SQL SUM前5個值然後加入結果

Question

我想返回排名前五位的投票數最多的項目，按投票計數排序。 我想最好的辦法是將每行的votes_up和votes_down相加，然後取其中的前5個並與另一個表加入。

這裏是我現有的表，Items.ID映射到Votes.item_ID：

Items 
+----+--------+---------+ 
| ID | name | site_ID | 
+----+--------+---------+ 
| 10 | box |  111 | 
| 11 | hammer |  222 | 
| 12 | drill |  333 | 
| 13 | nail |  444 | 
+----+--------+---------+ 

Votes 
+----+---------+----------+------------+ 
| ID | item_ID | votes_up | votes_down | 
+----+---------+----------+------------+ 
| 1 |  10 |  25 |   20 | 
| 2 |  11 |  200 |  100 | 
| 3 |  12 |  100 |   50 | 
| 4 |  13 |  50 |   20 | 
+----+---------+----------+------------+ 

這些都是結果，我想回去：

+--------+-------+ 
| name | votes | 
+--------+-------+ 
| hammer | 100 | 
| drill | 50 | 
| nail | 30 | 
| box |  5 | 
+--------+-------+ 

Answer 1

SELECT Items.name, votes_up-votes_down AS votes_num 
FROM  Items, Votes 
WHERE Items.id = Votes.item_id 
ORDER BY votes_num DESC 
LIMIT 5; 

數據庫第一做JOIN然後LIMIT，否則很可能會丟失元組。如果您使用的是MySQL或PostgreSQL，您可以使用EXPLAIN SELECT ...語句來驗證這一點。如果你想要做的前5個首先，你必須手工進行：

SELECT I.name, V.votes_num AS votes_num 
FROM Items I, 
    (SELECT item_ID, votes_up-votes_down AS votes_num 
    FROM Votes ORDER BY votes_num DESC LIMIT 5) V 
WHERE I.ID = V.item_ID 
ORDER BY votes_num DESC; 

結帳這個demo;它會返回你想要的結果。

NAME VOTES_NUM 
hammer 100 
drill 50 
nail 30 
box  5 

Answer 2

SELECT i.name AS name, v.votes_up-v.votes_down AS votes 
FROM items AS i 
JOIN votes AS v ON v.item_id=i.id 
ORDER BY votes DESC 
LIMIT 5 ; 

我覺得SELECT TOP 5是MS SQL語法。