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    what is the difference between a primary key and a unique key?

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    Difference Between Primary key and Unique key

    Difference Between Primary key and Unique key: The primary key is the minimum set of traits that distinguishes any row of a table. A unique key is an individual value that is used to protect duplicate values in a column. Visit to know more on Primary key Vs Unique key.

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    Difference Between Primary key and Unique key

    Difference Between Primary key and Unique key What Is Primary Key?

    The primary key is the minimum set of traits that distinguishes any row of a table. It cannot have NULL and duplicate values. The primary key is used to add integrity to the table.

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    In the case of a primary key, both Duplicate and NULL values are not valid. And, it can be utilized as foreign keys for different tables.

    What Is a Unique Key?

    A unique Key is an individual value that is used to protect duplicate values in a column. The foremost purpose of a unique key in a table is to prevent duplicate values. However, when it comes to a unique value, the primary key also includes it. So, there is one big difference that makes a unique key different, and it is: a unique key can have a NULL value which is not supported in a primary key.

    Difference Between Primary Key and Unique Key

    Parameters Primary Key Unique KeyBasic The primary key is accepted as a unique or sole identifier for every record in the table. When the primary key is not present in the table, then the unique key is also used as a unique identifier for recordsNULL In the case of a primary key, we cannot save NULL values. In the case of a unique key, we can save a null value, however, only one NULL value is supported.Purpose It supports entity integrity. It supports unique data.Index The primary key tends to generate a clustered index by default. The unique key tends to generate a non-clustered index.Number of Key Each table holds just one primary key. A table can hold more than one unique key.Value Modification With the primary key, we cannot modify or delete the values. With the unique key, we can modify the column values.Uses It is used to recognize specific records in the table. It anticipates storing duplicate entries in a column except for a NULL value.

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    Difference between Primary key and Unique key

    A Computer Science portal for geeks. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions.

    Difference between Primary key and Unique key

    Difficulty Level : Medium

    Last Updated : 12 Jul, 2022

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    Prerequisite – Keys in Relational Model

    A primary key is a column of table which uniquely identifies each tuple (row) in that table. Primary key enforces integrity constraints to the table. Only one primary key is allowed to use in a table. The primary key does not accept the any duplicate and NULL values. The primary key value in a table changes very rarely so it is chosen with care where the changes can occur in a seldom manner. A primary key of one table can be referenced by foreign key of another table.

    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. So, we can make Roll_number attribute as a primary key in this case.

    Unique key constraints also identifies an individual tuple uniquely in a relation or table. A table can have more than one unique key unlike primary key. Unique key constraints can accept only one NULL value for column. Unique constraints are also referenced by the foreign key of another table. It can be used when someone wants to enforce unique constraints on a column and a group of columns which is not a primary key.

    For better understanding of unique key we take Student table with Roll_number, Name, Batch, Phone_number, and Citizen_ID attributes.

    Roll number attribute is already assigned with the primary key and Citizen_ID can have unique constraints where each entry in a Citizen_ID column should be unique because each citizen of a country must have his or her Unique identification number like Aadhaar Number. 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.

    Key Differences Between Primary key and Unique key:

    Primary key will not accept NULL values whereas Unique key can accept NULL values.

    A table can have only one primary key whereas there can be multiple unique key on a table.

    A Clustered index automatically created when a primary key is defined whereas Unique key generates the non-clustered index.

    Comparison Chart:

    Parameter PRIMARY KEY UNIQUE KEY

    Basic Used to serve as a unique identifier for each row in a table. Uniquely determines a row which isn’t primary key.

    NULL value acceptance Cannot accept NULL values. Can accepts NULL values.

    Number of keys that can be defined in the table Only one primary key More than one unique key

    Index Creates clustered index Creates non-clustered index

    Auto Increment A Primary key supports auto increment value. A unique key does not supports auto increment value.

    Modification We cannot change or delete values stored in primary keys. We can change unique key values.

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    Difference between Primary Key and Unique key

    Difference between Primary Key and Unique key with mysql tutorial, examples, functions, programming, mysql, literals, procedure, regexp_like(), regexp_replace operator, regular expression, crud etc.

    Difference between Primary Key and Unique key

    Keys in MySQL are the column or set of columns used to construct a relationship between one or more than two tables. They are also used for accessing records from the table. Both keys provide a guaranteed uniqueness for a column or a set of columns in a table or relation. The main difference among them is that the primary key identifies each record in the table, and the unique key prevents duplicate entries in a column except for a NULL value. In this article, we are going to compare essential differences between Primary and Unique Keys based on various parameters. Before making a comparison, we will discuss in brief these keys.

    What is Primary Key?

    The primary key is a unique or non-null key that uniquely identifies every record in that table or relation. The primary key column cannot store duplicate values that mean primary key column values are always unique. It is also called a minimal super key; therefore, we cannot specify more than one primary key in any relationship. A primary key column of one table can be referenced by a foreign key column of another table.

    For example, we have a table named students with attributes such as Stud_ID, Roll_No, Name, Mobile, and Email.

    Here only the Roll_No column can never contain an identical and NULL value. We know every student has a unique roll number. Therefore two students can never have the same roll number. This feature helps to identify each record in the database uniquely. Hence, we can make the Roll_No attribute a primary key.

    Features of Primary Key

    The following are the essential primary key features:

    The primary key column cannot contain duplicate values.

    The primary key implements the entity integrity of the table.

    A table cannot have more than one primary key column.

    We can make the primary key from one or more table fields.

    The primary key column should have NOT NULL constraints.

    What is a Unique Key?

    The unique key is a single column or combination of columns in a table to uniquely identify database records. A unique key prevents from storing duplicate values in the column. A table can contain multiple unique key columns, unlike a primary key column. This key is similar to the primary key, except that one NULL value can be stored in the unique key column. The unique key is also called unique constraints and can be referenced by another table's foreign key.

    For example, let's consider the same table named students with attributes such as Stud_ID, Roll_No, Name, Mobile, and Email.

    Here Stud_ID can be assigned as a unique constraint because each student must have a unique identification number. If a student changes the college, then he or she would not have any student ID. In that case, the entry may contain a NULL value because a unique key constraint allows storing NULL, but it should be only one.

    Features of Unique key

    The following are the essential unique key features:

    We can construct the unique key from one or more table fields.

    A table can define multiple unique key columns.

    By default, a unique key is in non-clustered unique indexes.

    The unique constraint column can store NULL value, but only one NULL is allowed null per column.

    The foreign key can reference the unique constraint in preserving the uniqueness of a table.

    Key differences between Primary and Unique Key

    The following points explain the key differences between primary and candidate keys:

    A primary key can constitute one or more fields of a table to identify records in a table uniquely. On the other hand, a unique key prevents two rows from having duplicate entries in a column.

    A table cannot have more than one primary key in a relational database, while there can be multiple unique keys per table.

    A primary key column cannot contain NULL values, whereas a unique key can have NULL values, but only one NULL is allowed in a table.

    A primary key should be unique, but a unique key cannot necessarily be the primary key.

    The primary key by default is a clustered index where data is physically organized in the sequential index. In contrast, the unique key is a unique non-clustered index.

    The primary key implements entity integrity, whereas the unique key enforces unique data.

    Primary Key vs. Unique Key Comparison Chart

    The following comparison chart explains their main differences in a quick manner:

    Comparison Basis Primary Key Unique Key

    Basic The primary key is used as a unique identifier for each record in the table. The unique key is also a unique identifier for records when the primary key is not present in the table.NULL We cannot store NULL values in the primary key column. We can store NULL value in the unique key column, but only one NULL is allowed.

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