Why mysql is giving error “Not allowed to return a result set from a function”?

Question

I am trying to create a MySQL function using phpMyAdmin and getting this error.

#1415 - Not allowed to return a result set from a function

Answer 1

Because

SELECT left_id AS c1 FROM mlm_user_mst WHERE parent_id=a AND left_id>0; 

doesn't set the variable c1, it returns a set with a column named c1

You want

SELECT left_id INTO c1 FROM mlm_user_mst WHERE parent_id=a AND left_id>0; 

Similarly for c2.