sql為多行選擇最早的日期

Question

我有一個如下所示的數據庫;

circuit_uid   |  customer_name   | location      | reading_date | reading_time | amps | volts  |  kw  | kwh | kva  |  pf  |  key
--------------------------------------------------------------------------------------------------------------------------------------
cu1.cb1.r1    | Customer 1       | 12.01.a1      | 2012-01-02   | 00:01:01     | 4.51 | 229.32 | 1.03 |  87 | 1.03 | 0.85 |    15
cu1.cb1.r1    | Customer 1       | 12.01.a1      | 2012-01-02   | 01:01:01     | 4.18 | 230.3 | 0.96 |  90 | 0.96 | 0.84 |    16
cu1.cb1.s2    | Customer 2       | 10.01.a1      | 2012-01-02   | 00:01:01     | 7.34 | 228.14 | 1.67 | 179 | 1.67 | 0.88 | 24009
cu1.cb1.s2    | Customer 2       | 10.01.a1      | 2012-01-02   | 01:01:01     | 9.07 |  228.4 | 2.07 | 182 | 2.07 | 0.85 | 24010
cu1.cb1.r1    | Customer 3       | 01.01.a1      | 2012-01-02   | 00:01:01     | 7.32 | 229.01 | 1.68 | 223 | 1.68 | 0.89 | 48003 
cu1.cb1.r1    | Customer 3       | 01.01.a1      | 2012-01-02   | 01:01:01     | 6.61 | 228.29 | 1.51 | 226 | 1.51 | 0.88 | 48004

我想要做的是產生一個結果，該結果具有從該日起最早（ min(reading_time) ）的每個客戶的KWH讀數，該日期將由用戶在Web表單中選擇。

結果將是/應該類似於;

Customer 1   87
Customer 2   179
Customer 3   223

這里顯示的每天行數超過了客戶數量，客戶數量也會增加，客戶數量也會定期變化。

我沒有太多的SQL經驗，我已經查看了子查詢等但我沒有找到如何根據每個客戶的最早讀數排列它然后只輸出kwh列。

這是在Redhat / CentOS上的PostgreSQL 8.4中運行的。

Answer 1

select customer_name,
       kwh,
       reading_date, 
       reading_time
from (
   select customer_name,
          kwh,
          reading_time,
          reading_date,
          row_number() over (partition by customer_name order by reading_time) as rn
   from readings
   where reading_date = date '2012-11-17'
) t
where rn = 1

作為備選：

select r1.customer_name,
       r1.kwh, 
       r1.reading_date,
       r1.reading_time
from readings r1
where reading_date = date '2012-11-17'
and reading_time = (select min(r2.reading_time)
                    from readings
                    where r2.customer_name = r1.customer_name
                    and r2.read_date = r1.reading_date);

但我希望第一個更快。

順便說一句：為什么要將日期和時間存儲在兩個單獨的列中？ 您是否意識到使用timestamp列可以更好地處理這個問題？

Answer 2

這應該是最快的解決方案之一：

SELECT DISTINCT ON (customer_name)
       customer_name, kwh  -- add more columns as needed.
FROM   readings
WHERE  reading_date = user_date
ORDER  BY customer_name, reading_time

似乎是另一個應用：

• 選擇每個GROUP BY組中的第一行？

Answer 3

   SELECT rt.circuit_uid ,  rt.customer_name, rt.kwh
   FROM READING_TABLE rt JOIN  
       (SELECT circuit_uid, reading_time
       FROM READING_TABLE
       WHERE reading_date = '2012-01-02'
       GROUP BY customer_uid
       HAVING MIN(reading_time) = reading_time) min_time
   ON (rt.circuit_uid = min_time.circuit_uid 
      AND rt.reading_time = min_time.reading_time);

參數化上面查詢中的reading_date值。