I came across the following SQL in a book:
CREATE TABLE 'categories'(
id SMALLINT NOT NULL AUTO INCREMENT,
category VARCHAR(30) NOT NULL,
PRIMARY KEY('id'),
UNIQUE KEY 'category'('category')
)ENGINE=MyISAM DEFAULT CHARSET = utf8;
I was wondering is there a reason why I would need a PRIMARY and UNIQUE KEY in the same table? I guess, underlying that question is, what is the difference between PRIMARY and UNIQUE keys?
The relational model says there's no essential difference between one key and another. That is, when a relation has more than one candidate key, there are no theoretical reasons for declaring that this key is more important than that key. Essentially, that means there's no theoretical reason for identifying one key as a primary key, and all the others as secondary keys. (There might be practical reasons, though.)
Many relations have more than one candidate key. For example, a relation of US states might have data like this.
State Abbr Postal Code
--
Alabama Ala. AL
Alaska Alaska AK
Arizona Ariz. AZ
...
Wyoming Wyo. WY
It's clear that values in each of those three columns are unique--there are three candidate keys.
If you were going to build a table in SQL to store those values, you might do it like this.
CREATE TABLE states (
state varchar(15) primary key,
abbr varchar(10) not null unique,
postal_code char(2) not null unique
);
And you'd do something like that because SQL doesn't have any other way to say "My table has three separate candidate keys."
I didn't have any particular reason for choosing "state" as the primary key. I could have just as easily chosen "abbr" or "postal_code". Any of those three columns can be used as the target for a foreign key reference, too.
And as far as that goes, I could have built the table like this, too.
CREATE TABLE states (
state varchar(15) not null unique,
abbr varchar(10) not null unique,
postal_code char(2) not null unique
);
I'm surprised that nobody mentionned that a primary key can be referenced as foreign key into other tables.
Also an unique constraint allows NULL values.
The reason you need two uniqueness restrictions (one being the Primary Key) is that you are using Id as a surrogate key. I.e., it is an arbitrary value that has no meaning in relation to the data itself. Without the unique key (or colloquially known as "business key" i.e, a key that the user would recognize as being enforced), a user could add two identical category values with different arbitrary Id values. Since users should never see the surrogate key, they would not know why they are seeing a duplicate even though the database would think they are different.
When using surrogate keys, having another unique constraint on something other than the surrogate key is critical to avoid duplicate data.
Depending on who you talk to and how they read the specification, Unique keys( which is redundant by the way. A "key" is by definition unique) are also not supposed to allow nulls. However, one can also read the specifications as saying that Unique constraints, unlike Primary Key constraints, are in fact supposed to allow nulls (how many nulls are allowed also varies by vendor). Most products, including MySQL, do allow nulls in Unique constraints whereas Primary Key constraints do not.
Similarity
Both a PRIMARY and UNIQUE index create a constraint that requires all values to be distinct (1).
Difference
The PRIMARY key (implicitly) defines all key columns as NOT NULL; additionally, a table can only have one primary key.
(1) Each NULL value is considered to be distinct.
A UNIQUE constraint and PRIMARY key both are similar and it provide unique enforce uniqueness of the column on which they are defined.
Some are basic differences between Primary Key and Unique key are as follows.
Primary key
Primary key cannot have a NULL value.
Each table can have only single primary key.
Primary key is implemented as indexes on the table. By default this index is clustered index.
Primary key can be related with another table's as a Foreign Key.
We can generate ID automatically with the help of Auto Increment field. Primary key supports Auto Increment value.
Unique Constraint
Unique Constraint may have a NULL value.
Each table can have more than one Unique Constraint.
Unique Constraint is also implemented as indexes on the table. By default this index is Non-clustered index.
Unique Constraint cannot be related with another table's as a Foreign Key.
Unique Constraint doesn't support Auto Increment value.
You can find detailed information from: http://www.oracleinformation.com/2014/04/difference-between-primary-key-and-unique-key.html
Related
I found some legacy code, which sets two almost identical constraints (UNIQUE and PRIMARY KEY) for the primary key field.
Here is the code sample:
CREATE TABLE foofoo (
id NUMERIC(9) NOT NULL ,
bar VARCHAR(40) NOT NULL,
CONSTRAINT PK_foofoo PRIMARY KEY (id),
CONSTRAINT UNIQUE_foofoo UNIQUE(id)
)
I think it's redundant to have these two set and PRIMARY KEY would do the job.
Of course, I read what's the difference between these two constraints, but
what's the point of setting these two constraints for the same field?
There is no point in doing so. A primary key is always unique by nature. I would advise against making both indexes, as indexes comes with a cost (mainly disk space). Just create the PK and you'll be good!
There is no point on setting the exact same constraint as the PK.
A Primary Key is already making sure that this column is unique and indexed.
