Default Index is clustered : Default Index is no-clustered : The purpose of the primary key is to enforce entity integrity. Therefore, unique key columns may or may not contain NULL values but primary key columns cannot contain NULL values.Dr.Roshan G. Ragel, is a Doctorate in Computer Science and Engineering and Member of IET (UK) and IEEE. SQL allows primary key to be made up of one or more columns and each column that is included in the primary key is implicitly defined to be NOT NULL.

Primary Key: Unique Key: There can be one primary key in a table : There can be multiple unique keys in the table : It does not allow null columns. TRIGGERS are stored programs that are fired by Oracle engine...In this article, we will see how to create tables, modify tables and dropping tables in SQLite3...What is While Loop?

This means that, for each value in the primary or unique key column, only one record will be returned. A unique key is a group of one or more than one fields or columns of a table which uniquely identify database record.A unique key is the same as a primary key, but it can accept one null value for a table column. Primary keys can be used as foreign keys for other tables too.We have a table name employee which stores data of employees of a company. Unique constraints are referenced by the foreign key of other tables. You can easily identify each row of a table with student's roll number. One important property is that the unique keys do not enforce the NOT NULL constraint. It cannot be null.

Primary key allows each row in a table to be uniquely identified and ensures that no duplicate rows exist and no null values are entered.

Index. For better understanding of primary key we take table named as Student table, having attributes such as Roll_number, Name, Batch, Phone_number, Citizen_ID.The roll number attribute can never have identical and NULL value, because every student enrolled in a university can have unique Roll_number, therefore two students cannot have same Roll_number and each row in a table is uniquely identified with student’s roll number.

To define a primary key in SQL server, you would simply need to use the keywords ‘Primary Key’ after the column’s name. Most importantly, a table can have only a single primary key while it can have more than one unique key. But if student is migrated to another country in that case, he or she would not have any Citizen_ID and the entry could have a NULL value as only one NULL is allowed in the unique constraint.Attention reader! All the conclusions were drawn using MySQL 5.0.67 server.

Definition. Here are the important differences between primary key and unique key:   What is a union? Content: Primary key Vs Unique key

A unique key is made up of a single column or a set of columns. As the id of each employee is unique and no two employees can have the same Emp_id.Multiple unique keys can present in a table.

Duplicate and NULL (empty) values are not valid in the case of the primary key.

We can define Primary key constraint on temporary table and table variable. We can't delete primary key value from the parent table which is used as a foreign key in child table. Primary Key is a column that is used to uniquely identify each tuple of the table. Unique Key. Primary key enforces the implicit NOT NULL constraint. A primary key is sort of a unique key identifier that uniquely identifies a row within a database table, while a unique key identifies all possible rows that exist in a table and not just the currently existing rows.

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