What is a secondary key in SQL?

Asked By: Navjot Schmaltz | Last Updated: 3rd February, 2020
Category: technology and computing databases
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A secondary key is a non-identifying column or set of columns that can be used to find row in a table. It is non-identifying in the sense that multiple rows may satisfy an give secondary key value. Foreign keys are secondary keys as are any common filter and join columns especially if they are indexed.

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Also to know is, what is a secondary key in a database?

DEFINITION: A secondary key is made on a field that you would like to be indexed for faster searches. A table can have more than one secondary key. The main purpose of a database is to store and search for data. When databases become large, possibly with hundreds of thousands of records, they can take a while to search

Beside above, what is difference between primary and secondary key? 3 Answers. Primary Key: Is a single field chosen by the designer to uniquely identify a record in a table (relation), cannot be null (empty/unassigned). Secondary (or Alternative) Key: Is any field in the table that isn't selected to be any of the two types above.

People also ask, what is primary and secondary key in SQL?

Secondary Key is the key that has not been selected to be the primary key. However, it is considered a candidate key for the primary key. Therefore, a candidate key not selected as a primary key is called secondary key. Candidate key is an attribute or set of attributes that you can consider as a Primary key.

What is alternate key SQL?

An alternate key is a key associated with one or more columns whose values uniquely identify every row in the table, but which is not the primary key. For example, where the primary key for a table may be the employee id, the alternate key might combine the first, middle, and last names of the employee.

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Can foreign key have duplicate values?

Unlike primary keys, foreign keys can contain duplicate values. Also, it is OK for them contain NULL values. Indexes aren't automatically created for foreign keys; however, as a DBA, you can define them. A table is allowed to contain more than one foreign key.

Can I have two primary keys in a table?

A table can have only one primary key, which may consist of single or multiple fields. When multiple fields are used as a primary key, they are called a composite key. If a table has a primary key defined on any field(s), then you cannot have two records having the same value of that field(s).

What do you mean by normalization?

Normalization is a systematic approach of decomposing tables to eliminate data redundancy(repetition) and undesirable characteristics like Insertion, Update and Deletion Anomalies. It is a multi-step process that puts data into tabular form, removing duplicated data from the relation tables.

Can a primary key be a foreign key?

Primary keys always need to be unique, foreign keys need to allow non-unique values if the table is a one-to-many relationship. It is perfectly fine to use a foreign key as the primary key if the table is connected by a one-to-one relationship, not a one-to-many relationship.

What is difference between primary key and candidate key?


Candidate Key – A Candidate Key can be any column or a combination of columns that can qualify as unique key in database. There can be multiple Candidate Keys in one table. Primary Key – A Primary Key is a column or a combination of columns that uniquely identify a record. Only one Candidate Key can be Primary Key.

What is primary key and foreign key?

Primary key uniquely identify a record in the table. Foreign key is a field in the table that is primary key in another table. Primary Key can't accept null values. Foreign key can accept multiple null value.

What is foreign key in DBMS?

Definition: Foreign keys are the columns of a table that points to the primary key of another table. They act as a cross-reference between tables. For example: In the below example the Stu_Id column in Course_enrollment table is a foreign key as it points to the primary key of the Student table.

What is composite key in SQL?

A composite key is a combination of two or more columns in a table that can be used to uniquely identify each row in the table when the columns are combined uniqueness is guaranteed, but when it taken individually it does not guarantee uniqueness.

Can foreign key be null?

A foreign key containing null values cannot match the values of a parent key, since a parent key by definition can have no null values. However, a null foreign key value is always valid, regardless of the value of any of its non-null parts. A table can have many foreign keys.

What is Normalisation in SQL?


In brief, normalization is a way of organizing the data in the database. Normalization entails organizing the columns and tables of a database to ensure that their dependencies are properly enforced by database integrity constraints. It usually divides a large table into smaller ones, so it is more efficient.

What is the difference between primary and unique key?

Key Differences Between Primary key and Unique key:
Primary key will not accept NULL values whereas Unique key can accept one NULL value. A Clustered index automatically created when a primary key is defined whereas Unique key generates the non-clustered index.

Can a candidate key be null?

Primary key can't contain Null value, but Candidate key value can have one NULL value. If a column (part of candidate keys) contain one null value, then that column will remain as part of candidate key, but not be considered as primary key.

What is candidate key with example?

Candidate Key: The minimal set of attribute which can uniquely identify a tuple is known as candidate key. For Example, STUD_NO in STUDENT relation. The value of Candidate Key is unique and non-null for every tuple. For Example, {STUD_NO, COURSE_NO} is a composite candidate key for relation STUDENT_COURSE.

What is primary key and example?

A primary key is either an existing table column or a column that is specifically generated by the database according to a defined sequence. For example, students are routinely assigned unique identification (ID) numbers, and all adults receive government-assigned and uniquely-identifiable Social Security numbers.

What is a candidate key in SQL?


A candidate key is a column, or set of columns, in a table that can uniquely identify any database record without referring to any other data. Each table may have one or more candidate keys, but one candidate key is unique, and it is called the primary key.

What is null value?

The SQL NULL is the term used to represent a missing value. A NULL value in a table is a value in a field that appears to be blank. A field with a NULL value is a field with no value. It is very important to understand that a NULL value is different than a zero value or a field that contains spaces.

What is super key in DBMS?

Definition of Super Key in DBMS: A super key is a set of one or more attributes (columns), which can uniquely identify a row in a table.