Database

Database

What is a relational database?

• A database is a collection of information that is organized so that it can easily be accessed, managed, and updated. In one view, databases can be classified according to types of content: bibliographic, full-text, numeric, and images.

What is a relational model?

• The relational model (RM) for database management is an approach to managing data using a structure and language consistent with first-order predicate logic, first described in 1969 by Edgar F. Codd.
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Cocept one to one

• In a one-to-one relationship, each row in one database table is linked to 1 and only 1 other row in another table. In a one-to-one relationship between Table A and Table B, each row in Table A is linked to another row in Table B. The number of rows in Table A must equal the number of rows in Table B.
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Concept one to many

• In a one-to-many relationship, each row in the related to table can be related to many rows in the relating table. This allows frequently used information to be saved only once in a table and referenced many times in all other tables. In a one-to-many relationship between Table A and Table B, each row in Table A is linked to 0, 1 or many rows in Table B. The number of rows in Table A is almost always less than the number of rows in Table B.
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Concept many to many

• In a many-to-many relationship, one or more rows in a table can be related to 0, 1 or many rows in another table. In a many-to-many relationship between Table A and Table B, each row in Table A is linked to 0, 1 or many rows in Table B and vice versa. A 3rd table called a mapping table is required in order to implement such a relationship.
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