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A set of data is described by the same properties.
In a relational table, columns are attributes of a givendomain of existence, the rows are called tuples or records.

Attributes define the properties of each record. Thecardinality is the number of rows in the report, the grade is the number of columns. Within the table records are not ordered and no two records with all attributes equally between them.

To connect two tables together using the logical pointers, manufactured by a unique ID or a set of attributes thatuniquely define each record. In this way there isredundancy of information and access is fast.
The possibility exists that an attribute is not known, in this case we use the null value (null), which identifies this situation. The attribute is different from the zero reset aswith the first identifies a property unknown, with the second hand indicates a value consociuto and equal to zero.

The identity constraints guarantee the accuracy of the data. Constraints can be expressed on characteristicsinternal to the table or across tables. Constraints can be "dictated" to the DBMS when the database is created. In the time of insertion of each tuple, all the constraints are checked, and in the case where all are met is inserted in the database.

-domain constraints: identify limits of each column.
-tuple of constraints: it may or may not have a value in the tuple only if other conditions are present within the other attributes of the tuple.

This key is the unique ID that allows them to distinguishbetween two tuples (records) various. The key is theminimal set of these characteristics. A superkey is a keynot minimal.

The primary key is instead a key that can not be assignedthe value null.

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