General information Relational model
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Relationships between tables
In the case of relational data organization, record types are l inked by means of certain
record elements with matching contents. A table in a relational schema th us corresponds
to a record type in a CODASYL schema. For example, to link a table with another table by
means of record element contents, the identifying primary key of the first table must also be
included as a (redundant) foreign key in the second table (see figure 3, DEPARTMENT and
EMPLOYEE tables).
A foreign key must match the values of another table’s primary key. Foreign keys create
logical links between tables. Foreign keys are defined by the users themselves.
Tables are the only fixed data structures in the relational model. Most relational database
systems neither monitor nor maintain the logical relationships bet ween tables. Users create
these relationships by means of values when processing the database, i.e. the tables ar e
linked logically on the basis of their contents. Users are thus not restricted by structures
prescribed by the system.
Data manipulation and retrieval in relational databases
The relational model defines basic database functions whic h are analogous to set opera-
tions. Thus, in addition to the principle of processing data in the form of tables, another
major feature characterizing the relational model is a defined set of basic operations which
can be used to retrieve information from relational databases.
The relational model does not define a language for data manipulat ion and retrieval.
The main features of data manipulation languages (DML) in relational systems are the set-
oriented operations projection, selection and join and the absence of any predefined struc-
tures. In other words, relational data manipulation languages descr ibe the desired result
itself, instead of describing how that result is to be obtained, i.e. users no longer need to
specify how the desired result is to be ascerta ined from the database - they need merely
state what they want to see.
The relational model is supported in UDS/ SQL through the inclusion of SQL (Structured
Query Language), a language for relational database system s.