firebird-mirror/doc/ambiguity.txt

Alias handling and ambiguous field detecting has been improved in FB2.0.

Changes:
a) When an alias is present on a table, that alias must be used or no alias at 
   all, the tablename only is not valid anymore.
b) Fields without qualifier can now be used in a higher scope level. Own scope 
   level is checked first and ambiguous field checking is done on scope level.

Author:
    Arno Brinkman <firebird@abvisie.nl>

Examples:
a)
  1. 
    When an alias is present it must be used or no alias at all must be used.
    This query was allowed in FB1.5 and earlier versions:
    
    SELECT
      RDB$RELATIONS.RDB$RELATION_NAME
    FROM
      RDB$RELATIONS R
    
    but will now correctly report an error that the field
    "RDB$RELATIONS.RDB$RELATION_NAME" couldn't be found.
    
    Use this (preferred):
    
    SELECT
      R.RDB$RELATION_NAME
    FROM
      RDB$RELATIONS R
    
    or this statement:
    
    SELECT
      RDB$RELATION_NAME
    FROM
      RDB$RELATIONS R
    
  2. 
    The statement below will now correctly use the FieldID from the subselect 
    and from the updating table:
    
    UPDATE
      TableA
    SET
      FieldA = (SELECT SUM(A.FieldB) FROM TableA A WHERE A.FieldID = TableA.FieldID)
    
    In firebird it's possible to give an alias by an update statement, but by 
    many other database vendors this isn't supported. These SQL statements are
    now better interchangeable between other SQL database products.

  3. 
    This example didn't run correctly in FB1.5 and earlier:
    
    SELECT
      RDB$RELATIONS.RDB$RELATION_NAME,
      R2.RDB$RELATION_NAME
    FROM
      RDB$RELATIONS
      JOIN RDB$RELATIONS R2 ON
        (R2.RDB$RELATION_NAME = RDB$RELATIONS.RDB$RELATION_NAME)
    
    If RDB$RELATIONS contains 90 records, it would return 90 * 90 = 8100 records, 
    but in FB2.0 it will correctly return 90 records.
    
    

b)
  1.
    This failed in FB1.5, but is possible in FB2.0:
    
    SELECT
      (SELECT RDB$RELATION_NAME FROM RDB$DATABASE)
    FROM
      RDB$RELATIONS

  2.
    Ambiguity checking in sub-selects. The query below did run on FB1.5 
    without reporting an ambiguity, but will report it in FB2.0:

    SELECT
      (SELECT
         FIRST 1 RDB$RELATION_NAME
       FROM
         RDB$RELATIONS R1
         JOIN RDB$RELATIONS R2 ON
           (R2.RDB$RELATION_NAME = R1.RDB$RELATION_NAME))
    FROM
      RDB$DATABASE