[mysql] difference between primary key and unique key

I'm using mysql database. I have a confusion between primary key and unique key.

Please help me where should I create primary and unique key. I mean in which situation we create unique key or primary key .

I know this question is several years old but I'd like to provide an answer to this explaining why rather than how

Purpose of Primary Key: To identify a row in a database uniquely => A row represents a single instance of the entity type modeled by the table. A primary key enforces integrity of an entity, AKA Entity Integrity. Primary Key would be a clustered index i.e. it defines the order in which data is physically stored in a table.

Purpose of Unique Key: Ok, with the Primary Key we have a way to uniquely identify a row. But I have a business need such that, another column/a set of columns should have unique values. Well, technically, given that this column(s) is unique, it can be a candidate to enforce entity integrity. But for all we know, this column can contain data originating from an external organization that I may have a doubt about being unique. I may not trust it to provide entity integrity. I just make it a unique key to fulfill my business requirement.

There you go!

Simply Primary Key is a unique and can't be null, unique can be null and may not be unique.

A primary key’s main features are:

It must contain a unique value for each row of data. It cannot contain null values. Only one Primary key in a table.

A Unique key’s main features are:

It can also contain a unique value for each row of data.

It can also contain null values.

Multiple Unique keys in a table.

difference between Primary Key and Unique Key

Both Primary key and Unique Key are used to uniquely define of a row in a table. Primary Key creates a clustered index of the column whereas a Unique creates an unclustered index of the column.

A Primary Key doesn’t allow NULL value, however a Unique Key does allow one NULL value.

Primary Keys

The main purpose of the primary key is to provide a means to identify each record in the table.

The primary key provides a means to identity the row, using data within the row. A primary key can be based on one or more columns, such as first and last name; however, in many designs, the primary key is an auto-generated number from an identity column.

A primary key has the following characteristics:

1. There can only be one primary key for a table.
2. The primary key consists of one or more columns.
3. The primary key enforces the entity integrity of the table.
4. All columns defined must be defined as NOT NULL.
5. The primary key uniquely identifies a row.
6. Primary keys result in CLUSTERED unique indexes by default.

Unique Keys

A unique key is also called a unique constraint. A unique constraint can be used to ensure rows are unique within the database.

Don’t we already do that with the primary key? Yep, we do, but a table may have several sets of columns which you want unique.

In SQL Server the unique key has the following characteristics:

1. There can be multiple unique keys defined on a table.
2. Unique Keys result in NONCLUSTERED Unique Indexes by default.
3. One or more columns make up a unique key.
4. Column may be NULL, but on one NULL per column is allowed.
5. A unique constraint can be referenced by a Foreign Key Constraint.

source : here

A primary key must be unique.

A unique key does not have to be the primary key - see candidate key.

That is, there may be more than one combination of columns on a table that can uniquely identify a row - only one of these can be selected as the primary key. The others, though unique are candidate keys.

Unique key :- It should be used when you have to give unique value.In the case of unique key it means null values are also allowed.Unique keys are those keys which are unique and non similar in that column like for example your pet name.it can be nothing like null and if you are asking in context of database then it must be noted that every null is different from another null in the database.EXCEPT-SQL Server where null=null is true

primary key :- It should be used when you have to give uniquely identify a row.primary is key which unique for every row in a database constraint is that it doesn't allow null in it.so, you might have seen that the database have a column which is auto increment and it is the primary key of the table. plus it can be used as a foreign key in another table.example can be orderId on a order Table,billId in a bill Table.

now coming back to situation when to use it:-

1) primary key in the column which can not be null in the table and you are using as foreign key in another table for creating relationship

2) unique key in table where it doesn't affect in table or in the whole database whether you take the null for the particular column like snacks in the restaurant it is possible you don't take snacks in a restaurant

If your Database design is such that their is no need of foreign key, then you can go with Unique key( but remember unique key allow single null value ).

If you database demand foreign key then you leave with no choice you have to go with primary key.

To see the difference between unique and primary key visit here

For an organization or a business, there are so many physical entities (such as people, resources, machines, etc.) and virtual entities (their Tasks, transactions, activities). Typically, business needs to record and process information of those business entities. These business entities are identified within a whole business domain by a Key.

