanqing    | NULL   |    90 |
| a          | NULL   |  NULL |
| a          | NULL   |  NULL |
| b          | NULL   |  NULL |
+------------+--------+-------+
11 rows in set (0.00 sec)

2.1.1.2 不使用as关键字

不使用as关键字直接在字段名之后指定自定义的显示字段名称，字段之间用英文逗号隔开。

mysql> select name 姓名 ,grade 成绩 from student;
+------------+--------+
| 姓名       | 成绩   |
+------------+--------+
| songjiang  |     40 |
| wuyong     |    100 |
| qinming    |     90 |
| husanniang |     88 |
| sunerniang |     66 |
| wusong     |     86 |
| linchong   |     92 |
| yanqing    |     90 |
| a          |   NULL |
| a          |   NULL |
| b          |   NULL |
+------------+--------+
11 rows in set (0.00 sec)

2.2 按条件查询

数据库中包含大量的数据，当我们需要根据需求获取指定的数据，或者对查询的数据重新进行排列组合时，就要在SELECT语句中指定查询条件对查询结果进行过滤。
SELECT语句中最常见的是使用WHERE子句指定查询条件，查询语法为

select 字段名1, 字段名2,... from 表名 where 条件表达式

2.2.1 关系运算符

使用关系运算符对查询结果进行过滤。
常用的关系运算符如下表所示：

2.2.1.1 范例

mysql> select * from student;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
|  8 | yanqing    |    90 | NULL   |
| 21 | b          |  NULL | NULL   |
+----+------------+-------+--------+
9 rows in set (0.00 sec)
mysql> select id, name From student where id = 4;
+----+------------+
| id | name       |
+----+------------+
|  4 | husanniang |
+----+------------+
1 row in set (0.00 sec)

mysql> select name, gender from student where name = 'wuyong';
+--------+--------+
| name   | gender |
+--------+--------+
| wuyong | M      |
+--------+--------+
1 row in set (0.06 sec)

mysql> select name 姓名, grade 成绩 from student where grade > 80;
+------------+--------+
| 姓名       | 成绩   |
+------------+--------+
| wuyong     |    100 |
| qinming    |     90 |
| husanniang |     88 |
| wusong     |     86 |
| linchong   |     92 |
| yanqing    |     90 |
+------------+--------+
6 rows in set (0.05 sec)

上述三个例子简单实用关系运算符进行查询结果的过滤

2.2.2 IN关键字

带In关键字的语法为

select * | 字段名1， 字段名2，... from 表名 
where 字段名 [NOT] IN (元素1， 元素2， 元素3, ...)  

2.2.2.1 范例

mysql> select * from student;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
|  8 | yanqing    |    90 | NULL   |
| 21 | b          |  NULL | NULL   |
+----+------------+-------+--------+
9 rows in set (0.00 sec)

mysql> select name, grade from student where name in ('wuyong', 'songjiang');
+-----------+-------+
| name      | grade |
+-----------+-------+
| songjiang |    40 |
| wuyong    |   100 |
+-----------+-------+
2 rows in set (0.00 sec)

mysql> select id, grade, name, gender from student where id in (1, 2, 3);
+----+-------+-----------+--------+
| id | grade | name      | gender |
+----+-------+-----------+--------+
|  1 |    40 | songjiang | M      |
|  2 |   100 | wuyong    | M      |
|  3 |    90 | qinming   | M      |
+----+-------+-----------+--------+
3 rows in set (0.00 sec)

mysql> select id, grade, name, gender from student where id not in (1, 2, 3);
+----+-------+------------+--------+
| id | grade | name       | gender |
+----+-------+------------+--------+
|  4 |    88 | husanniang | F      |
|  5 |    66 | sunerniang | F      |
|  6 |    86 | wusong     | M      |
|  7 |    92 | linchong   | M      |
|  8 |    90 | yanqing    | NULL   |
| 21 |  NULL | b          | NULL   |
+----+-------+------------+--------+
6 rows in set (0.00 sec)

IN 关键字适用于离散的数值点集合范围的查询。

2.2.3 Between AND关键字

带BETWEEN AND关键字的查询—判断某个字段的值是否在指定的范围之内
语法为：

SELECT *|{字段名1,字段名2,……}
FROM 表名
WHERE 字段名 [NOT] BETWEEN 值1 AND 值2

2.2.3.1 between and实例

mysql> select * from student;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
|  8 | yanqing    |    90 | NULL   |
| 21 | b          |  NULL | NULL   |
+----+------------+-------+--------+
9 rows in set (0.00 sec)    
mysql> select * from student where id between 2 and 4;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
+----+------------+-------+--------+
3 rows in set (0.00 sec)

mysql> select * from student where id not  between 2 and 4;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
|  8 | yanqing    |    90 | NULL   |
| 21 | b          |  NULL | NULL   |
+----+------------+-------+--------+
6 rows in set (0.00 sec)

