GET FILE='C:\temp\data1.sav'.
Sort cases by id.
***Match the active data1 with data2 ***.
match files file=*/in=demo/file='C:\temp\data2.sav'/in=zip/by id.
execute.
****check contribution from both files to matched file***.
****by cross-tabulation zip by demo***.
crosstabulation zip by demo.
select if (demo=1 and zip=1).
SAVE OUTFILE='C:\temp\data_merge.sav'.