show data that exists in both MYSQL tables and/or R objects

Question

Hi guys, i think its an regular problem, answered several times, but I just don't know how to ask the question right =(

in MySQL:
i have 2 tables with some kind of strings inside, now i want:
1. the data that occur in both tables
2. the data from a that is not in table b

same in R:
i have 2 R data.frame s and i want:
1. the data that occur in a and b
2. the data that occur in a but not in b

Answer 1

in mysql you could do this to get the data in both tables

SELECT * FROM table1 t1 INNER JOIN table2 t2 ON t1.id = t2.id

and do this to get the tada from a that is not in table b

SELECT * FROM table1 t1 LEFT JOIN table2 t2 ON t1.id = t2.id
WHERE t2.id IS NULL

Answer 2

In R:

a <- data.frame(V1=sample(letters[1:3],20,TRUE),V2=rnorm(20))
b <- data.frame(V1=sample(letters[2:4],20,TRUE),V2=rnorm(20))

# the data that occur in a and b
(ab <- merge(a,b,by="V1"))

# the data that occur in a but not in b 
aNOTb <- merge(a,b,by="V1",all=TRUE)
(aNOTb <- aNOTb[is.na(aNOTb$V2.y),])