Joins in MS SQL Server

A database comprises tables and each table in case of RDBMS is called a relation.

Let us consider a sample database named University and it has two tables named Student and Marks. If a user wants to transfer a certain set of rows, insert into select statement is used along with the query. But if a user wants a whole set of rows from both tables or if a user wants to extract only a selected set of rows from both tables, joins are a perfect match to use in query.

Joins are clauses that extracts data from two tables to make a meaningful set of new data. Joins are used for a maximum of two tables only. The data that is extracted from tables forms a new table or relation that is different from previous tables that is being used for data extraction. Microsoft version of SQL supports different types of joins like – left join, right join, self join, inner join, full outer join, and cross join.

Example –
The two tables namely Student and Marks are of the same database University. If a user wants to join all rows from both tables, query must be given as –

select * 
from student full join marks

Table – Student

Name Rollno Course
Aisha 111 CSE
Mina 112 EEE
Shilpa 113 Mech

Table – Marks

Name Age GPA
Aisha 18 9.5
Mina 19 8.9
Shilpa 18 7.7

We can see that tables have been joined without missing out on a single column. To extract only a few rows, we can use other joins according to convenience.

Name Rollno Course Name Age GPA
Aisha 111 CSE Aisha 18 9.5
Mina 112 EEE Mina 19 8.9
Shilpa 113 Mech Shilpa 18 7.7

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