Couple of things I have found, if your source data is in a .qvd file then sample wont work. If it is an inline or autogenerated type table it will work. Also if your source is in an .xls file it will work... Finally, it doesn't return an 'exact' 10% sample... The 1st 3 examples below work, the 4th (ZIPtable) returns all records.
Table1: SAMPLE 0.50 LOAD Rand() AS Random1 AUTOGENERATE 1000000;
Table2: SAMPLE 0.90 LOAD Rand() AS Random2 AUTOGENERATE 1000000;