Per input file:

Once per command:
        /BY var_list[({D|A}] [var_list[({D|A})]…]

MATCH FILES merges sets of corresponding cases in multiple input files into single cases in the output, combining their data.

MATCH FILES shares the bulk of its syntax with other PSPP commands for combining multiple data files. See Common Syntax, above, for an explanation of this common syntax.

How MATCH FILES matches up cases from the input files depends on whether BY is specified:

When MATCH FILES creates an output case, variables that are only in files that are not present for the current case are set to the system-missing value for numeric variables or spaces for string variables.