通过使用 SQL，可以为表名称或列名称指定别名。
基本上，创建别名是为了让列名称的可读性更强。
列的SQL别名语法
SELECT column_name AS alias_name
FROM table_name;
表的SQL别名语法
SELECT column_name(s)
FROM table_name AS alias_name;
AS 关键词可以忽略不写，用空格隔开即可。
演示数据库
在本教程中，我们将使用 LintCode 样本数据库。
下面是选自 "teachers" 教师表的数据：
下面是 "courses" 课程表的数据：
1 列的别名实例Ⅰ
下面的 SQL 语句指定了两个别名，一个是 name 列的别名为 ‘ teacher_name’，一个是 country 列的别名为 ‘ teacher_country’。
提示：
如果别名包含空格，要求使用双引号或方括号，建议不使用空格，而使用 '_'。
SELECT name AS teacher_name, country teacher_name
FROM teachers;
执行结果输出
mysql> SELECT name AS teacher_name, country teacher_name
-> FROM teachers;
+------------------+--------------+
| teacher_name | teacher_name |
+------------------+--------------+
| Eastern Heretic | CN |
| Northern Beggar | CN |
| Western Venom | USA |
| Southern Emperor | CN |
| Linghu Chong | CN |
+------------------+--------------+
5 rows in set (0.00 sec)
练习题：列的别名Ⅰ
查询所有教师的 id 和姓名（name），并为 name 起别名为 teacher_name。
SELECT id, name AS teacher_name
FROM teachers;
执行结果输出
+----+------------------+
| id | teacher_name |
+----+------------------+
| 1 | Eastern Heretic |
| 2 | Northern Beggar |
| 3 | Western Venom |
| 4 | Southern Emperor |
| 5 | Linghu Chong |
+----+------------------+
2 列的别名实例Ⅱ
在下面的 SQL 语句中，我们把两个列（age 和 country）结合在一起，并创建一个名为 "teacher_info" 的别名。
SELECT name, CONCAT(age, ', ', country) AS teacher_info
FROM teachers;
其中
CONCAT 方法可以将所有参数进行拼接成一个字符串。
执行结果输出
mysql> SELECT name, CONCAT(age, ', ', country) AS teacher_info
-> FROM teachers;
+------------------+--------------+
| name | teacher_info |
+------------------+--------------+
| Eastern Heretic | 20, CN |
| Northern Beggar | 21, CN |
| Western Venom | 28, USA |
| Southern Emperor | 21, CN |
| Linghu Chong | 18, CN |
+------------------+--------------+
5 rows in set (0.00 sec)
练习题：列的别名Ⅱ
查询所有教师的 id 和姓名（name）和 age，将 age 后面拼接一个字符 ‘ 岁’ ，并为 age 起别名为 teacher_age
SELECT id, name, CONCAT(age, '岁') AS teacher_age
FROM teachers;
执行结果输出
+----+------------------+-------------+
| id | name | teacher_age |
+----+------------------+-------------+
| 1 | Eastern Heretic | 20岁 |
| 2 | Northern Beggar | 21岁 |
| 3 | Western Venom | 28岁 |
| 4 | Southern Emperor | 21岁 |
| 5 | Linghu Chong | 18岁 |
+----+------------------+-------------+
3 表的别名实例
下面的 SQL 语句查询所有课程的 id ，课程名和对应教师的姓名。我们使用 "teachers" 和 "courses" 表，并分别为它们指定表别名 "t" 和 "c"（通过使用别名让 SQL 更简短）：
-- 不使用别名写法 --
SELECT courses.id, courses.name, teachers.name
FROM teachers, courses where teachers.id = courses.teacher_id;
-- 使用别名的写法 --
SELECT c.id, c.name, t.name
FROM teachers t, courses c where t.id = c.teacher_id;
执行输出结果
mysql> SELECT c.id, c.name, t.name
-> FROM teachers t, courses c where t.id = c.teacher_id;
+----+-------------------------+------------------+
| id | name | name |
+----+-------------------------+------------------+
| 1 | Senior Algorithm | Southern Emperor |
| 2 | System Design | Western Venom |
| 3 | Django | Western Venom |
| 4 | Web | Southern Emperor |
| 5 | Big Data | Eastern Heretic |
| 6 | Artificial Intelligence | Western Venom |
| 7 | Java P6+ | Western Venom |
| 8 | Data Analysis | Eastern Heretic |
| 10 | Object Oriented Design | Southern Emperor |
| 12 | Dynamic Programming | Eastern Heretic |
+----+-------------------------+------------------+
10 rows in set (0.00 sec)
练习题：表的别名
查询所有课程的 id ，课程名和对应教师的姓名。我们使用 "teachers" 和 "courses" 表，并分别为它们指定表别名 "t" 和 "c"，因为课程名和教师姓名的列名都是 'name' ，要求为课程名起别名为 'course_name'，为教师姓名起别名为 'teacher_name' ：
SELECT c.id, c.name course_name, t.name teacher_name
FROM teachers t, courses c where t.id = c.teacher_id;
执行输出结果
+----+-------------------------+------------------+
| id | course_name | teacher_name |
+----+-------------------------+------------------+
| 1 | Senior Algorithm | Southern Emperor |
| 2 | System Design | Western Venom |
| 3 | Django | Western Venom |
| 4 | Web | Southern Emperor |
| 5 | Big Data | Eastern Heretic |
| 6 | Artificial Intelligence | Western Venom |
| 7 | Java P6+ | Western Venom |
| 8 | Data Analysis | Eastern Heretic |
| 10 | Object Oriented Design | Southern Emperor |
| 12 | Dynamic Programming | Eastern Heretic |
+----+-------------------------+------------------+
4 AS 总结
在下面的情况下，使用别名很有用：
