>Change one of "Val 3" by another value... and it will not be present in your crsTest.

No it will not, It just compares Key fields. It is easy when you work with tables with just two fields to include them both in join condition, but I doubt you have such case.

lcSql  = [SELECT ]
lcJoin = []
FOR lnFor = 1 TO FCOUNT([MasterTable])
    lcFld   = FIELD(lnFor,[MasterTable])
    lcSql   = lcSql   + IIF(lnFor = 1, [],[,]) + [NVL(MasterTable.]+lcFld+[, TempTable.]+lcFld+[) AS lcFld]
    lcJoin  = lcJoin  + IIF(lnFor = 1, [],[ AND ]) + [MasterTable.]+lcFld+[ == TempTable.]+lcFld
NEXT
lcSql   = lcSql + [ FROM MasterTable ]+;
                  [FULL JOIN TempTable ON ]+lcJoin+;
                  [ WHERE MasterTable.KeyFld IS NULL OR TempTable.KeyFld IS NULL]+;
                  [ INTO CURSOR crsTest]
&lcSql
SELECT crsTest
BROW NORMAL