Catlink has:
ab_id  catagory
56     animal
56     mineral
57     animal
58     animal
58     mineral
58     vegetable

catagory has:
animal
vegetable
mineral

To get what you want you can use a LEFT OUTER JOIN as in;

SELECT Catagory.Catagory, CatLink.Ab_id ;
  FROM Catagory LEFT OUTER JOIN CatLink ;
    ON Catagory.Catagory = CatLink.Catagory ;
  ORDER BY 2 ;
 INTO CURSOR MyResult

GO TOP
LOCATE FOR ab_id = 57

* Now process the records until ab_id does not = 57.

When you sue the where clause you are eliminating the records that don't have a 57 in Ab_id including those that have the null there. Don't filter in the select, rather filter after the select.