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In a relational database it is used as a way to uniquely identify a record in a table. It also allows tables within the database to reference the specific record within the table.
For example, a common practice is to have the database automatically determine the Primary Key with an auto-incrementing value starting at 1 upon inserting each new record.
The reason for uniquely identifying the record is there could be cases where the data between two records are exactly the same, but they need to be separate as they do represent different things.
For example, let's say you have a database containing the name of a person "John Smith". What would happen if you added "John Smith" into the database multiple times without a Primary Key? By providing the Primary Key as shown below, you can now reference the correct "John Smith".
Id Name
1 John Smith
2 John Smith
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How a primary key is fixed in a sql table?
A primary key field in a sql table is created using the PRIMARY KEY keyword. ex: CREATE TABLE tbl_employee ( emp_num VARCHAR(10), emp_name VARCHAR(100), PRIMARY KEY (emp_num)) The above script creates a table called tbl_employee and sets the emp_num field as the primary key
Does primary key allows null value?
No. A primary key requires a value because it is the field that all other values in the row depend on.
What does the primary key in a data base do?
We know that primary key is the one that is not null and is always unique. A primary key uniquely identifies a record in a table.
Candidate key and primary key differnece?
All Primary keys are definitely Candidate Keys. A Candidate key is one which can be used as a Primary key that is not null and unique. That is one of the candidate keys can be chosen as a primary key.A Candidate key is a Unique Key and it can be used to find out any particular Tuple (row) in a table. The following are the differences between A Candidate key and a Primary Key: 1) A Unique key can be null but not a Primary key 2) On a table we can have only 1 primary key but 'N' number of unique keys.
What is primary key constraint?
Primary Key is a Constraint Used to avoid Duplicate entries in database table and you define primary key the column doesn't allow NULL values.
