oracle over(partition BY) 函数用法

sql:

SELECT u.user_id,

    u.user_name,

    u.mail_receive_time

  FROM mail_user u,

    mail_interior i

  WHERE u.mail_id          = i.mail_id

  AND i.mail_creator       = '8440'

  AND u.mail_status       <> 3

  AND u.user_id           <> i.mail_creator

  AND u.mail_receive_time IS NOT NULL

  ORDER BY u.mail_receive_time DESc;

 前：


-

--去重复后

SELECT u.user_id,u.user_name,u.mail_receive_time

FROM

  (SELECT a.* ,

    row_number() over(partition BY a.user_id order by a.mail_receive_time DESC)rn

  FROM

    (SELECT u.user_id,

      u.user_name,

      u.mail_receive_time

    FROM mail_user u,

      mail_interior i

    WHERE u.mail_id          = i.mail_id

    AND i.mail_creator       = '8440'

    AND u.mail_status       <> 3

    AND u.user_id           <> i.mail_creator

    AND u.mail_receive_time IS NOT NULL

    ) a

  )u

WHERE rn=1

order by u.mail_receive_time desc;