Can we have primary key and unique key on same column?
A table can have only one primary key whereas there can be multiple unique key on a table. A Clustered index automatically created when a primary key is defined whereas Unique key generates the non-clustered index.
Can a column be primary and unique?
A primary key column cannot contain NULL values, whereas a unique key can have NULL values, but only one NULL is allowed in a table. A primary key should be unique, but a unique key cannot necessarily be the primary key.
What will happen if I keep primary key or unique on a column?
Since both primary key and unique columns do not accept duplicate values, they can be used for uniquely identifying a record in the table. This means that, for each value in the primary or unique key column, only one record will be returned.
Can we use unique key as primary key?
Purpose: Primary Key is used to uniquely identify a row but a unique key is used to prevent duplicate values in a column. Existence: A table can have only one primary key but it can have multiple unique keys.
What is the main difference between primary key and unique key?
The primary key is accepted as a unique or sole identifier for every record in the table. In the case of a primary key, we cannot save NULL values. In the case of a unique key, we can save a null value, however, only one NULL value is supported.
What is difference between primary key and unique key and foreign key?
A primary key is used to ensure data in the specific column is unique. A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables.
Is unique key and candidate key are same?
Candidate key is a set of columns which can uniquely identify the values in a table and can act as a unique key. One of these candidate keys will become the primary key and the rest will become alternate keys. Unique key as the name suggests , is used to uniquely identify a value in a table.
What is difference between unique key and unique index?
Unique Key: It is a constraint which imposes limitation on database. That limitation is it will not allow duplicate values . For example if you want to select one column as primary key it should be NOT NULL & UNIQUE. Unique Index: It is a index which improves the performance while executing queries on your data base.
What is unique key example?
A unique key is a set of one or more than one fields/columns of a table that uniquely identify a record in a database table. You can say that it is little like primary key but it can accept only one null value and it cannot have duplicate values.
What is difference between primary key and unique constraints?
PRIMARY KEY constraint differs from the UNIQUE constraint in that; you can create multiple UNIQUE constraints in a table, with the ability to define only one SQL PRIMARY KEY per each table. Another difference is that the UNIQUE constraint allows for one NULL value, but the PRIMARY KEY does not allow NULL values.
What is the difference between primary key and candidate key?
The difference here is that a primary key acts as a minimal super key. Thus, a relation can only have one primary key. On the other hand, multiple candidate keys (two or more) can take place in any relationship. The attributes in a candidate key may contain a NULL value that opposes the primary key in it.
What is the relation between primary key and candidate key?
Primary Key is a unique and non-null key which identify a record uniquely in table. A table can have only one primary key. Candidate key is also a unique key to identify a record uniquely in a table but a table can have multiple candidate keys.