第一次和第二次在PostgreSQL中出現行ID

Question

假設我們有一個包含日期的ID列表。我們想知道第一次和第二次ID出現的時間。大約第一次，我創建了一個查詢，這是

SELECT year, mon, COUNT(id) AS sum_first_id 
FROM (
    SELECT DISTINCT 
     ON (id) DATE, id 
    FROM TABLE 
    GROUP BY 2, 1 
    ) AS foo 
GROUP BY 2, 1 
ORDER BY 1, 2; 

我認爲這是有效的。但是我怎樣才能找到IDs第二次出現？

Answer 1

比方說，你有表table_x：

select * 
from table_x 
order by 1, 2 

id | date  
----+------------ 
    1 | 2015-06-04 
    1 | 2015-06-05 
    1 | 2015-06-14 
    2 | 2015-06-05 
    2 | 2015-06-08 
    2 | 2015-06-10 
    2 | 2015-06-17 
    2 | 2015-06-22 
(8 rows) 

要選擇N個第一元素組使用row_number()功能：

select id, date 
from (
    select id, date, row_number() over (partition by id order by date) rn 
    from table_x 
    order by 1, 2 
    ) sub 
where rn <= 2 

id | date  
----+------------ 
    1 | 2015-06-04 
    1 | 2015-06-05 
    2 | 2015-06-05 
    2 | 2015-06-08 
(4 rows) 

---

它不會出現你的查詢是正確的。

SELECT year, mon, COUNT(id) AS sum_first_id -- what is year, mon? 
FROM (
    SELECT DISTINCT 
     ON (id) DATE, id 
    FROM TABLE 
    GROUP BY 2, 1       -- should be order by 2, 1 
    ) AS foo 
GROUP BY 2, 1 
ORDER BY 1, 2;