The half-keyed JOIN is a simpler version, wherein the INDEX is the right-hand recordset in the JOIN. Just as with the full-keyed JOIN, the join condition must use the key fields in the INDEX to do its work. The purpose of the half-keyed JOIN is the same as the full-keyed version.

In fact, a full-keyed JOIN is, behind the curtains, actually the same as a half-keyed JOIN then a FETCH to retrieve the base dataset records. Therefore, a half-keyed JOIN and a FETCH are semantically and functionally equivalent, as shown in this example code (contained in the IndexHalfKeyedJoin.ECL file):

IMPORT $;

r1 := RECORD
  $.DeclareData.Layout_Person;
  $.DeclareData.Layout_Accounts;
END;
r2 := RECORD
  $.DeclareData.Layout_Person;
  UNSIGNED8 AcctRecPos;
END;

r2 Xform2($.DeclareData.Person.FilePlus L, 
          $.DeclareData.IDX_Accounts_PersonID R) := TRANSFORM
  SELF.AcctRecPos := R.RecPos;
  SELF := L;
END;

J2 := JOIN($.DeclareData.Person.FilePlus(PersonID BETWEEN 1 AND 100),
           $.DeclareData.IDX_Accounts_PersonID,
           LEFT.PersonID=RIGHT.PersonID,
           Xform2(LEFT,RIGHT));		

r1 Xform3($.DeclareData.Accounts L, r2 R) := TRANSFORM
  SELF := L;
  SELF := R;
END;
F1 := FETCH($.DeclareData.Accounts,
            J2,  
            RIGHT.AcctRecPos,
            Xform3(LEFT,RIGHT));

OUTPUT(F1,ALL);

This code produces the same result set as the previous example.

The advantage of using half-keyed JOINs over the full-keyed version comes in where you may need to do several JOINs to fully perform whatever process is being run. Using the half-keyed form allows you to accomplish all the necessary JOINs before you explicitly do the FETCH to retrieve the final result records, thereby making the code more efficient.