@cmd{ADD FILES}, @cmd{MATCH FILES}, and @cmd{UPDATE} commands. The
following sections describe details specific to each command.
-Each of these commands reads two or more input files and combines
-them. The command's output becomes the new active dataset. The input
-files are not changed on disk.
+Each of these commands reads two or more input files and combines them.
+The command's output becomes the new active dataset.
+None of the commands actually change the input files.
+Therefore, if you want the changes to become permanent, you must explicitly
+save them using an appropriate procedure or transformation (@pxref{System and Portable File IO}).
The syntax of each command begins with a specification of the files to
be read as input. For each input file, specify FILE with a system
system-missing value for numeric variables or spaces for string
variables.
+These commands may combine any number of files, limited only by the
+machine's memory.
+
@node ADD FILES
@section ADD FILES
@vindex ADD FILES
@itemize @bullet
@item
When a match is found, then the values of the variables present in the
-transaction file replace those variable's values in the new active
+transaction file replace those variables' values in the new active
file. If there are matching cases in more than more transaction file,
@pspp{} applies the replacements from the first transaction file, then
from the second transaction file, and so on. Similarly, if a single
transaction file has cases with duplicate @subcmd{BY} values, then those are
applied in order to the master file.
-When a variable in a transaction file has a missing value or a string
+When a variable in a transaction file has a missing value or when a string
variable's value is all blanks, that value is never used to update the
master file.