CREATE TABLE table_203_594 (id NUMBER, "canal" CLOB, "length  ( miles ) " NUMBER, "locks" NUMBER, "max length  ( ft ) " NUMBER, "width  ( ft ) " NUMBER, "year opened" NUMBER, "year abandoned" NUMBER, "year restored" NUMBER)

crinan canal is only half of what other canal
SELECT "canal" FROM table_203_594 WHERE "length (miles)" = (SELECT "length (miles)" FROM table_203_594 WHERE "canal" = 'crinan canal') / 0.5
SELECT "canal" FROM "table_203_594" WHERE "length (miles)" = (SELECT "length (miles)" FROM "table_203_594" WHERE "canal" = 'crinan canal') / NULLIF(0.5, 0)