Search results
Results from the WOW.Com Content Network
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 ...
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 index to police this constraint.
Every relation/table has a primary key, this being a consequence of a relation being a set. [19] 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 ...
A key that may become the primary key. Primary The key that is selected as the primary key. Only one key within an entity is selected to be the primary key. This is the key that is allowed to migrate to other entities to define the relationships that exist among the entities.
A candidate key is a minimal superkey, [1] i.e., a superkey that doesn't contain a smaller one. Therefore, a relation can have multiple candidate keys, each with a different number of attributes. [2] Specific candidate keys are sometimes called primary keys, secondary keys or alternate keys.
Index funds or ETFs: Which are better? Determining whether an index fund or ETF is better is difficult because the answer depends on the specific funds being discussed and your goals as an investor.
A simple B+ tree example linking the keys 1–7 to data values d 1-d 7. The linked list (red) allows rapid in-order traversal. This particular tree's branching factor is =4. Both keys in leaf and internal nodes are colored gray here. By definition, each value contained within the B+ tree is a key contained in exactly one leaf node.
Upgrade to a faster, more secure version of a supported browser. It's free and it only takes a few moments: