What is the difference between primary key and unique constraints in SQL - DeveloperIndian
#difference_between_primary_key_and_unique_constraints #sql_constraints #oracle mysql #what is constraint in sql
Primary key cannot have NULL value, the unique constraints can have NULL values.
There is only one primary key in a table, but there can be multiple unique constrains.
UNIQUE: This constraint when specified with a column, tells that all the values in the column must be unique. That is, the values in any row of a column must not be duplicate.
PRIMARY KEY: A primary key is a field which can uniquely identify each row in a table. And primary key constraint is used to specify a field in a table as primary key which is always contain unique data.
Below is the syntax to create constraints at the time of creating the table.
CREATE TABLE demo_table
(
column1 data_type(size) constraint_name,
column2 data_type(size) constraint_name,
column3 data_type(size) constraint_name,
....
);
demo_table: Name of the table to be created.
data_type: Type of data that can be stored in the field .
constraint_name: Name of the constraint which is used by us. for example- NOT NULL, UNIQUE, PRIMARY KEY etc.