A way to enforce error-checking and formating in data-types. Usually the rules for data integrity is created when a table is specified. Examples:
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Null -- a data field may or may not be allowed to be Null.
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Unique -- a data field may or may not be allowed to be uniquely identified among all the other raws in a table
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Primary key -- a data field being a primary key must be unique
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Referential integrity -- inserting or updating a field's value on table A must depend on if it matches the value of a field on another table B.
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