semi-join

I have two tables with a single key column. Keys in table a are subset of all keys in table b. I need to select keys from table b that are NOT in table a. Here is a citation from Hive manual: "LEFT SEMI JOIN implements the uncorrelated IN/EXISTS subquery semantics in an efficient way. As of Hive 0.13 the IN/NOT IN/EXISTS/NOT EXISTS operators are supported using subqueries so most of these JOINs don't have to be performed manually anymore. The restrictions of using LEFT SEMI JOIN is that the right-hand-side table should only be referenced in the join condition (ON-clause), but not in WHERE- or

How do I perform a semi-join with data.table? A semi-join is like an inner join except that it only returns the columns of X (not also those of Y), and does not repeat the rows of X to match the rows of Y. For example, the following code performs an inner join: x <- data.table(x = 1:2, y = c("a", "b")) setkey(x, x) y <- data.table(x = c(1, 1), z = 10:11) x[y] # x y z # 1: 1 a 10 # 2: 1 a 11 A semi-join would return just x[1] Matt Dowle More possibilities : w = unique(x[y,which=TRUE]) # the row numbers in x which have a match from y x[w] If there are duplicate key values in x, then that needs :

I have a votes table: votes ----------------- userid gameid ------- -------- a 1 a 2 a 3 b 1 b 2 and a games table: games ---------------- gameid title ------ ------ 1 foo 2 bar 3 fizz 4 buzz What kind of a join would I use to perform the query "Select * from games where [user A voted on the game]"? I've tried following Jeff's guide , but I'm not getting the expected results. You would use an INNER join to establish the relationship between the common gameid field; select votes.userid, games.title from games inner join votes on (votes.gameid = game.gameid) where votes.userid = 'a' This gets