例子展示了使用between and关键字和not between and关键字的用法。

注意：between and关键字指定的两个值得范围是左右包含的。

2.2.4 带Distinct关键字

Distinct关键字的语句用于去除查询结果的重复记录。有时候，处于对数据的分析需求，需要过滤掉查询记录中重复的值。语法如下：

SELECT DISTINCT 字段名 FROM 表名;

2.2.4.1 Distinct示例

mysql> select * from student;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
|  8 | yanqing    |    90 | NULL   |
| 21 | b          |  NULL | NULL   |
+----+------------+-------+--------+
9 rows in set (0.00 sec)

mysql> select gender from student;
+--------+
| gender |
+--------+
| M      |
| M      |
| M      |
| F      |
| F      |
| M      |
| M      |
| NULL   |
| NULL   |
+--------+
9 rows in set (0.00 sec)
mysql> select distinct gender from student;
+--------+
| gender |
+--------+
| M      |
| F      |
| NULL   |
+--------+
3 rows in set (0.06 sec)

2.2.4.2 Distinct作用于多个字段

Distinct作用多个字段，只有DISTINCT关键字后面指定的国歌字段值都相同，才会被认为是重复记录。

SELECT DISTINCT 字段名1,字段名2,…… 
FROM 表名;

首先插入一条数据，以达到多字段重复的效果

mysql> insert into student (name, grade, gender) VALUES ('songjiang', 20, 'M');
Query OK, 1 row affected (0.07 sec)

示例：

mysql> select * from student;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
|  8 | yanqing    |    90 | NULL   |
| 21 | b          |  NULL | NULL   |
| 22 | songjiang  |    20 | M      |
+----+------------+-------+--------+
10 rows in set (0.00 sec)

mysql> select * from student where name = 'songjiang';
+----+-----------+-------+--------+
| id | name      | grade | gender |
+----+-----------+-------+--------+
|  1 | songjiang |    40 | M      |
| 22 | songjiang |    20 | M      |
+----+-----------+-------+--------+
2 rows in set (0.00 sec)

mysql> select distinct name, gender from student;
+------------+--------+
| name       | gender |
+------------+--------+
| songjiang  | M      |
| wuyong     | M      |
| qinming    | M      |
| husanniang | F      |
| sunerniang | F      |
| wusong     | M      |
| linchong   | M      |
| yanqing    | NULL   |
| b          | NULL   |
+------------+--------+
9 rows in set (0.00 sec)

在下述查询中，仅仅看到了一条’songjiang’, ‘M’的数据

mysql> select * from student;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
|  8 | yanqing    |    90 | NULL   |
| 21 | b          |  NULL | NULL   |
| 22 | songjiang  |    20 | M      |
+----+------------+-------+--------+
10 rows in set (0.00 sec)

mysql> select distinct name, gender from student;
+------------+--------+
| name       | gender |
+------------+--------+
| songjiang  | M      |
| wuyong     | M      |
| qinming    | M      |
| husanniang | F      |
| sunerniang | F      |
| wusong     | M      |
| linchong   | M      |
| yanqing    | NULL   |
| b          | NULL   |
+------------+--------+
9 rows in set (0.00 sec)

仅仅显示一条数据，表明已经过滤了重复记录。

2.2.5 空值查询

判断某些列是否有空值。
在数据表中，某些列的值可能为空值NULL，空置不同于0，也不同于空字符串。在MySQL中，使用IS NULL关键字来判断字段的值是否为空值，其语法格式为：

SELECT *|字段名1,字段名2,……
FROM 表名
WHERE 字段名IS [NOT] NULL

2.2.5.1 空值查询示例

mysql> select * from student;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
|  8 | yanqing    |    90 | NULL   |
| 21 | b          |  NULL | NULL   |
+----+------------+-------+--------+
9 rows in set (0.00 sec)

mysql> select * from student where grade is null;
+----+------+-------+--------+
| id | name | grade | gender |
+----+------+-------+--------+
| 21 | b    |  NULL | NULL   |
+----+------+-------+--------+
1 row in set (0.00 sec)

mysql> select * from student where gender is null;
+----+---------+-------+--------+
| id | name    | grade | gender |
+----+---------+-------+--------+
|  8 | yanqing |    90 | NULL   |
| 21 | b       |  NULL | NULL   |
+----+---------+-------+--------+
2 rows in set (0.00 sec)

mysql> select * from student where gender is not null;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
+----+------------+-------+--------+
7 rows in set (0.00 sec)

