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Question

我有一個小問題，用下面的查詢：

SELECT 
    country.country_id AS "COUNTRY_ID", count(con.medal_id) AS "GOLD" 
FROM 
    country country 
LEFT OUTER JOIN 
    contestant con on con.country_id = country.country_id 
AND con.medal_id = 1 
LEFT OUTER JOIN 
    event e on e.event_id = con.event_id 
WHERE e.og_id = 1 
GROUP BY 
    country.country_id 
ORDER BY 
    country.country_id 

我試圖讓我的查詢，顯示如下所示：

COUNTRY_ID  GOLD 
---------- ---------- 
     1   1 
     2   2 
     3   0 
     4   0 

然而，我目前的查詢，其顯示的：

COUNTRY_ID  GOLD 
---------- ---------- 
     1   1 
     2   2 

如果我改變WHERE e.og_id = 1成AND e.go_id = 1

個

結果將是以下：

COUNTRY_ID  GOLD 
---------- ---------- 
     1   1 
     2   5 
     3   0 
     4   3 

這個結果等於沒有這條線：

LEFT OUTER JOIN 
    event e on e.event_id = con.event_id 
AND e.og_id = 1 

如果有人需要更多信息，請讓我知道。

謝謝你的幫助。

Answer 1

移動條款e.og_id = 1到JOIN應該做的伎倆：

SELECT 
    country.country_id AS "COUNTRY_ID", count(con.medal_id) AS "GOLD" 
FROM 
    country country 
LEFT OUTER JOIN contestant con 
    on con.country_id = country.country_id AND con.medal_id = 1 
LEFT OUTER JOIN event e 
    on e.event_id = con.event_id AND e.og_id = 1 
GROUP BY 
    country.country_id 
ORDER BY 
    country.country_id 

或者，您可以明確允許空值：

SELECT 
    country.country_id AS "COUNTRY_ID", count(con.medal_id) AS "GOLD" 
FROM 
    country country 
LEFT OUTER JOIN contestant con 
    on con.country_id = country.country_id AND con.medal_id = 1 
LEFT OUTER JOIN event e 
    on e.event_id = con.event_id 
WHERE e.og_id = 1 or e.og_id IS NULL 
GROUP BY 
    country.country_id 
ORDER BY 
    country.country_id 

Answer 2

有WHERE會過濾掉創建空值由左連接。移動where到加盟條件：

SELECT 
    country.country_id AS "COUNTRY_ID", count(con.medal_id) AS "GOLD" 
FROM 
    country country 
     LEFT OUTER JOIN contestant con 
     on con.country_id = country.country_id 
     AND con.medal_id = 1 
    LEFT OUTER JOIN 
     event e on e.event_id = con.event_id 
     AND e.og_id = 1 
GROUP BY 
    country.country_id 
ORDER BY 
    country.country_id; 

或者，移動過濾器放回where子句，處理在LEFT JOIN返回NULL值的情況。

country country 
     LEFT OUTER JOIN contestant con 
     on con.country_id = country.country_id 
    LEFT OUTER JOIN 
     event e on e.event_id = con.event_id 
    WHERE 
     (con.country_id IS NULL OR con.medal_id = 1) 
     AND 
     (e.event_id IS NULL OR e.og_id = 1) 

更多關於這behaviour here