In SQL, a primary key constraint is used to define a column or a set of columns that uniquely identify each row in a table. It is a type of constraint that helps enforce data integrity by preventing duplicate or null values in the primary key column(s).
The primary key constraint is specified at the time of creating a table, either through the CREATE TABLE statement or the ALTER TABLE statement. A primary key can be made up of one or more columns, and each column must contain unique values and cannot be null.
For example, let’s say we have a table named “employees” with columns “emp_id,” “first_name,” “last_name,” and “email.” We can set the “emp_id” column as the primary key, which means that every row in the table must have a unique “emp_id” value.
To create a primary key constraint using the CREATE TABLE statement, we can specify the PRIMARY KEY keyword followed by the column name(s) in parentheses after the column definition.
CREATE TABLE employees ( emp_id INT PRIMARY KEY, first_name VARCHAR(50), last_name VARCHAR(50), email VARCHAR(100) );
Alternatively, we can also use the ALTER TABLE statement to add a primary key constraint to an existing table.
ALTER TABLE employees ADD CONSTRAINT pk_emp_id PRIMARY KEY (emp_id);
In addition to ensuring data integrity, a primary key constraint also helps improve performance when querying the table. Since the primary key is unique for each row, it can be used to quickly locate and retrieve specific rows using indexes.
In summary, the primary key constraint in SQL is used to ensure that each row in a table is uniquely identified by a column or set of columns. It helps enforce data integrity and improve performance when querying the table.