2.2.6 带Like关键字的查询

有时候需要对字符串进行模糊匹配，例如查询student表中name字段值’b’开头的记录，为了完成类似功能，Mysql提供了Like关键字进行模糊查询。LIKE关键字可以判断两个字符串是否相匹配。语法如下：

SELECT *|{字段名1,字段名2,……}
FROM 表名
WHERE 字段名 [NOT] LIKE '匹配字符串';

匹配字符串指定用来匹配的字符串，其值可以是一个普通字符串，也可以是一个包含百分号%或者下划线_的通配字符串。两个符号在匹配过程中有特殊含义。

2.2.6.1 百分号%

%通配符用于匹配任意长度的字符串，包括空字符串。例如，字符串’c%’可以匹配以c开头的任意长度的字符串，包括’c’, ‘ct’, ‘cnt’, ‘current’。
查询完整结果集

mysql> select * from student;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
|  8 | yanqing    |    90 | NULL   |
| 21 | b          |  NULL | NULL   |
| 22 | songjiang  |    20 | M      |
+----+------------+-------+--------+
10 rows in set (0.00 sec)

查询以name以’s’开头的学生信息

mysql> select * from student where name Like 's%';
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  5 | sunerniang |    66 | F      |
| 22 | songjiang  |    20 | M      |
+----+------------+-------+--------+
3 rows in set (0.00 sec)

百分号通配符可以出现在通配符字符串中的任意位置。
查询student表中name字段以‘s’开头以‘g’开头的id和name

mysql> select id, name from student where name like 's%g'
    -> ;
+----+------------+
| id | name       |
+----+------------+
|  1 | songjiang  |
|  5 | sunerniang |
| 22 | songjiang  |
+----+------------+
3 rows in set (0.00 sec)

查询name中包含字符’y’的学生id和name

mysql> select id, name from student where name like '%y%';
+----+---------+
| id | name    |
+----+---------+
|  2 | wuyong  |
|  8 | yanqing |
+----+---------+
2 rows in set (0.00 sec)

2.2.6.2 下划线_

下划线通配符_仅仅匹配单个字符，如果要匹配多个字符，需要使用多个下划线通配符。
查询student表中name字段值以字符串‘wu’开始，以字符串‘ong’结束，并且两个字符串之间只有一个字符的记录。

mysql> select * from student where name like 'wu_ong';
+----+--------+-------+--------+
| id | name   | grade | gender |
+----+--------+-------+--------+
|  2 | wuyong |   100 | M      |
|  6 | wusong |    86 | M      |
+----+--------+-------+--------+
2 rows in set (0.00 sec)

查询student表中字段值包含7个字符，并且以字符串’ing’结束的记录

mysql> select * from student where name like '____ing';
+----+---------+-------+--------+
| id | name    | grade | gender |
+----+---------+-------+--------+
|  3 | qinming |    90 | M      |
|  8 | yanqing |    90 | NULL   |
+----+---------+-------+--------+
2 rows in set (0.00 sec)

如果要匹配的字符串包含%或者下划线_，怎么进行呢？

注意：百分号和下划线是通配符，它们在通配字符串中有特殊含义，因此，如果要匹配字符串中的百分号和下划线，就需要在通配字符串中使用右斜线(“\”)对百分号和下划线进行转义，例如，“\%”匹配百分号字面值，“_”匹配下划线字面值。

2.2.7 带逻辑运算符的多条件查询

在使用select语句查询数据时，有时候为了使查询结果更加精确，可以使用多个查询条件。

2.2.7.1 AND关键字

在MySQL中提供了AND关键字，使用AND关键字可以连接两个或者多个查询条件，只有满足所有条件的记录才会被返回。

SELECT *|{字段名1,字段名2,……}
FROM 表名
WHERE 条件表达式1 […… AND 条件表达式n];

查询student表中id字段小于5，并且gender字段为’F’的学生姓名

mysql> select id, name, gender from student where id < 5 AND gender = 'F';
+----+------------+--------+
| id | name       | gender |
+----+------------+--------+
|  4 | husanniang | F      |
+----+------------+--------+
1 row in set (0.09 sec)

查询student表中id字段在1，2，3，4之间，name字段值以字符串‘ng’结尾，并且grade字段小于80的学生记录

mysql> select * from student where id in (1, 2, 3, 4) and name like '%ng' and grade < 80;
+----+-----------+-------+--------+
| id | name      | grade | gender |
+----+-----------+-------+--------+
|  1 | songjiang |    40 | M      |
+----+-----------+-------+--------+
1 row in set (0.07 sec)

2.2.7.2 OR关键字

在使用SELECT语句查询数据时，也可食用OR逻辑运算符连接多个查询条件。与AND不同，在使用OR关键字时，只要记录满足任意一个条件就会被查询出来，其语法格式为:

