CREATE TABLE table_name_23 (partner VARCHAR, surface VARCHAR, date VARCHAR)

What is the name of a partner that played on a hard surface on 18 April 2011?
SELECT partner FROM table_name_23 WHERE surface = "hard" AND date = "18 april 2011"
SELECT "partner" FROM "table_name_23" WHERE "18 april 2011" = "date" AND "hard" = "surface"