Challenge. Split database 500 records in two smaller datasets, randomly
Easiest solution would be to store the datasets into tables.
After you use your query to get the first dataset:

 INSERT INTO dataset1 SELECT * FROM foo ORDER BY rand() LIMIT 125


then do:

 INSERT INTO dataset2 SELECT foo.* FROM foo 
   LEFT JOIN dataset1 d1 ON foo.key=d1.key WHERE d1.key IS NULL