A qualifier for a column-name can be a:
When you qualify a column name, you enter the qualifier, followed immediately by a period (.), then the column-name. The format for qualifying a column name is shown in the below:
qualifier.column-name
Whether a column-name may be qualified depends on its context. In some forms of the COMMENT ON statement, a column-name must be qualified. 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. For a discussion of these purposes, see Column-Name Qualifiers to Avoid Ambiguity and Column-Name Qualifiers in Correlated References.
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