When using the SET statement to combine datasets, how is the data combined?

When using the SET statement in SAS to combine datasets, the data is combined vertically in the order specified in the SET statement. This means that the observations from the datasets are stacked one on top of the other, with the first dataset listed in the SET statement appearing first in the resulting dataset, followed by the second dataset, and so on. This stacking preserves the observation order as defined by the programmer, ensuring that the data from each dataset retains its original sequence when concatenated.

Additionally, all variables that exist in both datasets will be included in the combined dataset. If a variable is present in one dataset but not in the other, SAS will still include it in the resulting dataset, filling in missing values with periods for observations from the dataset lacking that variable. This behavior allows users to seamlessly merge multiple datasets while controlling the order and structure of the resulting combined dataset.