CREATE TABLE table_15988 ("Class" CLOB, "Operator" CLOB, "No. Built" FLOAT, "Year Built" CLOB, "Cars per Set" FLOAT, "Unit nos." CLOB)

What year was number 7 built?
SELECT "Year Built" FROM table_15988 WHERE "No. Built" = '7'
SELECT "Year Built" FROM "table_15988" WHERE "No. Built" = '7'