I am reading about primary keys and at a lot of tutorials, technical blogs etc., and I found that a primary key cannot be null. I think it's totally wrong because I was able to insert null value in the column.
I suppose a primary key can have a not null value only if the column is declared as not null. But again this is not a feature of primary keys.
My question is why do we have a concept of primary key because I find only one difference between primary key and unique key is that "Primary key can be declared only on one column whereas unique key can be declared on multiple columns". So my understanding is that why can't we also declare the primary key as a unique key if we don't have any other difference.
I suppose a primary key can have a not null value only if the column
is declared as not null.But again this is not a feature of primary
key.
Primary key can't have a null values. By definition of primary key, it is UNIQUE and NOT NULL.
My another question is that why do we have a concept of primary key
because I find only one difference between primary key and unique key
is that "Primary key can be declared only on one column whereas unique
key can be declared on multiple columns"
This is completely wrong. You can create primary key on multiple columns also, the difference between Primary Key and Unique Key is Primary Key is not null and Unique key can have null values.
The main purpose of primary key is to identify the uniqueness of a row, where as unique key is to prevent the duplicates, following are the main difference between primary key and unique key.
Primary Key :
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 Key :
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.
I suggest you read this primary key and unique key
You forgot Indexing. When it comes to large data to find particular data raw it need to travel through memory record by record. To overcome that the concept of indexing is there. Primary key helps in this. So it will help to get your data access faster. After that there is concept of binary search which will helps further in that task.
A primary key is a special relational database table column (or combination of columns) designated to uniquely identify all table records.
A primary key’s main features are:
It must contain a unique value for each row of data.
It cannot contain null values.
A primary key is either an existing table column or a column that is specifically generated by the database according to a defined sequence.
The primary key concept is critical to an efficient relational database. Without the primary key and closely related foreign key concepts, relational databases would not work.
A primary key, also called a primary keyword, is a key in a relational database that is unique for each record.
One Table Have Only One Primary Key.
Related
(interview question)
if i have a table without primary key can i use unique key as a primary key or not ?
A primary key has three properties:
The key values uniquely define each row.
None of the key values are NULL.
There is only one per table.
A unique key satisfies the first of these conditions. If it satisfies the second, then it is a candidate primary key. And, if so, it could be chosen as the primary key if no other primary key is already defined. However, unique constraints allow NULL values, so this is not always true.
I am making a car rental system. I have a table with the information about cars and in the table I have 2 attributes one called VIN(which is a unique identification number) and I also have ULP(Unique License Plate), because they are both unique and you cannot have two primary keys in one table, will they both be classed together as Composite Primary Keys
No. More likely one will be the primary key and the other will be an alternate key.
A composite key is when the combination of two columns make the row unique. In your case you have two unique columns, which is not the same thing.
A primary key has three attributes:
It is never NULL.
It is unique.
There is only one per table.
Other keys (or combinations of keys) with these attributes are candidate primary keys. You can choose any of them that you want for the primary key of the table. Or, you can create a synthetic primary key yourself.
A composite primary key is when the primary key has more than one key. You could create a composite key from your two fields, but that does not seem necessary.
Instead, you have two candidate primary keys and you can choose either of them as the primary key for your table. Or declare the columns as NULL and unique and have an auto-incremented key.
How does one work around the issue of not matching an existig primary key or unique constraint when creating a FK in SSMS?
I have two tables namely : user and firstdb where both fields have the same datatype. In user table I have two primary keys, and in the foreign key table I have one field which I want to make it the target of a foreign key.
I tried to have one field as a primary key in the user table, the relationship worked fine at that time but once I set the second field to primary it forces me to delete the existing relationship for some reason.
I tried to create a relationship like this in MySQL, it worked fine with me, but it doesn't work.
You seem to have a profound misunderstand of what a primary key is. A primary key (singular) is one or more columns that uniquely identify each row (and none of the columns are NULL). When you click "primary key" by two columns, you are generating a composite primary key.
