What is the difference between primary unique and foreign key constraints and indexes

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What is the difference between primary  unique and foreign key constraints  and indexes    I work on Oracle 10g and SQL Server 2008

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1 A primary key is a set of one or more attributes that uniquely identifies tuple within relation    2 A foreign key is a set of attributes from a relation scheme which can be uniquely identify tuples fron another relation scheme

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Primary key mainly prevent duplication and shows the uniqueness of columns  Foreign key mainly shows relationship on two tables

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Primary Key  identify uniquely every row it can not be null  it can not be a duplicate   Foreign Key  create relationship between two tables  can be null  can be a duplicate  nbsp

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A primary key is a column or a set of columns that uniquely identify a row in a table  A primary key should be short  stable and simple  A foreign key is a column  or set of columns  in a second table whose value is required to match the value of the primary key in the original table  Usually a foreign key is in a table that is different from the table whose primary key is required to match  A table can have multiple foreign keys  The primary key cannot accept null values  Foreign keys can accept multiple  We can have only one primary key in a table  We can have more than one foreign key in a table  By default  Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index  Foreign keys do not automatically create an index  clustered or non-clustered  You can manually create an index on a foreign key

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Primary Key and Unique Key are Entity integrity constraints  Primary key allows each row in a table to be uniquely identified and ensures that no duplicate rows exist and no null values are entered   Unique key constraint is used to prevent the duplication of key values within the rows of a table and allow null values   In oracle one null is not equal to another null     KEY or INDEX refers to a normal non-unique index  Non-distinct values for the index are allowed  so the index may contain rows with identical values in all columns of the index  These indexes don t enforce any structure on your data so they are used only for speeding up queries  UNIQUE refers to an index where all rows of the index must be unique  That is  the same row may not have identical non-NULL values for all columns in this index as another row  As well as being used to speed up queries  UNIQUE indexes can be used to enforce structure on data  because the database system does not allow this distinct values rule to be broken when inserting or updating data  Your database system may allow a UNIQUE index on columns which allow NULL values  in which case two rows are allowed to be identical if they both contain a NULL value  NULL is considered not equal to itself   though this is probably undesirable depending on your application  PRIMARY acts exactly like a UNIQUE index  except that it is always named  PRIMARY   and there may be only one on a table  and there should always be one  though some database systems don t enforce this   A PRIMARY index is intended as a way to uniquely identify any row in the table  so it shouldn t be used on any columns which allow NULL values  Your PRIMARY index should always be on the smallest number of columns that are sufficient to uniquely identify a row  Often  this is just one column containing a unique auto-incremented number  but if there is anything else that can uniquely identify a row  such as  countrycode  in a list of countries  you can use that instead  FULLTEXT indexes are different to all of the above  and their behaviour differs more between database systems  Unlike the above three  which are typically b-tree  allowing for selecting  sorting or ranges starting from left most column  or hash  allowing for selection starting from left most column   FULLTEXT indexes are only useful for full text searches done with the MATCH     AGAINST   clause    see Differences between INDEX  PRIMARY  UNIQUE  FULLTEXT in MySQL

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Key index   A key is an aspect of a LOGICAL database design  an index is an aspect of a PHYSICAL database design   A key corresponds to an integrity constraint  an index is a technique of physically recording values that can be usefully applied when enforcing those constraints   Primary foreign   A  primary  key is a set of attributes whose values must form a combination that is unique in the entire table   There can be more than one such set    1 key   and the word  primary  is a remnant from the earlier days when the designer was then forced to choose one of those multiple keys as being  the most important relevant one    The reason for this was primarily in combination with foreign keys    Like a  primary  key  a  foreign  key is also a set of attributes   The values of these attributes must form a combination that is an existing primary key value in the referenced table   I don t know exactly how strict this rule still applies in SQL today   The terminology has remained anyway   Unique   keyword used to indicate that an index cannot accept duplicate entries   Unique indexes are obviously an excellent means to enforce primary keys   To the extent that the word  unique  is used in contexts of LOGICAL design  it is superfluous  sloppy  unnecessary and confusing   Keys  primary keys  that is  are unique by definition

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Here are some reference for you   Primary  amp  foreign key Constraint      Primary Key  A primary key is a field   or combination of fields that uniquely   identify a record in a table  so that   an individual record can be located   without confusion       Foreign Key  A foreign key  sometimes   called a referencing key  is a key   used to link two tables together    Typically you take the primary key   field from one table and insert it   into the other table where it becomes   a foreign key  it remains a primary   key in the original table     Index  on the other hand  is an attribute that you can apply on some columns so that the data retrieval done on those columns can be speed up

[sql-server] What is the difference between primary, unique and foreign key constraints, and indexes?

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