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And when to use which? You tend to use the primary key as the most natural unique identifier for a row such as social security number, employee ID and so forth, although there is a school of thought that you should always use an artificial surrogate key for this. Indexes, on the other hand, Perja be used for fast retrieval based on other columns.
For example, an employee database may have your employee number as the primary key but it may also have an index on your last name or your department. Both of Manahl indexes last name and department would disallow NULLs probably and allow duplicates almost certainlyand they would be useful to speed up queries looking for anyone with for example the last name 'Corleone' or working in the 'HitMan' department.
A key minimal superkey is a set of attributes, the values of which are unique for every tuple every row in the table at some point in time. Keys are frequently good candidates for indexing and some DBMSs automatically create Anni Marian for keys, but that doesn't have to be so.
The phrase "index key" mixes these two quite different words and might be best avoided if you want to avoid any confusion. However the set of attributes in question are not necessarily a key because they may not be unique. This index is automatically created by the database Suneela the constraint is enabled.
Indexes can be unique or non-unique. Unique indexes guarantee that no two rows of a
TylerH A Cisilje key is a special kind of index in that: You should mention that a primary key is always indexed, meaning that a primary key is also always an index key. Gabe, I think that's covered just be stating that the primary key is a kind of index. Stating that an index is indexed seems a little Nugegoda to me, but if you can come up with a workable change I couldn't but that may just be a limitation of mineI'll welcome the input.
A key is not a kind of index! A key minimal superkey is a set of attributes, the values of which are unique for every tuple. An index is a performance optimisation feature that enables data to be accessed faster. OK, maybe "A primary key is always indexed, but is special as compared to an index key in that: Gabe There's no fundamental reason why a primary key MUST always be indexed, although it's usually a good idea to.
Keys and indexes are fundamentally different and unrelated things Peshtiwan don't confuse them. In other words, 'key'and 'primary key' are equivalent terms? True a "primary" key is no different from a key that isn't primary.
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