Question

我需要在2012年12月到2014年11月之间获取数据。

每个月我只需要1500行。

例如：

SELECT * FROM data WHERE YEAR(submit_date) = 2012 AND MONTH(submit_date) = 12 limit 1500;
SELECT * FROM data WHERE YEAR(submit_date) = 2013 AND MONTH(submit_date) = 1 limit 1500;
SELECT * FROM data WHERE YEAR(submit_date) = 2013 AND MONTH(submit_date) = 2 limit 1500;
SELECT * FROM data WHERE YEAR(submit_date) = 2013 AND MONTH(submit_date) = 3 limit 1500;
and until Nov 2014

有没有办法将SQL查询编写得更小？

Answer 1

此处有一些选项列表：http://www.xaprb.com/blog/2006/12/07/how-to-select-the-firstleastmax-row-per-group-in-sql/

恕我直言最好的一个是使用行计数器：

set @num := 0, @type := '';

select id, name, submit_date,
  @num := if(@type = CONCAT(YEAR(submit_date), MONTH(submit_date)), @num + 1, 1) as row_number,
  @type := CONCAT(YEAR(submit_date), MONTH(submit_date)) as dummy
from data force index(IX_submit_date)
group by id, name, submit_date
having row_number <= 2;

你可以在这里测试一下：http://sqlfiddle.com/#!2/e829c/13（我为2个元素剪切，而不是1500个）

Answer 2

我认为你正在寻找一个GROUP BY条款。我需要知道更多，以给你一个明确的答案。但是下面的pseduo查询可能会指导您朝着正确的方向发展。

SELECT *, SUM(some_field) 
FROM data
GROUP BY MONTH(submit_date)

或者，如果您只需要1500行，请选择按日期排序的前1500名

SELECT TOP(1500) *
FROM data
WHERE submit_date > '12-01-2012' AND submit_date < '11-01-2014'
ORDER BY MONTH(submit_date)

使用MySQL，您可以使用LIMIT

SELECT *
FROM data
WHERE submit_date > '12-01-2012' AND submit_date < '11-01-2014'
ORDER BY MONTH(submit_date)
LIMIT 0,1500;

Answer 3

你几乎可以像拥有它一样，只需在你的查询之间添加一个UNION。但是你仍然需要每月创建1个查询。

否则，您需要枚举返回的行。您需要首先订购并枚举您的记录，然后您可以选择该选项以仅获得前X.不确定您是否要包括上个月。

SET @prev_date='';
SELECT * FROM (
SELECT IF(@prev_date=submit_date, @incr := @incr+1, @incr:=1) AS row_num, 
data.*, 
(@prev_date := submit_date) AS set_prev_date 
FROM data WHERE submit_date BETWEEN "2012-12-01" AND "2014-11-30" 
ORDER BY submit_date
) tmp WHERE row_num<1500;

限制每个月的行数？

3 个答案: