MySQL 查詢各科前三的數據

 

 

 

 方法一：

SELECT sc.*,c.`
Cname` ,COUNT(sc.C)
FROM sc
LEFT JOIN sc a
ON sc.C = a.C AND sc.score >= a.score
LEFT JOIN course c
ON sc.C = c.C
WHERE sc.C = c.C
GROUP BY
sc.C,sc.S,sc.score
HAVING COUNT(sc.C)>=4
ORDER BY a.C,a.score DESC

解析：

先copy 一個sc 表中的數據，與原sc 進行對比。統計每個score 大於其他數據的計數。再按sc.C,sc.S進行分組

SELECT sc.* ,COUNT(sc.C) 
FROM sc  
LEFT JOIN sc a
ON sc.C = a.C AND sc.score >= a.score
GROUP BY sc.C,sc.S

 

 可以看到，第一行數據的count計數為 6，由於我們要取每科前三名，sc表中共有student S編號7個，

自身不納入count進行計數，則為6個需要取出前3--計數大於3的值。

SELECT sc.* ,COUNT(sc.C) 
FROM sc  
LEFT JOIN sc a
ON sc.C = a.C AND sc.score >= a.score
GROUP BY
sc.C,sc.S,sc.score
HAVING COUNT(sc.C)>3
ORDER BY a.C,a.score DESC

方法二：

SELECT a.* FROM
sc a
WHERE
(SELECT COUNT(*)
FROM sc
WHERE sc.C =a.C
AND a.score<sc.score) <3
ORDER BY a.C,a.score DESC

解析：

與方法一同理，復制一個表進行count計數