在聚合查询中计算具有特定条件的行

Question

我有这个查询来获得PlayerSessions 和reconnect = TRUE 的数量，按Player.country 分组：

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

我想修改它以不仅显示重新连接的会话计数，还显示总计数，例如：

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

这可能吗？如果可以，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

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

Answer 3

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

我只是重写了查询。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，因为此查询中总会有 PlayerSession 行（除非没有会话）

Answer 4

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