1. Self-joins Using PROC SQL
Chapter 3 Supplement
Self-joins Using PROC SQL

2. Self-Joins
Self-joins, or Reflexive Joins are used to compare values within a data set
Examples typically use equality operations to create a join
Interesting short-cut to create choose(n,2) pairs

3. 2011 SEC Football Scores Date Winning Team Winning Score Losing Team
Losing Score
11/19/2011
Alabama 45
Georgia Southern 21
Arkansas 44
Mississippi State 17
LSU 52
Ole Miss 3
Tennessee 27
Vanderbilt
11/12/2011
South Carolina
Florida 12

4. Self-Joins
Note that USC’s winning score on November 12 matches Mississippi State’s losing score on November 19
We can record all such instances using a reflexive join
The reflexive join can look like an outer union

5. Self-Join Syntax
proc sql; select winner.wscore, winner.date, winner.wteam, loser.date, loser.lteam from secscores as winner, secscores as loser where winner.wscore=loser.lscore; quit;
Most examples make sure a record doesn’t join with itself, but that can’t happen here (no ties in football).

6. Self-Join Syntax We do not have to use equality operators: proc sql;
select winner.wscore, winner.date, winner.wteam, loser.date, loser.lteam
from secscores as winner, secscores as loser
where winner.wscore lt loser.lscore;
quit;
I have another example just for fun. End of Day 9 (50 minutes).