Table: world_taekwondo_championships_1991
Columns: rank
nation
gold
silver
bronze
total

Question: what is the silver when the nation is japan and the total is less than 1?
SQL: SELECT SUM("silver") FROM world_taekwondo_championships_1991 WHERE "nation" = 'japan' AND "total" < 1

Question: what is the bronze when silver is 1, gold is 0 the rank is 11 and the total is more than 1?
SQL: SELECT MAX("bronze") FROM world_taekwondo_championships_1991 WHERE "silver" = 1 AND "gold" = 0 AND "rank" = '11' AND "total" > 1

Question: what is the rank when silver is 0, bronze is more than 1 and the total is more than 3?
SQL: SELECT "rank" FROM world_taekwondo_championships_1991 WHERE "silver" = 0 AND "bronze" > 1 AND "total" > 3

Question: what is the silver for mexico?
SQL: SELECT "silver" FROM world_taekwondo_championships_1991 WHERE "nation" = 'mexico'