Table: olympic_womens_400m_hurdles
Columns: rank
heat
name
nationality
result

Question: What is Melaine Walker's Rank?
SQL: SELECT SUM("rank") FROM olympic_womens_400m_hurdles WHERE "name" = 'melaine walker'

Question: What is the Name of the Player with a Rank of 23 or less, Heat of 4 and Result of 55.91?
SQL: SELECT "name" FROM olympic_womens_400m_hurdles WHERE "heat" = 4 AND "rank" < 23 AND "result" = '55.91'

Question: What is the Heat of the Player with a Rank of 18 or less and Result of 55.15?
SQL: SELECT "heat" FROM olympic_womens_400m_hurdles WHERE "rank" < 18 AND "result" = '55.15'