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SQL SERVER Questions And Answers Page 1

1. What is an Entity?

The basic data item stored in database is called entity. An entity can be any object, item, place, person, concept, or activity about which data is stored.

2. What is an attribute?

An attribute is a property of an entity. It describes a part of an entity. Entity could have one or more attributes.

3. What is ER diagram?

An Entity Relationship Diagram is diagrammatic representation of the logical structure of a database system.

4.Describe the concept of keys.

Candidate key
An attribute that uniquely identifies a row is called candidate key. It is also called das surrogate key.
Primary key
A candidate key that you choose to identify rows uniquely is called a primary key.
Alternate key
If there are multiple candidate keys in a table, the candidate keys that are chosen as primary key are called the alternate keys.
Composite key
When the key that uniquely identifies the rows of a table is made up of more than one attribute, it is called as a composite key.
Foreign key
Two tables can be related using a common attribute. When a primary key of one table is also available as an attribute in another related table it is called a foreign key.

5.What are joins?

Sometimes, data from multiple tables is to be displayed using select statement. For this purpose, the tables in use must have a column that is equated. This is termed as simple join or multiple join.Sometimes, you might want to display all records from one table and some from another. This type of join is called an outer join. An outer join is only possible between two tables.
There are two types of outer join, namely left and right. In a left outer join, all the rows of the first table named in the FROM clause are displayed. In a right outer join all the rows from the second table mentioned in the FROM clause are displayed. In either case, all the matching rows from other table are displayed.

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