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In a Database, more specifically a relational model, columns of one table that relate to columns of another table are called "key columns".
There are two types of "key" columns. Foreign key and Primary Key.
The Primary key of a table is usually a column within a table that contains unique data which cannot be duplicated. Think of it as a "unique identifier" column. You can actually create an array of key columns in a table to represent a unique series of values as well. (so PKey 1,2,3 are unique and no group of PKey's 1,2,3 will be the same, even though sometimes any one of those columns may contain a single value that has been duplicated, the consolidated value of all 3 will never be replicated).
The Primary key of table A is linked to the Primary key of Table B for instance. Table B's Primary Key in this relationship is then called a "Foreign Key", meaning "The Primary Key of the OTHER table".
That's a pretty basic understanding of how they work. There is a little bit more involved but this is the gist of it.
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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.
Is the primary key field?
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
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.
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
