What is the difference between Primary and Unique key (in MySQL)? How these can be considered as a foreign key? please explain.
I tried creating a database table and don't know how to make a primary key as a foreign key. Does it take Joins concept where the separating attribute automatically create a foreign key?
A table in MySQL can only have one Primary Key at most, while you can create as many unique Keys or indexes as you want.
Also a primary key is not nullable while a unique key can have NULL as value.
But the biggest difference is the purpose:
You want to have a primary key because you need an identifier
The unique Key/Index on the other hand is usefull to controll values that are automatically getting inserted into your table (e.g. to avoid duplicates where none are allowed)
If you want to use a column as a forgein key, you need to define it as a primary key first. Unique Constraint can not be related with other tables as a foreign key.
Related
I'm trying to understand something about the foreign key of a database.
I couldn't find a clear answer for this.
The definition of a foreign key goes something like this,
"Foreign key is a field or a group of fields in a table that refers to the primary key of another table"
OK I get that.
Then we learn about types of relationships we can have among tables using foreign keys.
In one-to-many(1:M) and many-to-many(M:N) relationships, there are duplicates in the foreign key.
But how can this be?
If a foreign key refers to a primary key and a primary key can't have duplicates, how can a foreign key can?
I'm confused.
Can someone please explain this to me?
Note - I'm expecting a common answer, not a DBMS specific one.
For example:
There is an employee table with a primary key column called EmployeeID. There is also a foreign key called SupervisorID, which references a different row in the same table.
Two employees work for the same supervisor. They will have different values in the EmployeeID field, but the same value in the SupervisorID field.
This example uses a reference back to the same table, but the concept is the same when the foreign key references a different table.
Primary key can be clustered or non clustered. It is unique and not null.
Suppose if I create an unique index/constraint and keyword NOT NULL then is this technically the same as primary key?
It depends what you mean by "technically" the same. The physical index is identical with a primary key, unique constraint, or unique index. A foreign key can reference a unique constraint or unique index as well as a primary key.
Differences that come to mind are:
Only one primary key is allowed per table
SQL Server will enforce the NOT NULL requirement for primary key columns but not for unique constraints/indexes
Unique indexes allow additional options such as INCLUDE and WHERE clauses, although cannot be referenced by FK constraints when a WHERE clause is specified
Let's say I create a table Clients. I define a primary key and a set of constraints, such as:
NOT NULL
Length > 5
UPPERCASE
and so on..
Now, I create another table, with a foreign key to Clients primary key.
Should I create the same CONSTRAINTS for the foreign key?
If I don't it wouldn't matter, since the value won't exist on the primary table in the first place:
Example: I don't create the constraints on the foreign key, and I try to add a value which length is lower than 5 characters, and is lowercase... The database will not find that value on the parent table, hence the value will not be recorded, so what is the point of setting the same set of constraints on the foreign table?
At least you should keep the Foreign key column as not null. Otherwise, you can have many NULL values coming into the child table.
Again, as #Dale Burrell, mentioned, PRIMARY KEY should be system generated, to enforce uniqueness. If you are going to create clustered index on the primary key column, it should be narrow, incrementing, not null, unique value for getting good performance.
You don't need the length or uppercase constraints on the foreign key. They'll be "implicitly checked" by the foreign key constraint (as you say, because the referenced data cannot exist).
But for nullability, it's a choice. In SQL Server, if one or more columns in the referencing table of a foreign key are NULL, the constraint isn't enforced.
So here, the question should instead be - are there rows in the other table which should validly not reference a row in the Clients table?
Others have advised that the PK should be system generated. Whilst I agree that it's often useful to do so, don't forget to also enforce the constraints on the real data. E.g. even if this column doesn't end up being your PK, maybe it needs a unique constraint on it to ensure that you don't end up with duplicates in your data.
Please correct if im wrong. And kindly point me to articles on this concept.
When we create a primary key, in the background there is automatically a unique index, clustered index, and a not null constraint created on that coloumn.
Does this also mean that if we create a not null constraint, [clustered index or non clustered index] and unique index on a column, then that column becomes a primary key?
I want to understand the core concept/relation between primary key, index and constrains.