SELECT *|{字段名1,字段名2,……}
FROM 表名
WHERE 条件表达式1 OR […… OR 条件表达式n];

示例1

mysql> select id, name, gender from student where id < 3 or gender='F';
+----+------------+--------+
| id | name       | gender |
+----+------------+--------+
|  1 | songjiang  | M      |
|  2 | wuyong     | M      |
|  4 | husanniang | F      |
|  5 | sunerniang | F      |
+----+------------+--------+
4 rows in set (0.00 sec)

示例2

mysql> select id, name, gender, grade from student where name like 'h%' or gender = 'F' or grade = 100;
+----+------------+--------+-------+
| id | name       | gender | grade |
+----+------------+--------+-------+
|  2 | wuyong     | M      |   100 |
|  4 | husanniang | F      |    88 |
|  5 | sunerniang | F      |    66 |
+----+------------+--------+-------+
3 rows in set (0.00 sec)

通过上述的查询示例，可以看到返回的记录只要满足其中任意一个条件都可以显示。

注意：OR关键字和AND关键字可以一起使用，需要注意的是，AND优先级高于OR，因此当两者在一起使用时，应该先运算AND量表的条件表达式，再运算OR两边的条件表达式。

2.3 为表和字段取别名

在查询数据时，可以为表和字段取别名，这个别名可以代替其指定的表和字段。

2.3.1 为表取别名

在查询操作时，如果表名很长使用起来不方便，这时可以为表取一个别名，用这个别名来代替表的名称。
这种语法如下：

SELECT * FROM 表名 [AS] 别名;

具体示例如下：

mysql> select * from student;
+----+------------+-------+--------+
| id | name       | grade | gender |
+----+------------+-------+--------+
|  1 | songjiang  |    40 | M      |
|  2 | wuyong     |   100 | M      |
|  3 | qinming    |    90 | M      |
|  4 | husanniang |    88 | F      |
|  5 | sunerniang |    66 | F      |
|  6 | wusong     |    86 | M      |
|  7 | linchong   |    92 | M      |
|  8 | yanqing    |    90 | NULL   |
| 21 | b          |  NULL | NULL   |
| 22 | songjiang  |    20 | M      |
+----+------------+-------+--------+
10 rows in set (0.00 sec)

mysql> select * from student AS stu where stu.gender = "M";
+----+-----------+-------+--------+
| id | name      | grade | gender |
+----+-----------+-------+--------+
|  1 | songjiang |    40 | M      |
|  2 | wuyong    |   100 | M      |
|  3 | qinming   |    90 | M      |
|  6 | wusong    |    86 | M      |
|  7 | linchong  |    92 | M      |
| 22 | songjiang |    20 | M      |
+----+-----------+-------+--------+
6 rows in set (0.00 sec)

mysql> select * from student stu where stu.gender = "M";
+----+-----------+-------+--------+
| id | name      | grade | gender |
+----+-----------+-------+--------+
|  1 | songjiang |    40 | M      |
|  2 | wuyong    |   100 | M      |
|  3 | qinming   |    90 | M      |
|  6 | wusong    |    86 | M      |
|  7 | linchong  |    92 | M      |
| 22 | songjiang |    20 | M      |
+----+-----------+-------+--------+
6 rows in set (0.00 sec)

在上述的代码片段中，为表student取别名为stu，并且在条件表达式中进行使用，很方便。
注意：AS关键字可有可无，也可以在From之后在写表名之后直接跟上起的别名。上述代码片段中第二个示例便是这种用法的显示。

2.3.2 为字段取别名

在操作sql表时，每条记录的列名并不是必须为定义时的列名，有时为了让所显示的查询结果更加直观，可以为字段取一个别名。Mysql中为字段起别名的格式如下：

SELECT 字段名 [AS] 别名[,字段名 [AS] 别名,……] FROM 表名

示例：

mysql> select name 姓名, gender as 性别  from student stu where stu.gender = "M";
+-----------+--------+
| 姓名      | 性别   |
+-----------+--------+
| songjiang | M      |
| wuyong    | M      |
| qinming   | M      |
| wusong    | M      |
| linchong  | M      |
| songjiang | M      |
+-----------+--------+
6 rows in set (0.00 sec)

在上述的代码片段中，可以看到查询出来的结果列名已经显示为姓名和性别了，这种自定义的方式使得sql查询更加的人性化。并且上述的查询语句同时包含了为表和字段起别名的演示。

3 参考

https://download.csdn.net/download/lk142500/10670417

4 总结

mysql语法是sql类数据库查询的基础，学会使用这些基础知识构建出需要的select语句，熟悉语法规则，孰料使用数据库查询语句，对于程序员是必备的开发技能。本文档包含mysql对于单表的基本操作，对于熟悉mysql数据库，尤其是命令行操作有很好的帮助。