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Unique constraint. A unique constraint can be defined on columns that allow nulls, in which case rows that include nulls may not actually be unique across the set of columns defined by the constraint. Each table can have multiple unique constraints. On some RDBMS a unique constraint generates a nonclustered index by default.
Entity integrity is concerned with ensuring that each row of a table has a unique and non-null primary key value; this is the same as saying that each row in a table represents a single instance of the entity type modelled by the table.
Indexes are used to police database constraints, such as UNIQUE, EXCLUSION, PRIMARY KEY and FOREIGN KEY. An index may be declared as UNIQUE, which creates an implicit constraint on the underlying table. Database systems usually implicitly create an index on a set of columns declared PRIMARY KEY, and some are capable of using an already-existing ...
A table (called the referencing table) can refer to a column (or a group of columns) in another table (the referenced table) by using a foreign key. The referenced column(s) in the referenced table must be under a unique constraint, such as a primary key. Also, self-references are possible (not fully implemented in MS SQL Server though [5]).
Entity integrity concerns the concept of a primary key. Entity integrity is an integrity rule which states that every table must have a primary key and that the column or columns chosen to be the primary key should be unique and not null. Referential integrity concerns the concept of a foreign key. The referential integrity rule states that any ...
The Eagles won their playoff opener against the Packers Sunday. Here's why their top wideout was seen reading on the sideline.
In the relational model of databases, a primary key is a designated attribute that can reliably identify and distinguish between each individual record in a table.The database creator can choose an existing unique attribute or combination of attributes from the table (a natural key) to act as its primary key, or create a new attribute containing a unique ID that exists solely for this purpose ...
Image source: The Motley Fool. Equinix (NASDAQ: EQIX) Q4 2024 Earnings Call Feb 12, 2025, 5:30 p.m. ET. Contents: Prepared Remarks. Questions and Answers. Call ...