As far as I'm aware, you can only assign primary keys and unique columns to foreign keys... yet I have a table that has a primary key between two columns:
alter table NAME add constraint PK primary key(VALUE1, VALUE2)
I'm trying to make Value1 a foreign key in another table, but it's not recognizing it as a primary key or unique - obviously because the primary key is shared between two values... So what do I do from here? I'm pretty new to SQL syntax...
You are correct that you can only assign primary keys and unique columns to foreign keys. I am not much aware of the business requirement here but ideally, you should be having a third table which has the VALUE1 as a primary key. If not you should create one.
The main idea is that you can't link a foreign key to a value that can hold duplicates on the referenced table. So if your main table has a compound key (more than 1 column), linking the foreign key to one (or many but not all) of it's columns would be linking the table to more than one row (since that column might have duplicates by itself).
If you really need to establish the link between the two then you have a problem, either:
Your primary key isn't really 2 or more columns. You can read about normalizing your database (in standard normal forms) to solve this.
Your relationship between the tables isn't 1 to N (it's N to M). You can't add a foreign key, you will have to create a 3rd table with both primary keys to link them.
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
Is it advisable to have table A having two foreign keys from table B? My thinking is that if one of the foreign key in table A is a primary key, then there is no need to add additional foreign key in table A.
I could see it possibly being valid. If table B has a foreign key to A's Primary key, you are saying every B must point to some A. but you could also have a foreign key to a nullable field in A, meaning A must exist and have data for the second field in your key.
I'm attempting to setup a foreign key in table cell_lines that will reference the topographic_region column of the composite primary key in table topographic_regions.
Each time I run the last three lines of code trying to add the foreign key, I receive Error Code 1215: cannot add foreign key constraint.
Now, the foreign key column name (topographic_region) in cell_lines only matches one of the composite primary key column names in topographic_regions, the other composite primary key column name being topographic_region_id. Do I usually need to address both components of a composite primary key when creating a foreign key?
A follow up problem is that I've actually already tried addressing both components of a composite primary key using a composite foreign key constraint, and I was still presented with an Error Code 1215: cannot add foreign key constraint.
What can I do to solve this problem, and is there anymore information you would like me to provide in order to do so? I'm happy to respond.
Thanks for reading. I'm very new to mySQL.
create table topographic_regions(
topographic_regions_id int not null auto_increment,
topographic_region int(10),
karyotypes varchar(255),
constraint pk_topographicID primary key (topographic_regions_id, topographic_region)
);
create table cell_lines(
cell_lines_id int not null auto_increment,
cell_line varchar(50),
topographic_region int(10),
constraint pk_cellID primary key (cell_lines_id, cell_line)
);
alter table cell_lines
add foreign key (topographic_region)
references topographic_regions(topographic_region);
This is the problem with composite PKs. In fact, your autonumber topographic_region_id will be unique and you should use that for the PK, and the FK. topographic_region sounds like it is also unique so you should add a unique index to it.
A foreign key is some columns whose subrow values have to appear as subrow values in another table where they are a candidate key. If you really had a foreign key from cell_lines to topographic_regions then cell_lines would have a topographic_region_name column and you would need:
alter table cell_lines
add foreign key (topographic_regions_id, topographic_region)
references topographic_regions(topographic_regions_id, topographic_region);
I suspect that (topographic_regions_id, topographic_region) is not a candidate key of topographic_regions and that topographic_regions_id is enough to identify a region just because you decided cell_lines doesn't have a topographic_region column. Although it may be that a cell line doesn't identify a particular topographic region. But then topographic_regions_id is not a foreign key in cell_lines, since it isn't a key in topographic_regions. (There is no easy way in SQL to constrain that some columns' subrow values have to appear as subrow values in another table where they are not a superset of a candidate key.)
If topographic_regions_id is unique in topographic_regions then it is a candidate key and should be declared UNIQUE NOT NULL. If topographic_region_name is unique in topographic_regions then it is a candidate key and should be declared UNIQUE NOT NULL. If either is a candidate key then (topographic_regions_id, topographic_region) is not a candidate key. You can pick one candidate key to be declared PRIMARY KEY which just means UNIQUE NOT NULL. Ditto for cell_line. Since your _id columns are auto_increment, I suspect they are unique, in which case neither table has a composite candidate key.
(All this assuming none of your columns can be NULL.)
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.