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

Different types of Joins are:

• Inner join/Simple Join
• Left Outer join/Left Join
• Right Outer Join/Right join
• Full Outer join
• Cross Join
• Self Join

Inner Join

• In SQL if a defined condition is valid the inner join will select all the matching rows and columns from both tables.

Syntax

Select column_1, column_2 column_3 from table_1 innerjoin table_2 ON table_1.column = table_2.column;

Left Join

• In left join all records are retrived from left table(table1) and matched rows and column from right table(table2).
• If there is no matching rows or columns, left join returns NULL.

Syntax

Select column_1, column_2, columns from Table_1 left join table_2 ON table_1.column_name = table_2.column name.

Right Join or Right Outer Join

• Records from the right table are retrived in RIGHT JOIN and the matched rows or columns from the left table(table1).
• If there is no matching rows or columns it will return NULL.

Syntax

Select column_1, column_2, cloumn(s) FROM table_1 right join table_2 ON table_1.column_name= table2.column_name.

Full Join or Full Outer Join

• This join is a combination of both Left join and right join.
• The joined tables returns all the records from both the tables.
• If no matches are found in the table, this join will return a NULL Value.
• It is also called as Full Outer Join.

Syntax

Select column_1, column_2, column(s) from table_1 Full join table_2 ON table_1.column_name = table_2.column_name;

Cross Join

• It is also known as Cartesian Join which returns cartesian product of two or more joined tables.
• Cross join produces a table that merges each rows from the first table with each rows of second table.
• No condition ir required for CROSS JOIN.

Syntax

Select * from table_1 cross join table_2;  
Or,
Select column1, column2, column3 from table_1, table_2; 

Self Join

• Tables formed by joining itself is called Self Join.
• It makes a temporary naming of atleast one table in SQL Statement.

Syntax

Select column1, column2, column(s) from table_1 Tbl1, table_2 Tbl2 where condition;  

]]> https://www.wikitechy.com/interview-questions/sql/what-is-a-join-in-sql-what-are-the-types-of-joins/feed/ 0 https://www.wikitechy.com/interview-questions/dbms/what-is-joins-in-dbms/ https://www.wikitechy.com/interview-questions/dbms/what-is-joins-in-dbms/#respond Fri, 03 Jun 2022 11:15:35 +0000 https://www.wikitechy.com/interview-questions/?p=4414

• DBMS join is a binary operation that allows to combine join product and selecting in one single statement.
• If data needs to be retrived 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:

• To return rows from both tables based on a satisfying condition we can use inner join.
• It is default join type and is the most widely used join operation.
• Inner join can classified into three subtypes
  • Theta Join
  • Natural Join
  • Equi Join

Theta Join:

• It is represented by symbol θ which allows us join two tables based on condition.
• This join works for all comparison operators
• This join is also called as 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 same name and domain in this join.
• In this join,two relations should have atleast one common attributes 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 equivalence condition, EQUI join can be used.
• This join is the most difficult join to be done on an RDBMS because RDBMS have essential performance problems.

Outer Join:

• This join returns all the attributes of both the tables depending on certain conditions.
• If there is no attributes present for any one of the tables it returns NULL irrespective of 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 for it 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