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A primary key is the identifier in a table. It cannot contain values that are null, and it has to be unique for every record. For example, a driver's license number could be a primary key in a relational database table. Every driver is assigned to one unique identifier, or driver's license number, and no two driver's license numbers are identical.
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Difference between composite and primary key?
There are two types of keys in any database; composite and primary keys. Composite key differs from primary key in that it contains more than one column while primary key is composed of only one field and cannot have a null value.
What is Relationship between a primary and a foreign key?
A primary key is one or more colums in a table whose values would uniquely identify a row in that table. A foreign key is a one or more columns in one table that are used to reference rows in another table. In a properly designed 3NF schema, the foreign key columns should correspond to the primary key columns of the table being referenced.
Is a foreign key field always on the one side of a one-to-many relationship between two tables?
No. The foreign key is always on the many side. A foreign key is a field that is a primary key in another table, not in the table it is in. It can therefore be repeated in the table it is, so it can act as the many side. In its own table, it is the primary key, and only appears once.
Diffentiate between full and partial dependency?
Full Dependency:Given a relation R and functional dependency x->y (y is fully functionally dependent on x)there is no any z->y ,where Z is a proper subset of xPartial Dependency:If any proper subset of the key determine any of the non-key attributes then there exist a partial dependency p q->c d (p q is the primary key)p->cq->cp->dq->d
What are the difference between superkey and candidate key?
A superkey is defined as a set of attributes of a relation for which it holds that in all relations assigned to that variable there are no two distinct tuples (rows) that have the same values for the attributes in this set. As an example Code: Roll Number | First Name | Last Name CSU0001 | Shabbir | Bhimani | CSU0002 | SomeName | SurName | CSU0003 | Larry | page | Now here we have the following as super keys 1. Roll Number | First Name 2. Roll Number | First Name | Last Name Now in plain language Any Unique key with some Non Unique key combination is called a super key of the relationship. A candidate key of a relationship is a set of attributes of that relationship such that there are no two distinct tuples with the same values for these attributes. In simple example candidate key is a minimal superkey, i.e. a superkey of which no proper subset is also a superkey. Since a relation is a set(no duplicate elements), it holds that every relation will have at least one candidate key (because the entire heading is always a superkey). For practical reasons RDBMSs usually require that for each relation one of its candidate keys is declared as the primary key Quote: For example, Given an employee table consisting of the columns: employeeID name job and departmentID we could use the employeeID in combination with any or all other columns of this table to uniquely identify a row in the table. Examples of superkeys in this table would be {employeeID, Name}, {employeeID, Name, job}, and {employeeID, Name, job, departmentID}. In a real database we don't need values for all of those columns to identify a row. We only need, per our example, the set {employeeID}. This is a minimal superkey - that is, a minimal set of columns that can be used to identify a single row. So, employeeID is a candidate key. Now, if employeeID is a candidate key then why not it is the superkey. Because employeeID can also uniquely identify the tuples. (2) In your example why Roll number is not the superkey as it is uniquely identifying the tuples? (3) Quote: Any Unique key with some Non Unique key combination is called a super key of the relationship Is it necessary that Unique key has to be combined with some Non Unique key to be called as a super key. Unique key is also a super key but the minimal super key is called candidate key and all candidate keys are super keys but the reverse is not true.
