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获取每组的最后一行

greatest-n-per-group mysql sql

user391986 · 技术社区 · 16 年前

如何对以下任务进行查询:

假设您以用户身份登录:1 我想在每次谈话中都有一行。对于我想得到的每一行, 对话中第一行的“主题” 第一行的“日期时间” “消息”此对话的最后一条消息,无论是谁写的

CREATE TABLE messages (
    ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    FromID INT NOT NULL,   
    ToID INT NOT NULL,    
    ConversationID INT NOT NULL,    
    Subject varchar(255), 
    Message varchar(255),
    DateTime DATETIME                                       
    ) ENGINE=InnoDB;


CREATE TABLE conversations (
    ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY                                    
    ) ENGINE=InnoDB;



INSERT INTO conversations (ID) VALUES (1), (2), (3);
INSERT INTO messages (FromID, ToID, ConversationID, Subject, Message, DateTime) VALUES (1,2, 1, "Hi", "This is a test message", "2010-08-08 16:23:48");        
INSERT INTO messages (FromID, ToID, ConversationID, Subject, Message, DateTime) VALUES (1,2, 1, "", "Hey again you have not answered", "2010-08-08 16:23:52");                                                                                                                                               
INSERT INTO messages (FromID, ToID, ConversationID, Subject, Message, DateTime) VALUES (2,1, 1, "", "Hi this is my answer", "2010-08-08 16:23:59");


INSERT INTO messages (FromID, ToID, ConversationID, Subject, Message, DateTime) VALUES (2,1, 2, "2.Hi", "2.This is a test message", "2010-08-08 16:25:48");        
INSERT INTO messages (FromID, ToID, ConversationID, Subject, Message, DateTime) VALUES (1,2, 2, "", "2.Hi back", "2010-08-08 16:25:52");                                                                                                                                               
INSERT INTO messages (FromID, ToID, ConversationID, Subject, Message, DateTime) VALUES (2,1, 2, "", "2.Hi this is my answer", "2010-08-08 16:25:59");


INSERT INTO messages (FromID, ToID, ConversationID, Subject, Message, DateTime) VALUES (2,1, 3, "3.Hi", "3.This is a test message", "2010-08-08 16:27:48");        
INSERT INTO messages (FromID, ToID, ConversationID, Subject, Message, DateTime) VALUES (1,2, 3, "", "2.Hi back", "2010-08-08 16:27:52");   
INSERT INTO messages (FromID, ToID, ConversationID, Subject, Message, DateTime) VALUES (1,2, 3, "", "2.Hello are you there?", "2010-08-08 16:27:59");

3 回复 | 直到 16 年前

Martin Smith 16 年前

SELECT M.ConversationID, 
MAX(CASE WHEN M.DateTime = X.FirstRow THEN M.Subject END) AS Subject,
CAST(COALESCE(MAX(CASE WHEN M.DateTime = X.LastRowSentByOtherUser 
                       THEN M.DateTime END),X.LastRow) AS DateTime)AS LastTime,
MAX(CASE WHEN M.DateTime = X.LastRow THEN M.Message END) AS Message,
MAX(CASE WHEN FromID = 1 THEN ToID ELSE FromID END) AS OtherParticipantId
FROM messages M
JOIN (
    SELECT ConversationID, MIN(DateTime) AS FirstRow, MAX(DateTime) AS LastRow,
    MAX(CASE WHEN FromID<>1 THEN DateTime END) AS LastRowSentByOtherUser
    FROM messages
    WHERE FromID=1 OR ToID=1
    GROUP BY ConversationID
) X ON X.ConversationID = M.ConversationID
AND (M.DateTime IN (X.FirstRow, X.LastRow, X.LastRowSentByOtherUser))
GROUP BY M.ConversationID
HAVING MAX(CASE WHEN M.DateTime = X.LastRowSentByOtherUser 
                   THEN M.DateTime END) IS NOT NULL

Community Mohan Dere 9 年前

看一看 ROW_NUMBER() in MySQL -你肯定能把这个应用到你的问题上。

gyromonotron 16 年前

您应该尝试如下操作:

SELECT 
   m1.Subject,
   m1.DateTime,
   m2.Message
FROM conversations c 
INNER JOIN 
   (SELECT MIN(ID) AS minID, 
           MAX(ID) AS maxID, 
           ConversationID
   FROM messages
   WHERE FromID = @userID OR ToID = @userID
   GROUP BY ConversationID) AS cGrouped
ON c.ConversationID = cGrouped.ConversationID
INNER JOIN messages m1 ON m1.ID = cGrouped.minID
INNER JOIN messages m2 ON m2.ID = cGrouped.maxID