What Is A Primary Key?
A primary key, also referred to as a primary keyword, is a key in relational database that is unique for each record. It is a unique identifier such as a driver license number, Social security number, telephone number (including area code), or vehicle identification number. A relational database must always have one and only one primary key. A primary key typically appears to be as columns in relational database tables. Primary keys must contain unique values. A primary key column cannot have NULL values. A table can have one primary key, which may consist of single or multiple fields. When multiple fields are used as a primary key, they are referred to as a composite key.
Facts About Primary Key
- A primary key is used to ensure data in the specific column is unique.
- It uniquely identifies a record in the relational database table.
- Only one primary key is allowed in a table in a table.
- It is a combination of UNIQUE and Not Null constraints.
- It does not allow NULL values cannot be deleted from the parent table.
- Its constraint can be implicitly defined on the temporary tables.
- Examples of primary keys include: Unique last name, Social security number, online username.
What Is A Secondary Key?
A secondary key represents a secondary value that is unique for each record that can be used to identify the record. You may have a primary key that is system generated and a secondary key that comes from the source or by some other process. You might have an invoice number that is generated by the system but you have a client specific identifier that is guaranteed unique. This is secondary key.
In other words, a secondary key provides a secondary reference point for objects whose primary keys do not adequately distinguish them for reference purposes. In the event that a primary key is not enough to distinguish an object, a secondary key can be used to render that object unique. It is processed and sorted in relation to a primary key, clarifying search terms so that only desired results appear when a table is consulted. This creates distinct, cleaner databases.
Facts About Secondary Key
- A secondary key provides a secondary reference point for objects whose primary keys do not adequately distinguish them for reference purposes.
- It is used for identification of rows but not usually unique.
- We can have multiple secondary key per table.
- In the event that a primary key is not enough to distinguish an object, a secondary key can be used to render that object unique.
- Attributes used for Secondary Key are not the ones used for Super key i.e secondary Key is not even be one of the Super key.
- Examples of secondary keys include: Street address number, Phone number, Middle name etc
Difference Between Primary Key And Secondary Key In Tabular Form
| BASIS OF COMPARISON | PRIMARY KEY | SECONDARY KEY |
| Description | The attribute that uniquely identifies a row or record in a relation is known as Primary key. | A field or combination of fields that is basis for retrieval is known as secondary key (mainly used for finding details from large data). |
| Use | It uniquely identifies a record in the relational database table. | It is used for identification of rows but not usually unique. |
| NULL Values | It does not allow NULL values cannot be deleted from the parent table. | Allows NULL values. |
| Number Of Keys | Only one primary key is allowed in a table in a table. | We can have multiple secondary key per table. |
| Examples | Examples of primary keys include: Unique last name, Social security number, Online username | Examples of secondary keys include: Street address number, Phone number, Middle name etc |
| Deletion | Cannot be deleted from the parent table. | Can be deleted from the parent table. |