SQL Reference

A qualifier for a column name may be a table, view, nickname, alias, or correlation name.

Whether a column name may be qualified depends on its context:

Depending on the form of the COMMENT ON statement, a single column name may need to be qualified. Multiple column names must be unqualified.
Where the column name specifies values of the column, it may be qualified at the user's option.
In all other contexts, a column name must not be qualified.

Where a qualifier is optional, it can serve two purposes. They are described under Column Name Qualifiers to Avoid Ambiguity and Column Name Qualifiers in Correlated References.

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