在MySQL GROUP BY中，如何選擇第n個值 - 不是最大值或最小值，而是第2個值？

Question

具體來說，我有多個與會話關聯的點擊，並且每個點擊都有一個日期時間。對於每個會話，我希望選擇字段'first_click_datetime'，'second_click_datetime'，'third_click_datetime'等。

到目前爲止，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; 

我怎樣才能獲得第二，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)上創建一個索引，以便快速工作。