As per RDBMS prospective, Key (a.k.a Candidate Key) is a value or set of values that uniquely identifies an entity.

For a DB-Table, there are so many keys are exist and might be eligible for Primary Key. So that all keys, primary key, unique key, etc are collectively called as Candidate Key. However, DBA selected a key from candidate key for searching records is called Primary key.

Difference between Primary Key and Unique key

1. Behavior: Primary Key is used to identify a row (record) in a table, whereas Unique-key is to prevent duplicate values in a column (with the exception of a null entry).

2. Indexing: By default SQL-engine creates Clustered Index on primary-key if not exists and Non-Clustered Index on Unique-key.

3. Nullability: Primary key does not include Null values, whereas Unique-key can.

4. Existence: A table can have at most one primary key, but can have multiple Unique-key.

5. Modifiability: You can’t change or delete primary values, but Unique-key values can.

For more information and Examples:

http://dotnetauthorities.blogspot.in/2013/11/Microsoft-SQL-Server-Training-Online-Learning-Classes-Integrity-Constraints-PrimaryKey-Unique-Key_27.html

Difference between Primary Key and Unique Key

+-----------------------------------------+-----------------------------------------------+
|                Primary Key              |                    Unique Key                 |
+-----------------------------------------+-----------------------------------------------+
| Primary Key can't accept null values.   | Unique key can accept only one null value.    |
+-----------------------------------------+-----------------------------------------------+
| By default, Primary key is clustered    | By default, Unique key is a unique            |
| index and data in the database table is | non-clustered index.                          |
| physically organized in the sequence of |                                               |
| clustered index.                        |                                               |
+-----------------------------------------+-----------------------------------------------+
| We can have only one Primary key in a   | We can have more than one unique key in a     |
| table.                                  | table.                                        |
+-----------------------------------------+-----------------------------------------------+
| Primary key can be made foreign key     | In SQL Server, Unique key can be made foreign |
| into another table.                     | key into another table.                       |
+-----------------------------------------+-----------------------------------------------+

You can find detailed information from:
http://www.dotnet-tricks.com/Tutorial/sqlserver/V2bS260912-Difference-between-Primary-Key-and-Unique-Key.html

A Primary key is a unique key.

Each table must have at most ONE primary key but it can have multiple unique key. A primary key is used to uniquely identify a table row. A primary key cannot be NULL since NULL is not a value.

A primary key has the semantic of identifying the row of a database. Therefore there can be only one primary key for a given table, while there can be many unique keys.

Also for the same reason a primary key cannot be NULL (at least in Oracle, not sure about other databases)

Since it identifies the row it should never ever change. Changing primary keys are bound to cause serious pain and probably eternal damnation.

Therefor in most cases you want some artificial id for primary key which isn't used for anything but identifying single rows in the table.

Unique keys on the other hand may change as much as you want.

Unique Key (UK): It's a column or a group of columns that can identify a uniqueness in a row.

Primary Key (PK): It's also a column or group of columns that can identify a uniqueness in a row.

So the Primary key is just another name for unique key, but the default implementation in SQL Server is different for Primary and Unique Key.

By Default:

1. PK creates a Clustered index and UK creates a Non Clustered Index.
2. PK is not null, but UK allows nulls (Note: By Default)
3. There can only be one and only one PK on a table, but there can be multiple UK's
4. You can override the default implementation depending upon your need.

It really depends what is your aim when deciding whether to create a UK or PK. It follows an analogy like "If there is a team of three people, so all of them are peers, but there will be one of them who will be a pair of peers: PK and UK has similar relation.". I would suggest reading this article: The example given by the author may not seem suitable, but try to get an overall idea.

http://tsqltips.blogspot.com/2012/06/difference-between-unique-key-and.html

• Think the table name is employe.
• Primary key
• Primary key can not accept null values. primary key enforces uniqueness of a column. We can have only one Primary key in a table.
• Unique key
• Unique key can accept null values. unique key also enforces uniqueness of a column.you can think if unique key contains null values then why it can be unique ? yes, though it can accept null values it enforces uniqueness of a column. just have a look on the picture.here Emp_ID is primary and Citizen ID is unique. Hope you understand. We can use multiple unique key in a table.