Question

我对以下查询有点问题：

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

我正在尝试将查询显示为以下内容：

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

然而，在我当前的查询中，它显示了这个：

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

如果我将WHERE e.og_id = 1更改为AND e.go_id = 1

结果将如下：

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

此结果与没有此行相同：

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，请告诉我。

感谢您的帮助。

Answer 1

将子句e.og_id = 1移动到JOIN应该可以解决问题：

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，您可以明确允许NULL：

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

WHERE将过滤掉左连接创建的空值。将where移动到连接条件：

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，将过滤器移回where子句，处理LEFT JOIN返回NULL的情况。

    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)

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