相关疑难解决方法(0)

给出两个数据框:

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)))

df1
#  CustomerId Product
#           1 Toaster
#           2 Toaster
#           3 Toaster
#           4   Radio
#           5   Radio
#           6   Radio

df2
#  CustomerId   State
#           2 Alabama
#           4 Alabama
#           6    Ohio

我怎样才能做数据库风格,即sql风格,加入？也就是说,我该怎么做:

• 一个内连接的df1和df2:
  只返回行中左表在右表匹配的密钥.
• 一个外连接的df1和df2:
  返回两个表中的所有行,从有右表中的匹配键左连接记录.
• 甲 …

如何使用data.table执行以下(直接使用sqldf)并得到完全相同的结果:

library(data.table)

whatWasMeasured <- data.table(start=as.POSIXct(seq(1, 1000, 100),
    origin="1970-01-01 00:00:00"),
    end=as.POSIXct(seq(10, 1000, 100), origin="1970-01-01 00:00:00"),
    x=1:10,
    y=letters[1:10])

measurments <- data.table(time=as.POSIXct(seq(1, 2000, 1),
    origin="1970-01-01 00:00:00"),
    temp=runif(2000, 10, 100))

## Alternative short names for data.tables
dt1 <- whatWasMeasured
dt2 <- measurments

## Straightforward with sqldf    
library(sqldf)

sqldf("select * from measurments m, whatWasMeasured wwm
where m.time between wwm.start and wwm.end")