在MySQL GROUP BY中，如何選擇第n個值（例如，不是max或min，而是第2個）？

Question

具體來說，我有多個與會話相關的點擊，並且每個點擊都有一個對應的日期時間。 對於每個會話，我想選擇“ first_click_datetime”，“ second_click_datetime”，“ third_click_datetime”等字段作為字段。

到目前為止，我有這樣的事情：

SELECT session_id, 
min(click_datetime) as first_click_datetime,
CASE
  when total_number_of_clicks = 2 then max(click_datetime)
/* when total_number_of_clicks >2 then ???? */
  else null
END as second_click_datetime
FROM table1 GROUP BY 1;

如何獲得第二，第三，第四等click_datetime？

謝謝！

Answer 1

SELECT  session_id,
        (
        SELECT  click_datetime
        FROM    table1 t1
        WHERE   t1.session_id = td.session_id
        ORDER BY
                session_id DESC, click_datetime DESC, id DESC
        LIMIT   1
        ) AS first_click,
        (
        SELECT  click_datetime
        FROM    table1 t1
        WHERE   t1.session_id = td.session_id
        ORDER BY
                session_id DESC, click_datetime DESC, id DESC
        LIMIT   1, 1
        ) AS second_click,

        (
        SELECT  click_datetime
        FROM    table1 t1
        WHERE   t1.session_id = td.session_id
        ORDER BY
                session_id DESC, click_datetime DESC, id DESC
        LIMIT   2, 1
        ) AS third_click
FROM    (
        SELECT  DISTINCT session_id
        FROM    table1
        ) td

在(session_id, click_datetime, id)上創建索引(session_id, click_datetime, id)以使其快速運行。