MySQL 8.0

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CREATE tABLe Courses (cid int,name varchar(50))
INSERT INTO Courses VALUEs (1,'a'),(2,'ab')
CREATE TABLe Students (sid int,last_name varchar(50),birthday date,major varchar(50))
INSERT INTO Students VALUEs (1,'testa','2020-01-01','testC'),(2,'testb','2020-02-01','testD')
CREATE TABLe Students_Courses (sid int,cid int)
INSERT INTO Students_Courses VALUES(1,1),(2,2)
    CREATE FUNCTION fHW_4_sampsoth (courseName varchar(30))
    RETURNS varchar(170)
    BEGIN
       DECLARE result varchar(170);
       
    if IFNULL(courseName,'') = '' then 
        set result = 'Please input a valid name';
    elseif 
       (select name from Courses where name like concat('%',courseName,'%') LIMIT 1) is null 
         then set result = concat('no course found for name: ', courseName);
    else 
       select group_concat(concat(last_name,':',TIMESTAMPDIFF(YEAR, birthday, CURDATE()),':',major)separator ',') into result from Students
       where sid in (select sid from Students_Courses where cid IN (select cid from Courses where name like concat('%',courseName,'%')) group by sid);
    END IF;       
    RETURN result;
    END 
SELECT fHW_4_sampsoth('a');


fHW_4_sampsoth('a')


testa:1:testC,testb:1:testD


    CREATE FUNCTION fHW_4_sampsoth1 (courseName varchar(30))
    RETURNS varchar(170)
    BEGIN
       DECLARE result varchar(170);
       
    if IFNULL(courseName,'') = '' then 
        set result = 'Please input a valid name';
    elseif 
       (NOT EXISTS(select 1 from Courses where name like concat('%',courseName,'%'))) 
         then set result = concat('no course found for name: ', courseName);
    else 
       select group_concat(concat(last_name,':',TIMESTAMPDIFF(YEAR, birthday, CURDATE()),':',major)separator ',') into result from Students
       where sid in (select sid from Students_Courses where cid IN (select cid from Courses where name like concat('%',courseName,'%')) group by sid);
    END IF;       
    RETURN result;
    END 
SELECT fHW_4_sampsoth1('a');


fHW_4_sampsoth1('a')


testa:1:testC,testb:1:testD


SELECT fHW_4_sampsoth1('c');


fHW_4_sampsoth1('c')


no course found for name: c