1. Putting tables together
Joins in SQL

2. In regular SAS -- MERGE
proc sort data=left ; by key ; run ; proc sort data=right ; by key ; run ; data both ; merge left (in=inleft) right (in=inright) ; by key ; < if … inleft, inright … then action > run ;

3. Three types of ‘Outer Joins’
Occasionally
Key is unique in both tables (keys/are)
Key is only variable in common (keys/are)
 Then do ‘natural join’
More typically
One table (left) is bigger than the other
Key is unique in smaller (right) table
 Then do ‘left join’

4. Outer Joins Occasionally Key may not be unique
Mismatches do not trigger corrective action
Then do ‘full join’
Natural, left, and full joins are ‘outer joins’
Tables do not need to be sorted

5. Inner Joins Can join multiple tables at once
Subsetted by WHERE conditions
Naturally constructs Cartesian product of all entries of one table with all of another table (and with any others…)
*** Cartesian products can get very big, very fast, so if WHERE conditions not correct, the resulting query can be astronomically large***