sql为多个行选择最早的日期 [英] sql select earliest date for multiple rows
问题描述
我有一个如下数据库:
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
我要做的是产生一个结果,该结果具有从最早的日期( min(reading_time)
),该日期将由用户通过网络表单选择。
What I am trying to do is produce a result that has the KWH reading for each customer from the earliest (min(reading_time)
) on that date, the date will be selected by the user in a web form.
结果应该是/应该类似于;
The result would be/should be similar to;
Customer 1 87
Customer 2 179
Customer 3 223
此处显示的行数超过了每天的行数,并且有更多的客户,并且客户数量会定期更改。
There are more than the number of rows per day shown here and there are more customers and the number of customers would change regularly.
我对SQL没有太多的经验,我研究过子查询等,但是我没有内容来弄清楚如何按照每个客户的最早阅读来排列它,然后输出 kwh
列。
I do not have much experience with SQL, I have looked at subqueries etc. but I do not have the chops to figure out how arrange it by the earliest reading per customer and then just output the kwh
column.
这在Redhat / CentOS的PostgreSQL 8.4中运行。
This is running in PostgreSQL 8.4 on Redhat/CentOS.
推荐答案
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
或者:
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);
但是我希望第一个更快。
But I'd expect the first one to be faster.
顺便说一句:为什么将日期和时间存储在两个单独的列中?您是否知道使用 timestamp
列可以更好地处理此问题?
Btw: why do you store date and time in two separate columns? Are you aware that this could be handled better with a timestamp
column?
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