Difference between Key Primary Key Unique Key and Index in MySQL

Question

When should I use KEY  PRIMARY KEY  UNIQUE KEY and INDEX

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Unique Key     More than one value can be null  No two tuples can have same values in unique key  One or more unique keys can be combined to form a primary key  but not vice versa    Primary Key   Can contain more than one unique keys  Uniquely represents a tuple

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PRIMARY KEY AND UNIQUE KEY are similar except it has different functions  Primary key makes the table row unique  i e  there cannot be 2 row with the exact same key   You can only have 1 primary key in a database table   Unique key makes the table column in a table row unique  i e   no 2 table row may have the same exact value   You can have more than 1 unique key table column  unlike primary key which means only 1 table column in the table is unique    INDEX also creates uniqueness  MySQL  example  will create a indexing table for the column that is indexed  This way  it s easier to retrieve the table row value when the query is queried on that indexed table column  The disadvantage is that if you do many updating deleting create  MySQL has to manage the indexing tables  and that can be a performance bottleneck    Hope this helps

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Unique Keys  The columns in which no two rows are similar  Primary Key  Collection of minimum number of columns which can uniquely identify every row in a table  i e  no two rows are similar in all the columns constituting primary key   There can be more than one primary key in a table  If there exists a unique-key then it is primary key  not  the  primary key  in the table  If there does not exist a unique key then more than one column values will be required to identify a row like  first name  last name  father name  mother name  can in some tables constitute primary key   Index  used to optimize the queries  If you are going to search or sort the results on basis of some column many times  eg  mostly people are going to search the students by name and not by their roll no   then it can be optimized if the column values are all  indexed  for example with a binary tree algorithm

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Primary key - we can put only one primary key on a table into a table and we can not left that column blank when we are entering the values into the table   Unique Key - we can put more than one unique key on a table and we may left that column blank when we are entering the values into the table      column take unique values  not same  when we applied primary  amp  unique key

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Primary key does not allow NULL values  but unique key allows NULL values   We can declare only one primary key in a table  but a table can have multiple unique keys  column assign

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KEY and INDEX are synonyms in MySQL  They mean the same thing  In databases you would use indexes to improve the speed of data retrieval  An index is typically created on columns used in JOIN  WHERE  and ORDER BY clauses    Imagine you have a table called users and you want to search for all the users which have the last name  Smith   Without an index  the database would have to go through all the records of the table  this is slow  because the more records you have in your database  the more work it has to do to find the result  On the other hand  an index will help the database skip quickly to the relevant pages where the  Smith  records are held  This is very similar to how we  humans  go through a phone book directory to find someone by the last name  We don t start searching through the directory from cover to cover  as long we inserted the information in some order that we can use to skip quickly to the  S  pages   Primary keys and unique keys are similar  A primary key is a column  or a combination of columns  that can uniquely identify a row  It is a special case of unique key  A table can have at most one primary key  but more than one unique key  When you specify a unique key on a column  no two distinct rows in a table can have the same value   Also note that columns defined as primary keys or unique keys are automatically indexed in MySQL

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The primary key is used to work with different tables  This is the foundation of relational databases  If you have a book database it s better to create 2 tables - 1  books and 2  authors with INT primary key  id   Then you use id in books instead of authors name    The unique key is used if you don t want to have repeated entries  For example you may have title in your book table and want to be sure there is only one entry for each title

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KEY and INDEX are synonyms   You should add an index when performance measurements and EXPLAIN shows you that the query is inefficient because of a missing index  Adding an index can improve the performance of queries  but it can slow down modifications to the table    You should use UNIQUE when you want to contrain the values in that column  or columns  to be unique  so that attempts to insert duplicate values result in an error   A PRIMARY KEY is both a unique constraint and it also implies that the column is NOT NULL  It is used to give an identity to each row  This can be useful for joining with another table via a foreign key constraint  While it is not required for a table to have a PRIMARY KEY it is usually a good idea

[mysql] Difference between Key, Primary Key, Unique Key and Index in MySQL

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