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**Relation**– A table with columns and rows.**Attribute**– A named column of a relation.**Domain**– The set of allowable values for one or more attributes.**Tuple**– A row of a relation.**Intension**– The structure of a relation (i.e. its attributes), together with a specification of the domains and any other restrictions on possible values.*The structure of a relation is usually fixed (the intension is static)*.**Extension**– (or**state**) The tuples of a relation.*The number of tuples in a relation, and their attribute values, change over time (the extension is dynamic)*.**Degree**– The number of attributes the relation contains.**Cardinality**– The number of tuples the relation contains.**Relational database**– A collection of normalized relations with distinct relation names.

A relation is a table with columns and rows; a relation schema is a named relation defined by a set of attribute and domain name pairs. A relational database schema is a set of relation schemas, each with a distinct name.

A relation has the following properties:

- has a name that is distinct from all other relation names in the relational schema;
- each cell contains exactly one atomic (single) value;
- each attribute has a distinct name;
- the values of an attribute are all from the same domain;
- each tuple is distinct; there are no duplicate tuples;
- the order of attributes has no significance;
- the order of tuples has no significance, theoretically.