Basic Language Elements › Column Names › Column-Name Qualifiers to Avoid Ambiguity › Avoiding Undefined or Ambiguous References

Avoiding Undefined or Ambiguous References

When a column-name refers to values of a column, exactly one object table must include a column with that name.

The following conditions are considered errors:

• No object table contains a column with the specified name. In this case, the reference is undefined.
• The column name is qualified by a table designator, but the designated table does not include a column with the specified name. Again, the reference is undefined.
• The name is unqualified and more than one object table includes a column with that name. In this case, the reference is ambiguous.

Avoid ambiguous references by qualifying a column-name with a uniquely defined table designator.

If the column is contained in several object tables with different names, the table names can be used as designators.

In the case that two or more of the object tables are instances of the same table and have the same name, you can use correlation-names to designate, unambiguously, the particular instances of the table.

For example, in the following FROM clause, X and Y are defined to refer, respectively, to the first and second instances of the table NAMETBL.

          .
          .
 FROM NAMETBL X, NAMETBL Y
          .
          .