SELECT a1, a2, (SELECT a3, a4 ,...,) exp FROM table
SELECT 123, 234, 345 , (SELECT sysdate FROM DUAL) test - sysdate FROM DUAL
SELECT 123, 234, 345 , (SELECT sysdate FROM DUAL) - sysdate FROM DUAL
SELECT 123, 234, 345 , (SELECT sysdate FROM DUAL) - sysdate test FROM DUAL