Alternate Key has all the properties to become a Primary Key and so is an alternate option. ALTERNATE Keys in SQL are defined using the SQL constraint UNIQUE.
What is an alternate key in SQL?
Alternate keys are those Candidate keys that were not chosen to be the Primary key of the table. Composite key is a Candidate key that consists of more than one attribute. Foreign key is an attribute which is a Primary key in its parent table but is included as an attribute in the host table.
Is unique key a constraint?
A unique constraint is the rule that the values of a key are valid only if they are unique. A key that is constrained to have unique values is called a unique key . A unique constraint is enforced by using a unique index.
Is foreign key a constraint in SQL?
The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables. The table with the foreign key is called the child table, and the table with the primary key is called the referenced or parent table. …
What is a alternate key in a database?
ALTERNATE KEYS is a column or group of columns in a table that uniquely identify every row in that table. A table can have multiple choices for a primary key but only one can be set as the primary key. All the keys which are not primary key are called an Alternate Key.
What is an alternate key constraint?
An alternate key is a key associated with one or more columns whose values uniquely identify every row in the table, but which is not the primary key. … Each alternate key can generate a unique index or a unique constraint in a target database.
What is key alternate key?
Alternate Key or Secondary Key is the key that has not been selected to be the primary key, but are candidate keys. However, it is considered a candidate key for the primary key. A candidate key not selected as a primary key is called alternate or secondary key.
What is difference between primary key and unique key in SQL?
Key Differences Between Primary key and Unique key:
Primary key will not accept NULL values whereas Unique key can accept NULL values. A table can have only one primary key whereas there can be multiple unique key on a table.
What is difference between candidate key and unique key?
Candidate key is a set of columns which can uniquely identify the values in a table and can act as a unique key. One of these candidate keys will become the primary key and the rest will become alternate keys. Unique key as the name suggests , is used to uniquely identify a value in a table. For example Emp_id etc.
Why unique key is different from primary key?
Both keys provide a guaranteed uniqueness for a column or a set of columns in a table or relation. The main difference among them is that the primary key identifies each record in the table, and the unique key prevents duplicate entries in a column except for a NULL value.
Can a foreign key reference a non primary key?
Yes, a foreign key can reference a non-primary key which is unique. A FOREIGN KEY constraint does not have to be linked only to a PRIMARY KEY constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table.
What is a surrogate key in database?
A surrogate key is a unique key for an entity in the client’s business or for an object in the database. Sometimes natural keys cannot be used to create a unique primary key of the table. This is when the data modeler or architect decides to use surrogate or helping keys for a table in the LDM.
Can a table have multiple primary keys?
Each table can only have one primary key. Access can automatically create a primary key field for you when you create a table, or you can specify the fields that you want to use as the primary key. This article explains how and why to use primary keys. To set a table’s primary key, open the table in Design view.
How do I create an alternate key in SQL?
Creating Alternate Keys
- Open the property sheet of a table and select the Columns tab.
- Select the column or columns to associate with the alternate key and click the Create Key tool. …
- Enter a name for the key. …
- [optional] Modify the default Constraint Name.
How do you declare an alternate key?
The UNIQUE keyword is used to define the alternate key, which indicates the value must be unique. Simple words to say Candidate keys that are not selected as primary key are called Alternate key. UNIQUE (col1,col2…n);
What is Oracle alternate key?
An alternate key (or secondary key) is any candidate key which is not selected to be the primary key (PK). You cant create an alternate key in Oracle, its just conceptual.