The relational model is very simple and elegant; a database is a collection of one or more relations, where each relation is a table with rows and columns. This simple tabular representation enables even novice users to understand the contents of a database, and it permits the use of simple, high-level languages to query the data.
The main construct for representing data in the relational model is a relation. A
relation consists of a relation schema and a relation instance. The relation instance is a table, and the relation schema describes the column heads for the table. We first describe the relation schema and then the relation instance. The schema specifies the relation's name, the name of each field (or column, or attribute), and the domain of each field. A domain is referred to in a relation schema by the domain name and has a set of associated values.
Students(sid: string, name: string, login: string, age: integer, gpa: real)
This says, for instance, that the field named sid has a domain named string. The set of values associated with domain string is the set of all character strings.
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