The preceding query was prepared by Mr. R.T.R.N. Reindeer, whose nose should now be even redder because he forgot to include any provision for attaching name and address data to the orders!
This could make for a very confusing evening for his boss, to say the least.
Your analysis shows that it would be more economical to handle this as follows: each site should send its order and name/address data to the North Pole separately, with the actual join being done there, since each child typically gets several orders filled.
To economize, it is decided to only send to the North Pole the name/address data for children who qualify for orders - i.e. if a given site serves 50,000 children but only 45,000 have been sufficiently good, then only these 45,000 name/address tuples should be sent north.
What SINGLE relational algebra operation should be used at each local site to extract the relevant tuples from the complete Child table?
Assume the result of the orders query is called QualifyingOrder.