Хм. Хотите новую строку? В этом случае
IF NOT EXISTS(SELECT 1 FROM emp WHERE fruits = 'mango')
INSERT INTO emp (fruits) VALUES ('mango')
Два способа сделать
1.IF NOT EXISTS (SELECT fruit FROM emp WHERE fruit='mango')
BEGIN
INSERT INTO emp(fruit) Values('mango');
END
2.INSERT INTO emp ('mango') SELECT distinct fruit FROM emp WHERE not exists (select fruit from emp as e Where emp.fruit = e.fruit);