How to retrieve a statistic sample from a database table

Question

Hi experts,
Let's say I have a 100.000 rows table and I want to take a representative statistical sample from that table.
Say, it should be 5% (or 5000 rows).
Question is: How to ensure that the sample is representative?
This is not guaranteed by UP TO N ROWS because the database selects without having an order by  or WHERE starting reading the first db blocks until it reaches  the limit of 5000 rows.
SELECT * UP TO 5000 rows into table .... from ZSAMPLE.
This means I miss mostly of the records in the table, hence the sample is not representative.
Do you ever encounter such a problem?

Answer 1

You could SELECT (with package size if desired) into an internal table.  Then:
Select * from table into internal table........
  package size 10000.  "not really necessary for 100000 row table
*Initialize a counter of type i.
  lv_counter = 0.
loop at the internal table.
  lv_counter = lv_counter + 1.
   if lv_counter lt 20. "1 in 20 = 5%
     delete internal table. "drop this row.
   elseif lv_counter eq 20.
     lv_counter = 0. 
endif.
endloop.
endselect.  "if you used package size.
at the end you would have kept every 1 out of every 20 records from your original table select.  Be sure to use a package size (if needed ) that is an exact multiple of 20.  To futher randomize the table, you could sort on some field that has random values, etc. before doing the every 20th record retention.