Difference between Primary Key and Foreign Key
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Your PRIMARY index should always be on the smallest number of columns that are sufficient to uniquely identify a row. Primary keys are defined through the PRIMARY KEY constraint and can be specified during the creation of the table. Foreign Key Constraints Some database management systems, such as SQL Server allow you to set up foreign key constraints.
A foreign key identifies a column or group of columns in one referencing table that refers to a column or group of columns in another referenced table — in our example above, the Employee table is the referenced table and the Employee Salary table is the referencing table. These help to enforce referential integrity. Fetching a particular data from those numerous records can be difficult at times or sometimes impossible.
Difference between Primary Key and Foreign Key - You would thenneed to determine which of these candidate keys would work best asyour primary key. The data that is stored on the server is a series of tables with columns.
Primary Key vs Candidate Key Though primary key is selected from the candidate keys there exist some difference between the primary key and the other candidate keys, which will be discussed in detail in this article. During this designing process, different with many relationships have to be created. In order to access these tables in a database, different types of keys are used in modern database designing languages such as , MSAccess, SQLite, etc. Out of these keys, candidate keys and primary keys have become essential in database designing practices. What is a Candidate Key? Candidate key is a single column or set of columns in a table of a database that can be used to uniquely identify any database record without referring to any other data. Each table of a database can have one or more than one candidate keys. A set of candidate keys can be created by using functional dependencies. There are some important features in a candidate key. The main purpose of a candidate key is to help to identify one single row out of millions of rows in a big table. Each candidate key is qualified to become a primary key. However, out of all the candidate keys, the most important and special candidate key will become the primary key of a table and it is the best among the candidate keys. What is a Primary Key? A primary key is the best candidate key of a table that is used to uniquely identify records that are stored in a table. When creating a new table in a database we are asked to select a primary key. Therefore, selection of a primary key for a table is the most critical decision that should be taken by a database designer. The most important constraint, which should be considered when deciding a primary key, is that the selected column of the table should only contain unique values, and it should not contain any NULL values. Some of the primary keys that are commonly used when designing tables are Social Security Number SSN , ID and National Identity Card Number NIC. The programmer should remember to choose a primary key carefully because it is difficult to change. Therefore, according to the programmers, the best practice of creating a primary key is to use an internally generated primary key such as Record ID created by AutoNumber data type of MS Access. If we try to insert a record into a table with a primary key that duplicates an existing record, the insertion will fail. The primary key value should not keep changing, so it is more important to keep a static primary key. A primary key is the best candidate key. What is the difference between Primary Key and Candidate Key? Therefore, the candidate key is not qualified for a primary key because the primary key should not contain any NULL values. Summary: Primary Key vs. Candidate Key Candidate key and primary key are essential keys that are used in designing databases in order to uniquely identify data in a record and make relationships among tables of a database. A table should contain only one primary key and may contain more than one candidate keys. Today, most databases are capable of auto generating their own primary key. Therefore, the primary key and candidate keys provide a numerous support to the.
This is one of the main. Foreign keys and referential integrity Foreign keys have a lot to do with the concept of referential integrity. Foreign Key Constraints Some database management systems, such as SQL Server allow you to set up foreign key constraints. The primary key consists of one or more columns whose data contained within is used to uniquely identify each row in the table. When creating a new table in a database we are asked to select a primary key. You must manually create an index on foreign keys. The Employee table below: Assuming that EmployeeID is the primary key, then we difference between primary and foreign key want to place a unique constraint on GovernmentNumber to ensure each employee has their own number. The primary key must be unique for each record as it acts as a unique identifier and it should not contain Null values. On the other hand, Foreign key doesn't create a clustered index in the table. It also makes it easy for foreign key values to refer back to corresponding primary key values, as we will learn about in the following section. Parent columns can have multiple child columns, but a child columncan only have one parent column. You can and should read a more in depth article on that concept here:.
