MySQL/Language/Alias

表达式与列可以使用AS定义别名。别名可用于ORDER BY, GROUP BY 或 HAVING子句，但不能用于WHERE 子句。例如：

 SELECT 
     CONCAT(last_name,' ', first_name) AS full_name,
     nickname AS nick 
 FROM
     mytable 
 ORDER BY
     full_name

表名也可以有别名。可以使用或忽略AS关键字。

 SELECT
     COUNT(B.Booking_ID), U.User_Location 
 FROM
     Users U
 LEFT OUTER JOIN
     Bookings AS B
 ON 
     U.User_ID    = B.Rep_ID AND
     B.Project_ID = '10'
 GROUP BY
     (U.User_Location)

自连接（self join）时表名的别名非常重要。

 SELECT
     p.name                                   AS parent,
     c.name                                   AS child,
     MIN((TO_DAYS(NOW())-TO_DAYS(c.dob))/365) AS minage
 FROM
     people AS p 
 LEFT JOIN
     people AS c 
 ON
     p.name=c.parent WHERE c.name IS NOT NULL
 GROUP BY
     parent HAVING minage > 50 ORDER BY p.dob;