what is the difference between primary key unique key and foreign key?
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Unique key can accept only one null value. By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index. ... Primary key can be made foreign key into another table. In SQL Server, Unique key can be made foreign key into another table.
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The primary key is a unique value that identifies each record. ... A natural key is a single column or set of columns that uniquely identifies a single record in a table, where the key columns are made up of real data. When I say “real data” I mean data that has meaning and occurs naturally in the world of data.
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['Super Key. ... ', 'Candidate Key. ... ', 'Primary Key. ... ', 'Alternate key. ... ', 'Composite/Compound Key. ... ', 'Unique Key. ... ', 'Foreign Key.']
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Secondary key is a non-unique field. One secondary key value may refer to many records. Candidate Key or Alternate key A relation can have only one primary key. It may contain many fields or combination of fields that can be used as primary key.
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By the SQL standard, a foreign key must reference either the primary key or a unique key of the parent table. If the primary key has multiple columns, the foreign key must have the same number and order of columns. Therefore the foreign key references a unique row in the parent table; there can be no duplicates.
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A primary key is a field in a table which uniquely identifies each row/record in a database table. Primary keys must contain unique values.
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Foreign Key: FK is a key in one table (child) that uniquely identifies a row of another table (parent). A FK is not-unique in the child table. ... Candidate Key: A candidate key can be uniquely used to identify a database record without any extraneous data. They are Not Null and unique.
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A primary key is a table column that serves a special purpose. Each database table needs a primary key because it ensures row-level accessibility. ... The values that compose a primary key column are unique; no two values are the same. Each table has one and only one primary key, which can consist of one or many columns.
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The difference between a UNIQUE constraint and a Primary Key is that per table you may only have one Primary Key but you may define more than one UNIQUE constraints. ... For MS SQL Server databases, a PRIMARY KEY will generate a unique CLUSTERED INDEX. A UNIQUE constraint will generate a unique NON-CLUSTERED INDEX.
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Primary Key is a unique and non-null key which identify a record uniquely in table. A table can have only one primary key. Candidate key is also a unique key to identify a record uniquely in a table but a table can have multiple candidate keys.
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A primary key is a special key used to uniquely identify records in a table, whereas a foreign key is used to establish relationship between two tables. Both are identical in structure but play different roles in relational database schema.
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A primary key uniquely identifies each record within a table and relates records to additional data stored in other tables. A natural key might require several fields to accomplish a unique identity for each record.
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By default there are no constraints on the foreign key, foreign key can be null and duplicate. while creating a table / altering the table, if you add any constrain of uniqueness or not null then only it will not allow the null/ duplicate values.
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A primary key is used to ensure data in the specific column is unique. A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. It uniquely identifies a record in the relational database table. ... Only one primary key is allowed in a table.
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A table can have only one primary key, which may consist of single or multiple fields. When multiple fields are used as a primary key, they are called a composite key. If a table has a primary key defined on any field(s), then you cannot have two records having the same value of that field(s).
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Sometimes database designers uses two unique keys on each table, a "business key," for example orderid, and a "counter", used as the primary key. ... The PK is chosen from a group of Alternate Keys (AKs), any one of which can uniquely identify any of the rows in the table. A primary key should have several characteristics.
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Primary and foreign keys are a way in which to constrain related data together to ensure data in your database remains consistent and to ensure no redundant data is in the database as a result of deleting a table or row in one table that affects data in other tables that may perhaps rely on that information.