what is the difference between primary key and unique key in sql server?
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Primary key will not accept NULL values whereas Unique key can accept one NULL value. A table can have only primary key whereas there can be multiple unique key on a table. A Clustered index automatically created when a primary key is defined whereas Unique key generates the non-clustered index.
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A primary key is used to ensure data in the specific column is unique. You can only set constraints with primary keys, by setting a foreign key to another column which creates a relationship with the column that has the primary key set. ... The id column is likely to be a primary key because is needs to be unique.
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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.
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Primary key does not allow NULL values, but unique key allows NULL values. ... You can have more than 1 unique key table column (unlike primary key which means only 1 table column in the table is unique). INDEX also creates uniqueness. MySQL (example) will create a indexing table for the column that is indexed.
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Difference between Primary Key and Foreign Key. Primary key uniquely identify a record in the table. Foreign key is a field in the table that is primary key in another table. ... By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index.
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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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1. The not null constraint is by default added to primary key, it means, primary key attribute cannot accept null values, whereas, the attribute declared as unique can accept null values. It is the major difference between the two.
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So primary key constraint means that what are the limitations which are imposed along with making any column as a primary key. The main difference is that the primary key column can only be created when you create/add the table or column, not later.
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No. 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 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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Primary keys must contain unique values. A primary key column cannot have NULL values. 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.
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You can only have one primary key per table, but multiple unique keys. Similarly, a primary key column doesn't accept null values, while unique key columns can contain one null value each. And finally, the primary key column has a unique clustered index while a unique key column has a unique non-clustered index.
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A unique index ensures that the values in the index key columns are unique. A unique constraint also guarantees that no duplicate values can be inserted into the column(s) on which the constraint is created. When a unique constraint is created a corresponding unique index is automatically created on the column(s).
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Introduction to MySQL primary key A primary key is a column or a set of columns that uniquely identifies each row in the table. ... If the primary key consists of multiple columns, the combination of values in these columns must be unique. A primary key column cannot have NULL values.
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Primary keys - Contain one or more columns whose combined values uniquely identify every row in a table. Each table can have only one primary key. Alternate keys - Contain one or more columns whose combined values uniquely identify every row in a table.
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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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A primary key is a candidate key that has been selected to be the unique identifier of each tuple in the relation. A foreign key is an attribute of a table that is also a primary key in another table. A composite key is a combination of two fields that together can uniquely identify a record.