獲取從最新的記錄中加入

Question

我有以下查詢：

SELECT f.name, MAX(cmt.created_at) AS CommentDate, cmt.comment AS LastComment 
FROM families f 
LEFT JOIN comments cmt ON f.id = cmt.commentable_id 
WHERE f.id =12212 

我得到評論的最後日期，而是獲得最後一個註釋（根據該日），我得到的第一一。那個怎麼樣？

Answer 1

試試這個方法：

SELECT f.name, cmt.created_at AS CommentDate, cmt.comment AS LastComment 
FROM families f 
LEFT JOIN comments cmt ON f.id = cmt.commentable_id 
WHERE f.id =12212 
and cmt.created_at = (
         SELECT MAX(cmt.created_at) 
         FROM families f 
         LEFT JOIN comments cmt ON f.id = cmt.commentable_id 
         WHERE f.id =12212 
        ) 

或

SELECT f.name, cmt.created_at AS CommentDate, cmt.comment AS LastComment 
FROM families f 
LEFT JOIN comments cmt ON f.id = cmt.commentable_id 
WHERE f.id =12212 
and cmt.created_at = (
         SELECT MAX(created_at) 
         FROM comments 
         WHERE commentable_id =12212 
        ) 

Answer 2

假設你的SQL Server上：您的查詢不應該工作，因爲沒有分組。

我想你需要的是採取TOP(1)訂購cmt.created_at DESC後;

SELECT TOP (1) f.name, cmt.created_at AS CommentDate, cmt.comment AS LastComment 
FROM families f 
LEFT JOIN comments cmt ON f.id = cmt.commentable_id 
WHERE f.id =12212 
ORDER BY cmt.created_at DESC 

對於MySQL我想你可以使用LIMIT代替TOP：

SELECT f.name, cmt.created_at AS CommentDate, cmt.comment AS LastComment 
FROM families f 
LEFT JOIN comments cmt ON f.id = cmt.commentable_id 
WHERE f.id =12212 
ORDER BY cmt.created_at DESC 
LIMIT 1 

Answer 3

這正是你的查詢要求：您選擇的名稱，最大評論日期則第一個被加入評論（在這種情況下，恰好是第一個）。您需要在其中放置額外的邏輯以獲得最新評論。

不同的SQL口味有這樣做的不同的方法，但下面應該到處工作：

select f.name, 
      cmt.created_at as CommentDate, 
      cmt.comment as LastComment 
from  families f 
left join comments cmt 
on  cmt.commentable_id = f.id 
left join comments latest_cmt 
on  latest_cmt.commentable_id = f.id 
and  latest_cmt.created_at > cmt.created_at 
where  f.id = 12212 
and  latest_cmt.id is null; 

我們假設評語表有一個id字段作爲主鍵。