如何按不同月份从R中的数据库加载数据

Question

我正在使用 RODBC 包从 R 中的数据库加载数据并根据月度数据进行分析。

由于数据非常庞大，我不能将所有数据一起加载，所以我的想法是逐月加载数据并使用循环进行分析，以下是逻辑：

table <- sqlQuery(channel, 
               " select * from db where date between 'yyyy-mm-dd' and 'yyyy-mm-dd' ##this period is 1 month##
               "
                 , believeNRows=FALSE) 

####run function on table#####
####then load next month data and run function again#####

我的问题是如何使用循环加载下个月数据并替换上个月表，以及当月数达到12时如何更改年数。

感谢您的帮助，祝您一切顺利。

Answer 1

这可能是一种方法：

dates <- seq(as.Date("2010-01-01"), by = "month", length.out = 13)
for (i in 1:(length(dates) -1)) {
  sqlQuery(channel, sprintf("select * from db where date between %s and %s;\n", dates[i],dates[i+1]), believeNRows=FALSE)
}

注意几点： 首先，必须根据您的需要编辑日期顺序，其次；查询一直到日期的倒数第二个元素（否则你会得到一个NA（所以在未来一个月）和第三个；如果你想这样做，这不会打印你发送的查询，更改（或添加）以下命令：

cat(sprintf("select * from db where date between %s and %s;\n", dates[i],dates[i+1]))

在这种情况下，此输出（将sqlQuery 切换为cat 会产生以下结果（注意简化循环仅适用于SQL 语句，而不是完整的sqlQuery 调用）：

dates <- seq(as.Date("2010-01-01"), by = "month", length.out = 13)
for (i in 1:(length(dates) -1)) {
  cat(sprintf("select * from db where date between %s and %s;\n", dates[i],dates[i+1]))
}
#select * from db where date between 2010-01-01 and 2010-02-01;
#select * from db where date between 2010-02-01 and 2010-03-01;
#select * from db where date between 2010-03-01 and 2010-04-01;
#select * from db where date between 2010-04-01 and 2010-05-01;
#select * from db where date between 2010-05-01 and 2010-06-01;
#select * from db where date between 2010-06-01 and 2010-07-01;
#select * from db where date between 2010-07-01 and 2010-08-01;
#select * from db where date between 2010-08-01 and 2010-09-01;
#select * from db where date between 2010-09-01 and 2010-10-01;
#select * from db where date between 2010-10-01 and 2010-11-01;
#select * from db where date between 2010-11-01 and 2010-12-01;
#select * from db where date between 2010-12-01 and 2011-01-01;