Description
By defining a column with Primary Key constraint then that primary key constraint won't acknowledge invalid and copy values. And a table can contain only one primary key.
Primary key = Unique + Not null
Unique constraints can acknowledge null values where as primary key won't acknowledge null and copy values.A table can have just a single primary key, which may comprise of single or different fields. At the point when different fields are utilized as an essential key, they are known as a composite key. In the event that a table has an essential key characterized on any field, then you can not have two records having a similar estimation of that field. Syntax
create table <table_name> (<column_name1>data type(size)constraint constraint name constraint type, <column_name2>data type(size),constraint(column_name1);
Table_name =>Any accurate table.
Column_name =>The operation that can be performed in the column of a table.
constraint_type =>constraint type will specify the type of constraint, that going to be executed in the database.
Examples
By viewing the below example, the concept of primary key constraint can be easily understood.
[c]
sql> create table prod13(prod_id int not null auto_increment,prod_name varchar(255)not null,primary key(prod_id));
Query OK, 0 rows affected (0.52 sec)
sql> insert into prod13 (prod_name) values ('mobile');
Query OK, 1 row affected (0.05 sec)
sql> insert into prod13 (prod_name) values ('laptop');
Query OK, 1 row affected (0.09 sec)
sql> insert into prod13 (prod_name) values ('system');
Query OK, 1 row affected (0.09 sec)
sql> select * from prod13;
+---------+-----------+
| prod_id | prod_name |
+---------+-----------+
| 1 | mobile |
| 2 | laptop |
| 3 | system |
+---------+-----------+
3 rows in set (0.00 sec)[/c]
In the above example, the column prod_id is given not null auto increment constraint and at the same time in another column prod_id is given primary key constraint. In such case, no need to insert each and every value, just by writing the prod_name, the prod_id values are displayed automatically.
Key Points
- Primary Key Constraint - Primary key constraint won't accept invalid and copied values