给定数据框
df1 <- data.frame(CustomerId=c(1:6),Product=c(rep("Toaster",3),rep("Radio",3)))
df2 <- data.frame(CustomerId=c(2,4,6),State=c(rep("Alabama",2),rep("Ohio",1)))
存储在列表中
dflist <- c(df1,df2)
如何在这些数据帧上运行 sqldf 查询(连接)?
失败的尝试:
test <- sqldf("select a.CustomerId, a.Product, b.State from dflist[1] a
inner join dflist[2] b on b.id = a.id")
test <- sqldf("select a.CustomerId, a.Product, b.State from dflist$df1 a
inner join dflist$df2 b on b.CustomerId = a.CustomerId")
最佳答案
如果将列表中的 data.frames 复制到新环境,则可以使用 sqldf
的 envir
参数或通过命名列表的元素,并使用 with
。
请注意以下几点:
- 我使用
list
而不是c
创建dflist
。
注意区别
str(c(df1,df2))
##List of 4
## $ CustomerId: int [1:6] 1 2 3 4 5 6
## $ Product : Factor w/ 2 levels "Radio","Toaster": 2 2 2 1 1 1
## $ CustomerId: num [1:3] 2 4 6
## $ State : Factor w/ 2 levels "Alabama","Ohio": 1 1 2
str(list(df1,df2))
##List of 2
## $ :'data.frame': 6 obs. of 2 variables:
## ..$ CustomerId: int [1:6] 1 2 3 4 5 6
## ..$ Product : Factor w/ 2 levels "Radio","Toaster": 2 2 2 1 1 1
## $ :'data.frame': 3 obs. of 2 variables:
## ..$ CustomerId: num [1:3] 2 4 6
## ..$ State : Factor w/ 2 levels "Alabama","Ohio": 1 1 2
- 我已经调整了 sql 查询以反射(reflect) data.frames 中的名称(按照您的第二种方法)
命名数据
dflist <- list(df1,df2)
names(dflist) <- c('df1','df2')
创建新的工作环境
# create a new environment
e <- new.env()
# assign the elements of dflist to this new environment
for(.x in names(dflist)){
assign(value = dflist[[.x]], x=.x, envir = e)
}
# this could also be done using mapply / lapply
# eg
# invisible(mapply(assign, value = dflist, x = names(dflist), MoreArgs =list(envir = e)))
# run the sql query
sqldf("select a.CustomerId, a.Product, b.State from df1 a
inner join df2 b on b.CustomerId = a.CustomerId", envir = e)
## CustomerId Product State
## 1 2 Toaster Alabama
## 2 4 Radio Alabama
## 3 6 Radio Ohio
使用 with
的更简单方法
您可以简单地使用 with
在本地进行计算(重要的是 dflist 是一个命名列表)
# this is far simpler!!
with(dflist,sqldf("select a.CustomerId, a.Product, b.State from df1 a
inner join df2 b on b.CustomerId = a.CustomerId"))
使用proto
的另一种简单方法
- 感谢@G.Grothendieck(请参阅评论
这使用了 proto
包,该包由 sqldf
加载
dflist <- list(a = df1, b = df2)
sqldf( "select a.CustomerId, a.Product, b.State from df1 a
inner join df2 b on b.CustomerId = a.CustomerId",
envir = as.proto(dflist))
使用数据表
或者您可以使用data.table
,它提供类似sql
的方法(请参阅FAQ 2.16)
library(data.table)
dflist <- list(data.table(df1),data.table(df2))
names(dflist) <- c('df1','df2')
invisible(lapply(dflist, setkeyv, 'CustomerId'))
with(dflist, df1[df2])
## CustomerId Product State
## 1: 2 Toaster Alabama
## 2: 4 Radio Alabama
## 3: 6 Radio Ohio
关于r - SQL 查询列表内的数据框,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/12346027/