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Sometimes a relation schema is taken to include a name.
and a named relation schema is effectively a relation variable or relvar for short (see Relation Variables below).
Under the definition of heading, the attributes of an element do not appear in any particular order either, nor, therefore do the elements of a tuple.
A similar comment does not apply here to SQL, which does define an ordering to the columns of a table.
A relation is thus a heading paired with a body, the heading of the relation being also the heading of each tuple in its body.