Table: fc_barcelona_players_0809
Columns: nat
name
moving_to
type
transfer_window
transfer_fee
source

Question: WHAT IS THE SOURCE WITH A TYPE OF TRANSFER AND MEX?
SQL: SELECT "source" FROM fc_barcelona_players_0809 WHERE "type" = 'transfer' AND "nat" = 'mex'

Question: WHAT IS THE NAME WITH BAYER LEVERKUSEN?
SQL: SELECT "name" FROM fc_barcelona_players_0809 WHERE "moving_to" = 'bayer leverkusen'

Question: WHAT NAME HAS A FREE TRANSFER FREE AND RETIRED?
SQL: SELECT "name" FROM fc_barcelona_players_0809 WHERE "transfer_fee" = 'free' AND "moving_to" = 'retired'

Question: WHAT TRANSFER FEE HAS A TYPE OF TRANSFER FOR DOS SANTOS?
SQL: SELECT "transfer_fee" FROM fc_barcelona_players_0809 WHERE "type" = 'transfer' AND "name" = 'dos santos'