SQL选择多行最早日期(sql select earliest date for multiple

我有一个类似如下的数据库;

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

我所试图做的是产生具有KWH读数从最早的每一个客户（结果min(reading_time)于该日），该日期将通过在网页表单的用户选择。

其结果将是/应该是类似;

Customer 1   87
Customer 2   179
Customer 3   223

还有比这里显示每天的行数更多，也有更多的客户和客户的数量将定期更换。

我没有与SQL太多的经验，我已经看过子查询等，但我没有挑起大梁弄清楚如何根据客户的最早的读数，并安排它，然后只输出kwh列。

这是在Redhat / CentOS的PostgreSQL的8.4运行。

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

作为备选：

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);

但是我希望第一个要快。

顺便说一句：你为​​什么存储日期和时间在两个单独的列？ 你知道，这可能处理具有更好的timestamp列？

Answer 2:

这应该是最快的可能的解决方案中：

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

似乎是另一个应用程序：

• 选择由组中每组第一排？

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);

参数在上面的查询的reading_date值。

文章来源: sql select earliest date for multiple rows