在SQL查詢

Question

 RATING 
[ id - pid = rate] 
[ 1 - 1 - 5 ] 
[ 2 - 1 - 5 ] 
[ 3 - 1 - 4 ] 
[ 4 - 1 - 1 ] 
[ 5 - 1 - 1 ] 

WHERE pid = 1; 
$r5 = 10; 
$r4 = 4; 
$r3 = 0; 
$r2 = 0; 
$r1 = 2; 
$tt = 16; 

從該表中有5分的評價公式我試圖使這個數學公式

($r5*5 + $r4*4 + $r3*3 + $r2*2 + $r1)/$tt 

，而不是調用所有的值到數組，並計算它，我該怎麼辦內查詢本身？哪一個實際上更快，最有效的網站？

Answer 1

您可以採用如下方案：

SELECT pid, 
    (((SUM(CASE WHEN rate = 5 THEN 5 ELSE 0 END) * 5) + 
    (SUM(CASE WHEN rate = 4 THEN 4 ELSE 0 END) * 4) + 
    (SUM(CASE WHEN rate = 3 THEN 3 ELSE 0 END) * 3) + 
    (SUM(CASE WHEN rate = 2 THEN 2 ELSE 0 END) * 2) + 
    (SUM(CASE WHEN rate = 1 THEN 1 ELSE 0 END) * 1))/SUM(rate)) AS c_rate 
FROM table_name 
WHERE pid = 1 
GROUP BY pid 

要格式化輸出，而不四捨五入您可以使用TRUNCATE結果。如果結果應舍入和截斷，也可以使用ROUND。

使用TRUNCATE解決方案：

SELECT pid, 
    TRUNCATE((((SUM(CASE WHEN rate = 5 THEN 5 ELSE 0 END) * 5) + 
    (SUM(CASE WHEN rate = 4 THEN 4 ELSE 0 END) * 4) + 
    (SUM(CASE WHEN rate = 3 THEN 3 ELSE 0 END) * 3) + 
    (SUM(CASE WHEN rate = 2 THEN 2 ELSE 0 END) * 2) + 
    (SUM(CASE WHEN rate = 1 THEN 1 ELSE 0 END) * 1))/SUM(rate)), 1) AS c_rate 
FROM table_name 
WHERE pid = 1 
GROUP BY pid 

使用ROUND解決方案：

SELECT pid, 
    ROUND((((SUM(CASE WHEN rate = 5 THEN 5 ELSE 0 END) * 5) + 
    (SUM(CASE WHEN rate = 4 THEN 4 ELSE 0 END) * 4) + 
    (SUM(CASE WHEN rate = 3 THEN 3 ELSE 0 END) * 3) + 
    (SUM(CASE WHEN rate = 2 THEN 2 ELSE 0 END) * 2) + 
    (SUM(CASE WHEN rate = 1 THEN 1 ELSE 0 END) * 1))/SUM(rate)), 1) AS c_rate 
FROM table_name 
WHERE pid = 1 
GROUP BY pid