12th November 2020:
PostgreSQL 13.1, 12.5, 11.10, 10.15, 9.6.20, & 9.5.24 Released!

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The mathematical concept underlying the relational model is
the set-theoretic *relation* which is a
subset of the Cartesian product of a list of domains. This
set-theoretic relation gives the model its name (do not confuse
it with the relationship from the *Entity-Relationship model*). Formally a domain is
simply a set of values. For example the set of integers is a
domain. Also the set of character strings of length 20 and the
real numbers are examples of domains.

The *Cartesian product* of domains
`D _{1}`,

For example, when we have `k`=2,
`D _{1}`=

A Relation is any subset of the Cartesian product of one or
more domains: `R` ⊆ `D _{1}` ×

For example `{(0,a),(0,b),(1,a)}` is a
relation; it is in fact a subset of `D _{1}` ×

The members of a relation are called tuples. Each relation of
some Cartesian product `D _{1}` ×

A relation can be viewed as a table (as we already did,
remember The Suppliers and
Parts Database where every tuple is represented by a row and
every column corresponds to one component of a tuple. Giving
names (called attributes) to the columns leads to the definition
of a *relation scheme*.

A *relation scheme* `R` is a finite set of attributes `A _{1}`,

Note:Arelation schemeis just a kind of template whereas arelationis an instance of arelation scheme. The relation consists of tuples (and can therefore be viewed as a table); not so the relation scheme.

We often talked about *domains* in
the last section. Recall that a domain is, formally, just a set
of values (e.g., the set of integers or the real numbers). In
terms of database systems we often talk of *data types* instead of domains. When we define a
table we have to make a decision about which attributes to
include. Additionally we have to decide which kind of data is
going to be stored as attribute values. For example the values
of `SNAME` from the table `SUPPLIER` will be character strings, whereas
`SNO` will store integers. We define
this by assigning a data type to each attribute. The type of
`SNAME` will be `VARCHAR(20)` (this is the SQL type for character strings of length <=
20), the type of `SNO` will be
`INTEGER`. With the assignment of a data
type we also have selected a domain for an attribute. The
domain of `SNAME` is the set of all
character strings of length <= 20, the domain of `SNO` is the set of all integer numbers.