在where子句

Question

我有一個子查詢，看起來像這樣使用子查詢：

SELECT field1, (
     SELECT date2 
     FROM table2 
     ORDER BY date2 DESC 
    ) AS last_contacted 
    FROM table1 
    ORDER BY last_contacted DESC 

但是，我得到記錄，其中last_contacted爲NULL。我不希望這些結果中，所以我認爲WHERE last_contacted IS NOT NULL但返回錯誤：

Unknown column 'last_contacted' in 'where clause'

所以，我怎麼能刪除那些從結果？

Answer 1

可以在GROUP BY，ORDER BY或HAVING子句中使用列別名。

請參閱mysql文檔。

http://dev.mysql.com/doc/refman/5.0/en/problems-with-alias.html

Answer 2

SELECT field1, (
    SELECT date2 
    FROM table2 
    ORDER BY date2 DESC 
) AS last_contacted 
FROM table1 
HAVING last_contacted NOT NULL 
ORDER BY last_contacted DESC