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  • 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

]]> https://www.wikitechy.com/interview-questions/dbms/what-is-joins-in-dbms/feed/ 0 What is the Difference between join and union ? https://www.wikitechy.com/interview-questions/sql/what-is-the-difference-between-join-and-union/ https://www.wikitechy.com/interview-questions/sql/what-is-the-difference-between-join-and-union/#respond Sun, 18 Jul 2021 20:27:46 +0000 https://www.wikitechy.com/interview-questions/?p=968

Union

    • The union operator combines the results of two or more queries into a single result set. But no.of columns must match in both/all the queries (and also the order) which are used for union.
    • We can union two tables irrespective of common field.
    • The UNION set operator is used for combining data from two tables which have columns with the same datatype.
      • Union – Returns with no duplicate rows.
      • Union all – Retruns with duplicate rows (No. of rows returned = No. of rows in Query1 + No. of rows in Query 2).
Union unionall except intersect
  • We cannot use the union operator within a create view statement.
  • We cannot use the union operator on text and image columns.
Union in SQL

Syntax

SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions]
UNION
SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions];

Join

  • Joins are used to extract information from more than one table based on the related column/coloums (PK and RFK) any no. of rows can be retrived based on matching colums.
  • We can join two tables by ‘join’ if they have common field.
  • A Join is used for displaying columns with the same or different names from different tables.
  • Different types of joins are Inner join=equi join=join , outer join (Right outer join/right join, Left outer join / left join), Cross join , and full outer join/outer join.
  • We can use join operator within a create view statement.
  • We can use the join operator on text and image columns.
Cross Join

Syntax

SELECT column-names
FROM table-name1 JOIN table-name2 
ON column-name1 = column-name2
WHERE condition
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