在where子句中使用子查询

Question

我有一个子查询，看起来像这样：

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

但是，我正在获取last_contacted为NULL的记录。 我不希望这些出现在结果中，所以我认为WHERE last_contacted IS NOT NULL但是返回错误：

Unknown column 'last_contacted' in 'where clause'

那么如何从结果中删除那些呢？

Answer 1

可以在GROUP BY，ORDER BY或HAVING子句中使用列别名。

请参考mysql文档。

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