What is Join in DBMS and what are its types ?

• DBMS join is a binary operation that allows combining join products and selecting in one single statement.
• If data needs to be retrieved from two or more tables, we can use joins to retrieve data.
• Tables in DBMS are associated with the primary key and foreign keys.

Types of Joins

Inner Joins: Theta, Natural, EQUI

Outer Join: Left, Right, Full

Inner Joins

• We can use an inner join to return rows from both tables based on a satisfying condition.
• It is the default join type and is the most widely used join operation.
• Inner join can classify into three subtypes

Theta Join

• It is represented by the symbol θ which allows us to join two tables based on condition.
• This join works for all comparison operators
• This join is also called Theta Join.

Syntax

A ⋈θ B

• Theta join can use any conditions in the selection criteria.

Natural Join

• This join does not use any comparison operators.
• Attributes should have the same name and domain in this join.
• In this join, two relations should have at least one common attribute between relations.
• It performs selection forming equality on those attributes which appear in both relations by eliminating the duplicate attributes.

EQUI join

• When Theta join uses an equivalence condition, the EQUI join can be used.
• This join is the most difficult join to be done on an RDBMS because RDBMS have fundamental performance problems.

Outer Join

• This join returns all the attributes of both tables depending on certain conditions.
• If there are no attributes present for any one of the tables, it returns NULL irrespective of a row of the table attribute.
• Outer Join is further classified as:
  • Left Outer Join
  • Right Outer Join
  • Full Outer Join

Left Outer Join

• If a left outer join is performed on two tables, it returns all the rows of the left table, even if there is no matching row for it in the right table.

Syntax

A Left Outer Join B

Right Outer Join

• It returns all the rows of the second table even if there is no matching row in the first table, performing Right Outer Join.

Syntax

A Right Outer Join B

Full Outer Join

• It returns all the rows of the first and second Table.

Syntax

A Full Outer Join B