I think it's redundant ...
Yes indeed it's redundant; since having primary key constraint on the column anyways will make sure that the column has only unique value. There is no point in defining an extra UNIQUE constraint on the same column.
when you declare primary then:
*PRIMARY KEY constraint uniquely identifies each record in a database table
*Primary keys must contain UNIQUE values
so their is no need to declare primary key unique because whenever u declare anything primary key then UNIQUE value is already attached with them.
For unique key:
*The UNIQUE constraint uniquely identifies each record in a database table.
*The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.
*A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.
The most important point is that
*Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.
In mysql when i take same as primary key and Unique then it gave me error
I am working on a project and I realized I am unsure about how to use multiple primary keys. I have a table named "User_Details" that has the details of Customer ID, email address and password. From my understanding, I can use both Customer ID and email address as the primary key. In this case do I use only one as Primary Key or both? If I use both, do they become composite primary keys?
(PS. I have other tables, where the foreign key is the customer ID)
You can only have one primary key, but you could definitely have other unique fields.
Usually using an integer / id as primary key is preferred over a string key, and an id is presumably auto assigned, where as email could change - which would be a problem for foreign key relations.
Since you already use customer Id as a foreign key in other tables, I would suggest you continue to do that.
You can only have one primary key, but you can have multiple columns in your primary key, alternatively you can also have Unique Indexes on your table, which will work a bit like a primary key in that they will enforce unique values, and will speed up querying of those values.
Easiest way tho is a Composite Primary Key which is a primary key made from two or more columns. For example:
CREATE TABLE userdata (
userid INT,
userdataid INT,
info char(200),
primary key (userid, userdataid),
);
Here is more info: Link
You can have a Composite Primary Key which is a primary key made from two or more columns. For example:
CREATE TABLE userdata (
userid INT,
userdataid INT,
info char(200),
primary key (userid, userdataid),
);
A table can have multiple candidate keys. Each candidate key is a column or set of columns that are UNIQUE, taken together, and also NOT NULL. Thus, specifying values for all the columns of any candidate key is enough to determine that there is one row that meets the criteria, or no rows at all.
Candidate keys are a fundamental concept in the relational data model.
It's common practice, if multiple keys are present in one table, to designate one of the candidate keys as the primary key. It's also common practice to cause any foreign keys to the table to reference the primary key, rather than any other candidate key.
I recommend these practices, but there is nothing in the relational model that requires selecting a primary key among the candidate keys.
I am very confused right now, maybe you can help me to understand the problem better regarding the question that can a table have two primary keys if yes then how ? And if no, then why?
You ask if you can have more than one primary key field and you most certainly can. You can have only one primary key, but that can consist of as many columns as you need to uniquely identify your rows.
Use something like this when you are creating your table:
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
where P_Td and LastName are columns in your table.
If you think you want more than one primary key, then the answer is "not really." You can have only one primary key. However, you can have as many indexes as you want that have a unique constraint on them. A unique index does pretty much the same thing as a primary key.
for example :-
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
)
Note: In the example above there is only ONE PRIMARY KEY (pk_PersonID). However, the value of the pk_PersonID is made up of two columns (P_Id and LastName).
No You cannot have two primary keys in one table, but you can have composite primary key
Because Primary key is an identity to the row and there can't be two IDs against a row.
A table can have many keys but by convention only one key per table is designated a "primary" one. Typically this may be the key deemed to be the "preferred" identifier for the information in that table or it may be a key singled out for some other specific purpose by the table's designer.
In principle whatever function or property you associate with the key designated "primary" could just as well be associated with any other key as well. Therefore for many practical purposes you could designate more than one such "primary" key if you so choose - but only if the limitations of any particular DBMS permit.
Under the relational model of data all keys are equal and there is no special function given to primary keys (in fact the relational usage of the term primary key originally referred to any and all keys of a relation and not just one key). Unfortunately many DBMSs don't respect this principle and may limit certain features to one and only one key of a table, making it necessary to be selective about which key gets designated as primary. So the answer to your question ought to be YES in principle. When you need to achieve it in some particular SQL DBMS the actual answer is: it depends.
You can only have 1 primary key - the range of keys that could all potentially be the primary key can be referred to as candidate keys. The one you select is the primary key, the other alternative keys can be implemented as unique constraints / indexes.
So whilst there is only 1 primary key, you can still ensure primality of other fields / combination of fields using the unique constraint / index.
No.The table have only on primary key. But that primary key can contain multiple field. Means when you create table and when you mention primary key, you can add more then one column which you want to.
for example
CREATE TABLE table_name ( col1 Datatype , col2 Datatype,col3 Datatype, col4 Datatype, PRIMARY KEY (col1,col2,col3) )
By this way you can add primary key in single table
On a table you can make indexes, which allow the internal database engine to process the contents of the affected columns (1 to many) for easy lookup. Because the engine is at that point already evaluating and sorting the contents of the fields, it can also easily ensure uniqueness of the values. Thus an index can span 1 to many rows, and optionally also be unique.
