How to delete multiple records

Dear all,

I execute this query on below:
     delete from PRODUCT     where exists (          select                product_id, item_id          from PRODUCT
     where               research_date < '2006-01-01'    )
 
this query deleted all records data in PRODUCT table. The subquery 
return only some records.
Tell me about other idea? and What's wrong from this query.

Best regards,

-Javanesevn