例如，在 MySQL GROUP BY 中，如何选择第 n 个值——不是最大值或最小值，而是第二个？

Question

具体来说，我有多个与会话关联的点击，并且每次点击都有一个对应的日期时间。对于每个会话，我想选择“first_click_datetime”、“second_click_datetime”、“third_click_datetime”等字段。

到目前为止，我有这样的事情：

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;

如何获取第 2 次、第 3 次、第 4 次等 click_datetime？

谢谢！

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) 上创建一个索引以使其快速工作。