Foreign key references need to be to the complete primary key. If you have a composite primary key, you need a composite foreign key.
Having single column foreign key references is one reason why the best type of primary key is an identity column. I am guessing that userid is such a column.
If you want uEmailId to be unique and non-NULL, then define it as unique and non-NULL. However, don't add it to the primary key!
What are the differences between Unique Key, Primary Key and Foreign Key with respect to concept of SQL?
How they are different from each other?
A PRIMARY Key and UNIQUE Key constraints both are similar and it provide unique enforce uniqueness of the column on which they are defined.
Primary Key
Primary key cannot have a NULL value.
Each table can have only one primary key.
By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index.
Primary key can be related with another table's as a Foreign Key.
We can generated ID automatically with the help of Auto Increment field. Primary key supports Auto Increment value.
Unique Key
Unique Constraint may have a NULL value.
Each table can have more than one Unique Constraint.
By default, Unique key is a unique non-clustered index.
Unique Constraint can not be related with another table's as a Foreign Key.
Unique Constraint doesn't supports Auto Increment value.
Foreign Key
Foreign key is a field in the table that is primary key in another table.
Foreign key can accept multiple null value.
Foreign key do not automatically create an index, clustered or non-clustered. You can manually create an index on foreign key.
We can have more than one foreign key in a table.
There are actual advantages to having a foreign key be supported with a clustered index, but you get only one per table. What's the advantage? If you are selecting the parent plus all child records, you want the child records next to each other. This is easy to accomplish using a clustered index.
Having a null foreign key is usually a bad idea. In the example below, the record in [dbo].[child] is what would be referred to as an "orphan record". Think long and hard before doing this.
Note: we use constraint for enforce data integrity
Primary Key
1)can't insert null value
2) one table have one primary key
Unique key
1) insert null value one at time
2)one table have multiple unique key
3) you can refereed as candidate key also
foreign key
1) maintain the relationship between two table and also multiple
Note: without any constraint you get data in multiple table but you can not get data peoperly
A note about Unique key
The parent table in a Primary Key-Foreign Key relation is normally called as Primary Key table but PK is not mandatory in a parent table. A unique key/constraint in parent table is sufficient. As PK is always unique, it is often used as foreign key in another table. see this SO post
I am very confused in difference between unique key and primary key, and unique key constraint and primary key constraint in SQL Server.
Can I define more than 1 primary key in my table?
Again I am not getting how to set unique key in my table and what is the practical use of it. Second thing, I found statement like this on internet
A PRIMARY KEY constraint automatically has a UNIQUE constraint
defined on it.
Note that you can have many UNIQUE constraints per table, but only one
PRIMARY KEY constraint per table.
Does it mean that when I create primary key, it automatically define the unique key on that, that is I can have only 1 null value inserted in it?
your primary key is always unique. you can add additional keys and the may or may not be unique. If i am not wrong, the primary key decides how the records are actually stored on the disk.
You cannot have more than one primary key on a table, but you can have as many (within reasonable technical limits) unique constraints on a table.
The primary key cannot contain nulls, a unique constraint does allow nulls to occur, but treats NULLs as values in this case (so for a single column constraint, only one null value can occur).
Both Primary Keys and Unique constraints may be the referenced key for a foreign key constraint.
Many relational purists believe that the introduction of "Primary Key" in SQL was a mistake - there's no real reason to select one key as "better" than any other key - if there are genuinely multiple keys for the table. You could abandon primary keys entirely and only use UNIQUE constraints, and treat all keys equally.
Ok:
you can have only one primary key for each table
the primary key is by definition unique and does not allow NULL values
On the other hand:
you can define as many unique indices as you like
The primary key is a very special thing - it defines the way to clearly and uniquely identify each row. Therefore it must be unique, and it cannot be NULL.
The unique index is just that - a mechanism to ensure something (a column, several columns toegther) are unique. You can have multiple of those constraints on a table.
Each unique index could be your primary key - it's a so-called candidate key. But only one of those can be picked and chosen to actually be your primary key.