The primary key is the one that is declared as the "primary" key. Just having the characteristics doesn't make a key "primary". It has to be explicitly declared as such.
Different databases implement primary keys in different ways. Although primary keys are usually implemented with a clustered unique index, that is not a requirement.
The primary key is exactly what its name suggests: "primary". Any other column or group of columns can be declared both unique and not null. That does not make them primary keys. In some databases, you could even define another column or group of columns as not null, unique and clustered -- without that being the primary key.
In summary:
You can have any number of unique indexes on a table.
You can have any number of unique indexes on non-NULL columns on a table.
You can have at most one clustered index. In almost all cases, this would be the primary key. But is not required in all databases.
You can have at most one primary key. In almost all cases, this would be clustered, although that is not required in all databases.
For more detail, you should refer to the documentation of the database you are using.
If you have multiple columns comprising non-NULL, unique keys, then only one is "primary" -- that one that has been explicitly declared as primary.
Why would you have a non-clustered primary key? I can give one scenario. Imagine a database where UUIDs are the keys for rows. The company does not want to use auto-generated sequence numbers, because they provide information in the number.
However, UUIDs are remarkably bad candidates for clustered indexes, because inserts are almost never at the end. In this case, you might want to design the table with a clustered auto-generated sequential key, to speed inserts You might make this key the primary key. But, you want all foreign key references to use the UUID -- and you want all foreign key references to be to the primary key of the table.
No.
All the columns could be added with Not null and Non-clustered index and Unique But only ONE column could be PK.
And the Unique allows NULL while Primary Key does not.
You might be talking about Candidate Key, here is the ref:
https://www.techopedia.com/definition/21/candidate-key
Is the primary key also a super key and a candidate key? Their definitions are lengthy but I wonder if this is true?
Please note that I'm not asking if they are the same term. I'm just asking in one direction, not the other way round.
Super Key - is a set of one or more columns that can be used to identify a record uniquely in a table
Candidate Key – can be any column or a combination of columns that can qualify as a unique key in database. There can be multiple Candidate Keys in one table. Each Candidate Key can qualify as a Primary Key. You can think of this as the "shortest" super key or minimal super key
Primary Key – is a column or a combination of columns that uniquely identify a record. Only one Candidate Key can be Primary Key.
For a Candidate Key to qualify as a Primary Key, it should be unique and non-null.
So, basically a primary key is just one of the candidate keys, which is a just a minimal super key.
According to dry definitions:
Your primary key is a super key by definition - you can not have two rows with the same primary key.
However, the primary key is not a natural constraint of your business, but an artificial constraint in your data store: for example, you could set a person's birthday as the primary key in your table, and never have two people who were born on the same day. That would be silly, but possible. In that case, the primary key of the table is not a super key of the domain.
However, your primary key is not necessarily a candidate key - you can add redundant columns to your primary key.
Different set of attributes which are able to identify any row in the database is known as super key. And minimal super key is termed as candidate key i.e. among set of super keys one with minimum number of attributes. Primary key could be any key which is able to identify a specific row in database in a unique manner. from this thread
And typing all three keys in google gives about 2,480,000 results
It depends.
The Primary key is the main key the table uses to identify between different elements. It is chosen from the candidate keys.
The candidate keys are all the keys that COULD be the primary key. All the keys that are unique and can be differentiated upon in the table.
The super key is a primary key with additional attributes, this extra information is used to uniquely identify an instance of the entity set.
A candidate key is the most minimal subset of fields that uniquely identifies a tuple. For example if you have a candidate key on the column "user_id" and "pet_id" you'll never have more than 1 tuple with the same user_id and pet_id and neither user_id nor pet_id individually will work as a unique identifier for the tuple.
A super key is a set of fields that contains a key. Using the above example where the combination of "user_id" and "pet_id" uniquely identifies a tuple if we added "pet_name" (which is not key because we can have multiple pets named "fluffy") it would be a super key. Basically it's like a candidate key without the "minimal subset of fields" constraint.
A primary key is a candidate key that you tell the DB to optimize on. There might be multiple ways of referring to a unique tuple (ie. multiple candidate keys) but you can specify one when you're creating the table that you will use the most frequently.
Yes we can simply say that a candiate key is primary key but it must be unique.