Putting tables together Joins in SQL

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 ;

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’

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

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***