在WHERE子句中使用子查詢的結果

Question

我嘗試在查詢的WHERE子句中使用子查詢的結果。我希望使用的屬性是last_contact。見下文。

SELECT forename, surname, type, 
     (SELECT MAX(completed_date) 
      FROM tblTasks 
      WHERE prospect_id = tblProspects.prospect_id AND completed = '1' 
     ) AS last_contact, 
     created_at 
    FROM tblProspects 
    WHERE hidden != '1' AND type='Prospect' AND last_contact > DATE_ADD(CURDATE(), INTERVAL -90 DAY) 
ORDER BY last_contact ASC 

我得到的SQL錯誤：＃1054 - 未知列 'last_contact' 在 'where子句'

任何幫助將不勝感激。

謝謝。

Answer 1

您需要使用HAVING條款，以便通過自定義的別名coulmn來優化搜索結果，它們不能在where子句中使用

SELECT 
forename, 
surname, 
type, 
(SELECT MAX(completed_date) FROM tblTasks WHERE prospect_id = tblProspects.prospect_id AND completed = '1') AS last_contact, 
created_at 
FROM tblProspects 
WHERE hidden != '1' AND type='Prospect' 
HAVING (
last_contact > DATE_ADD(CURDATE(), INTERVAL -90 DAY) 
OR last_contact IS NULL 
) 
ORDER BY last_contact ASC