#joins in R
data(mtcars)
mtcars$Model=rownames(mtcars)
mtcars$Brand = sapply(strsplit(mtcars$Model," "),FUN='[',n=1)
mtcars$Model = NULL
hp_mean = aggregate(
                    mtcars$hp,
                    by=list(mtcars$Brand),
                    FUN=mean
)

car_brand = data.frame(
                    Brand =c('Mazda','Toyota','Fiat','Volvo','Tata'),
                    country=c('Japan','Japan','Itely','Sweden','India')
)


# to join or merge we use merge() function
merge(x=mtcars,y=car_brand,by="Brand",all.x = FALSE, all.y = TRUE)

#for inner join 
#allx = FALSE, all.y = FALSE
#for left join
#allx = TRUE, all.y = FALSE
#for right join
#allx = FALSE, all.y = TRUE
#for outer join
#allx = TRUE, all.y = TRUE