Question

我有这个查询来获取PlayerSession的{{1}}个数，按reconnect = TRUE分组：

Player.country

我想修改它，不仅要显示重新连接的会话数，还要显示总数，例如：

SELECT
    country,
    COUNT(*) AS with_reconnect
FROM PlayerSession S LEFT JOIN Player P ON (P.id = S.player_id)
WHERE reconnect = TRUE
GROUP BY country

这是可能的，如果是这样，那么正确的语法是什么？

Answer 1

SELECT  Country,
        COUNT(*) AS Total,
        COUNT(CASE WHEN Reconnect = true THEN 1 END) AS With_Reconnect
FROM    PlayerSession S 
        LEFT JOIN Player P 
            ON P.id = S.player_id
GROUP BY country

Answer 2

以下就足够了

SELECT
    p.country,
    COUNT(*) AS total,
    SUM(IF(s.reconnect=TRUE,1,0)) AS with_reconnect
FROM PlayerSession s

INNER JOIN Player p
ON p.id = s.player_id

GROUP BY p.country

我刚刚重写了查询。每个PlayerSession都会有一个Player行，因此将其更改为INNER JOIN。此外，不需要CONCAT，因为此查询中始终存在PlayerSession行（除非没有会话）

Answer 3

SELECT
    country,
    COUNT(CASE WHEN reconnect = TRUE THEN S.player_id ELSE NULL END) AS with_reconnect,
    COUNY(*)
FROM PlayerSession S LEFT JOIN Player P ON (P.id = S.player_id) 
GROUP BY country

Answer 4

SELECT
    country,
    COUNT(*) AS total,
    sum(case when reconnect = TRUE then 1 else 0 end)  AS with_reconnect
FROM PlayerSession S LEFT JOIN Player P ON (P.id = S.player_id)
GROUP BY country

在聚合查询中计算具有特定条件的行

4 个答案: