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

how many miles long was the dingwall canal ?
SELECT "length (miles)" FROM table_203_594 WHERE "canal" = 'dingwall canal'
SELECT "length (miles)" FROM "table_203_594" WHERE "canal" = 'dingwall canal'