If a derived column list is not included in a SQL derived table, the names of the SQL derived table columns must match the names of the columns specified in the target list of the derived table expression. If a column name is not specified in the target list of the derived table expression, as in the case where a constant expression or an aggregate is present in the target list of the derived table expression, the resulting column in the SQL derived table has no name.
If a derived column list is included in a SQL derived table, it must specify names for all columns in the target list of the derived table expression. These column names must be used in the query block in place of the natural column names of the SQL derived table. The columns must be listed in the order in which they occur in the derived table expression, and a column name cannot be specified more than once in the derived column list.
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