CREATE TABLE table_31771 ("Nationality" TEXT, "No [a ]" TEXT, "Pos [a ]" TEXT, "From" TEXT, "School/club" TEXT, "Pts [b ]" FLOAT, "Reb [b ]" FLOAT, "Ast [b ]" FLOAT)

Which rebound was 8 and has and ast that was less than 57?
SELECT AVG("Reb [b ]") FROM table_31771 WHERE "No [a ]" = '8' AND "Ast [b ]" < '57'
SELECT AVG("Reb [b ]") FROM "table_31771" WHERE "Ast [b ]" < '57' AND "No [a ]" = '8'