存在差異和EXISTS

Question

我有一個這樣的查詢

SELECT ... FROM ... WHERE (SELECT EXISTS (SELECT...)) 

which did not return anything then i changed it to 

    SELECT ... FROM ... WHERE (EXISTS (SELECT...)) 

and i got my results, can anyone explain the difference? 

這裏完整的查詢，變更爲第40行：

SELECT 
    user_element_status.user_id, 
    user_element_status.element_id, 
    notifications.id 
FROM 
    notifications 
    INNER JOIN notification_element ON (notification_element.notification_id=notifications.id) 
    INNER JOIN user_element_status ON ( 
             user_element_status.element_id=notification_element.element_id 
            AND 
             case 
              when notifications.notstatus=0 then notifications.notification_status LIKE CONCAT('%',user_element_status.`status`,'%') 
              when notifications.notstatus=1 then notifications.notification_status NOT LIKE CONCAT('%',user_element_status.`status`,'%') 
             End 
            AND user_element_status.`status` != 'not_required' 
            AND 
             DATE_ADD(
             case 
              when notifications.notification_trigger='assigned' then user_element_status.assigned 
              when notifications.notification_trigger='first_launch' then user_element_status.firstlaunch 
              when notifications.notification_trigger='completed' then user_element_status.completedate 
              when notifications.notification_trigger='due' then user_element_status.duedate 
              when notifications.notification_trigger='credited' then user_element_status.creditcompletedate 
             End 
             , INTERVAL triggerdelay DAY) 
             - CURRENT_TIMESTAMP < 0 
            ) 
    INNER JOIN users ON (users.id=user_element_status.user_id AND users.is_active=1) 


WHERE 
    NOT EXISTS ( 
        SELECT * FROM alreadysent 
        WHERE 
         alreadysent.user_id=user_element_status.user_id AND 
         alreadysent.element_id=notification_element.element_id AND 
         alreadysent.notification_id=notification_element.notification_id 
       ) 
    AND 
    (EXISTS (
     SELECT 
      user_group.user_id, 
      element_group.element_id 
     FROM user_group 
      user_group 
      INNER JOIN users ON (users.id=user_group.user_id) 
      LEFT JOIN element_group ON (element_group.group_id=user_group.group_id) 
      LEFT JOIN elements ON (elements.group_id=user_group.group_id) 
      LEFT JOIN element_localization ON ((element_localization.element_id=element_group.element_id OR element_localization.element_id=elements.id) AND users.country_id=element_localization.country_id) 
      LEFT JOIN element_arealocalisation ON (element_arealocalisation.element_id = element_group.element_id OR element_arealocalisation.element_id=elements.id) 
      LEFT JOIN area_country ON (element_arealocalisation.area_id = area_country.area_id AND area_country.country_id=users.country_id) 
     WHERE 
      users.is_active=1 AND 
      user_group.user_id=user_element_status.user_id AND 
      (element_group.element_id=user_element_status.element_id OR elements.id=user_element_status.element_id) AND 
      (element_localization.country_id IS NOT NULL OR area_country.country_id IS NOT NULL) 
     )) 

Answer 1

我想到的第一個查詢是不正確的語法。請參閱http://www.techonthenet.com/sql/exists.php。這可能有幫助。

Answer 2

您是否檢查了Documentation;實際的語法是

SELECT .. FROM .. WHERE [NOT]EXISTS (Subquery); 

每在後您的編輯，你實際上是在做下面這是不是你在第一個查詢

WHERE 
    NOT EXISTS ( subquery) 
    AND 
    EXISTS (subquery) 

見@siride評論顯示什麼，唯一的區別我可以說是的AND (EXISTS (subquery))情況下，如果子查詢返回的行那麼它評價爲AND TRUE，而在AND (SELECT EXISTS (subquery))情況下，它會評估爲AND SELECT TRUE ... 但在兩種情況下EXISTS邏輯應該很好地工作，你的查詢應返回相同的預期結果設置，但根據你的帖子，你說第一個版本AND (SELECT EXISTS (subquery))不提取記錄......我真的懷疑。