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Primary keys are defined in the ISO SQL Standard, through the PRIMARY KEY constraint. The syntax to add such a constraint to an existing table is defined in SQL:2003 like this: ALTER TABLE < table identifier > ADD [ CONSTRAINT < constraint identifier > ] PRIMARY KEY ( < column name > [ { , < column name > } ...
A foreign key is defined as an attribute or set of attributes in a relation whose values match a primary key in another relation. The syntax to add such a constraint to an existing table is defined in SQL:2003 as shown below. Omitting the column list in the REFERENCES clause implies that the foreign key shall reference the primary key of the ...
In principle any key may be referenced by foreign keys. Some SQL DBMSs only allow a foreign key constraint against a primary key but most systems will allow a foreign key constraint to reference any key of a table.
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]).
A requirement of E. F. Codd in his seminal paper is that a primary key of an entity, or any part of it, can never take a null value. [1] The relational model states that every relation (or table ) must have an identifier, called the primary key (abbreviated PK), in such a way that every row of the same relation be identifiable by its content ...
Every relation/table has a primary key, this being a consequence of a relation being a set. [21] A primary key uniquely specifies a tuple within a table. While natural attributes (attributes used to describe the data being entered) are sometimes good primary keys, surrogate keys are often used instead. A surrogate key is an artificial attribute ...
We saw last season what Ford Field looks like for a Detroit Lions playoff game. It was perhaps the best atmosphere in the NFL. Imagine what it would be like if the Lions were two home wins from a ...
Database systems usually implicitly create an index on a set of columns declared PRIMARY KEY, and some are capable of using an already-existing index to police this constraint. Many database systems require that both referencing and referenced sets of columns in a FOREIGN KEY constraint are indexed, thus improving performance of inserts ...