为什么MySQL在简单的select语句上使用表别名会出错？

Question

I'm using phpmyadmin to test out some MySQL queries. I'm trying to write a larger, nested query, which is failing due to an unrecognized table alias, so I'm trying to debug smaller parts of it. However, I'm getting confusing errors when I try to use table aliases sometimes.

Can you explain why some of these queries throw errors?

SELECT * FROM table1 AS tablealias1 (works)

SELECT * FROM table1 GROUP BY userid (works)

SELECT * FROM table1 GROUP BY userid AS tablealias1 (error: #1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'AS tablealias1 LIMIT 0, 25' at line 1 )

SELECT * FROM table1 WHERE userid=1 (works)

SELECT * FROM table1 WHERE userid=1 AS tablealias1 (same error as above)

(SELECT * FROM table1 WHERE userid=1) AS tablealias1 (same error as above)

Answer 1

You alias things to:

• rename the column display's name
• give it a reference name for later use elsewhere in the query statement (whether you use it explicitly or implicitly--as long as it could be used elsehere)

If you're not doing either, an alias makes no sense. You can't alias a result set unless it's used inside a subquery, then you need an alias to reference it.

This will work:

 Select * FROM (SELECT * FROM table1 WHERE userid=1) AS tablealias1

as it implies

  Select tablealias1.* FROM (SELECT * FROM table1 WHERE userid=1) AS tablealias1

Alone, this is garbage:

  (SELECT * FROM table1 WHERE userid=1) AS tablealias1