Handbook of SAS DATA Step Programming

115Combining Data Sets

During the compilation phase, SAS reads the descriptor information

of the input data sets listed in the MERGE statement. SAS then creates

the PDV that contains all the variables from all the data sets along with

new variables created in the DATA step. In addition, SAS creates FIRST.

VARIABLE and LAST.VARIABLE for each variable listed in the BY

statement in the PDV.

During the execution phase, SAS examines the rst BY group in each input

data set listed in the MERGE statement to determine which BY group should

appear rst in the output data set. Then the DATA step starts to read obser-

vations from the rst BY group from each data set. When reading the rst

BY group, the DATA step reads the rst observations into the P ...