A primary key is a theoretically optional, though in practice mandatory, marker on a specific index that it's the eternally unique way of referencing a specific row in the table. It's usually either a GUID or an auto-increment integer (identity in SQL Server). The primary key itself is unique for any given table, and enforces a unique constraint by definition, but can optionally span multiple rows (a spanned index/key).
You could for example have a junction table containing only 2 fields, which are both foreign keys, and together form the primary key/index of the table.
You Can try FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table.
No.You cannot use more than 1 primary key in the table.for that you have composite key which is combination of multiple fields.
As you cannot define more than one column as Primary Key as below
create table test1 (col1 numeric(10) primary key, col2 numeric(10) primary key
,col3 numeric(10) primary key, col4 numeric(10))
It needs to be a composite key. Yes, we can have more than one column as primary key to solve some business requirements. Primary Keys ensures that the column(s) will not have duplicate values , Null in the table.
Below is the SQL to create a table with Composite Primary Key
CREATE TABLE track(
col1 numeric(10) , col2 numeric(10) ,col3 numeric(10) primary key, col4 numeric(10),
PRIMARY KEY (col1,col2,col3)
)
CREATE TABLE track(
col1 numeric(10) , col2 numeric(10) ,col3 numeric(10) , col4 numeric(10),
PRIMARY KEY (col1,col2,col3)
)
I'm using a MySQL database.
In which situations should I create a unique key or a primary key?
Primary Key:
There can only be one primary key constraint in a table
In some DBMS it cannot be NULL - e.g. MySQL adds NOT NULL
Primary Key is a unique key identifier of the record
Unique Key:
Can be more than one unique key in one table
Unique key can have NULL values
It can be a candidate key
Unique key can be NULL ; multiple rows can have NULL values and therefore may not be considered "unique"
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:
PK creates a Clustered index and UK creates a Non Clustered Index.
PK is not null, but UK allows nulls (Note: By Default)
There can only be one and only one PK on a table, but there can be multiple UK's
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
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
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.
Primary Key
Unique Key
A primary key can't accept NULL values
Unique key can accept NULL values, so problematic in the context of being unique
A primary key cannot contain duplicate values
A unique key also cannot contain duplicate values
We can have only one primary key in a table
We can have more than one unique key in a table
We can make a primary key from one or more table fields
We can also make a unique key from one or more table fields
By default, a primary key creates a clustered index
By default, a unique key creates a non-clustered unique index
It is used to identify each record in the table
It prevents storing duplicate entries in a column
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.
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.
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.
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!
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
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
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.
Simply Primary Key is a unique and can't be null, unique can be null and may not be unique.
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:
There can only be one primary key for a table.
The primary key consists of one or more columns.
The primary key enforces the entity integrity of the table.
All columns defined must be defined as NOT NULL.
The primary key uniquely identifies a row.
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:
There can be multiple unique keys defined on a table.
Unique Keys result in NONCLUSTERED Unique Indexes by default.
One or more columns make up a unique key.
Column may be NULL, but on one NULL per column is allowed.
A unique constraint can be referenced by a Foreign Key Constraint.
source : here
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.
How many primary keys are possible in a table in MySQL database.
You can't have several of what is called "primary". The answer is: one. A primary key can contain several columns, though. Then it's what you called a "composite primary key"
For this kind of question, you will always find an answer in the manual:
http://dev.mysql.com/doc/refman/5.5/en/create-table.html
You can only have one primary key and it can be composed (or not). Also you can have many unique indexes which are logicaly identical to primary keys (but some functions are not aviable for them)
you should understand primary key as "the" first index of a table, on many RDBMS it is mandatory to have a primary key in order to have other indexes
You can only have one primary key. From the MySQL documentation:
A PRIMARY KEY is a unique index where all key columns must be defined
as NOT NULL. If they are not explicitly declared as NOT NULL, MySQL
declares them so implicitly (and silently). A table can have only one
PRIMARY KEY. If you do not have a PRIMARY KEY and an application asks
for the PRIMARY KEY in your tables, MySQL returns the first UNIQUE
index that has no NULL columns as the PRIMARY KEY.
You posted a comment about composite primary keys. I suggest doing some reading from the MySQL manual to learn about them http://dev.mysql.com/doc/refman/5.5/en/create-table.html. Plus there are question here on SO about composite primary keys you just have to look for them.
One, hence "primary". You can have other "unique" keys/indexes on a table that signify uniqueness in that column/columns (and would likely be referred to as a candidate key).
One.
You can however use several fields to construct the primary key, if that is what you are looking for.