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Difference Between Join and Union in SQL

The SQL JOIN and UNION constructs are used to perform data retrieval operations on two or more tables.The article covers all information and the differences between JOIN and UNION.

Table of Contents

  1. What is the Union clause?
  2. Types of Union Clause in SQL
  3. What is the Join clause?
  4. Types Of Join Clause
  5. Differences Between JOIN and UNION In SQL

In an RDBMS (relational database management system), clauses are used to conduct actions on several tables (RDBMS). They generate a result by integrating information from more than one table. However, the methods for merging information from two or more relations vary in both sections. There are various methods for combining data from multiple sources. One of these choices is whether to employ Joins or Unions. But, before you choose over the other, one needs to grasp how they integrate data. Before establishing a comparison, let’s take a quick look at these clauses.

What is the Union clause?

The MySQL Union clause allows us to aggregate two or even more relations into a single result by performing several SELECT queries. It features a feature that removes duplicate rows from the result set by default.

The Union Clause in SQL must adhere to the following guidelines:

  • The arrangement and number of columns must be identical in all tables.
  • Each select query’s data type must be consistent with the appropriate locations.
  • In the SELECT queries, the column names must be in the same sequence.

Syntax

SELECT column_name(s) FROM table-name1    

UNION    

SELECT column_name(s) FROM table-name2;  

UNION Clause example

SELECT column list FROM tableA

UNION SELECT column list FROM tableB:

Types of Union Clause in SQL

UNION Clause is of two major types that are: the UNION and the UNION ALL. This enables us to create numerous SELECT queries, acquire the relevant results, and then merge them into a single, unified set. The basic difference is

  • UNION: only retains unique records.
  • UNION ALL: keeps all records.

What is the Join clause?

In MySQL, join is used with the SELECT command. To obtain information from several tables. It is used if we need to get records from many tables. It only returns entries from the tables which meet the stated criteria.

Syntax

SELECT column_name(s) FROM table_name1     

JOIN table_name2  ON conditions; 

JOIN Clause Example

SELECT column list FROM TableA

INNER JOIN TableB ON join condition;

Types Of Join Clause

This clause can be  LEFT JOIN, INNER JOIN FULL OUTER JOIN, or RIGHT JOIN.

  • INNER JOIN: 

The INNER JOIN command returns records or rows with identical values and is utilised to get data from both tables.

  • LEFT OUTER JOIN: 

A LEFT OUTER JOIN returns the matched rows from both tables. If no records from the left table match, it displays those with null values.

  • RIGHT OUTER JOIN:

The RIGHT  JOIN functions similarly to the LEFT  JOIN. A RIGHT OUTER JOIN picks records from the right side of the table and compares them to rows from the left table. The RIGHT JOIN produces a result set containing all rows there on the right side of the table, regardless of whether they have corresponding rows in the left table.

  • SELF JOIN:

The SELF JOIN function joins a database to itself. The SELF JOIN is equivalent to joining two copies of a single table. Although the table isn’t copied, SQL executes the operation as if it were. Self-join is performed by utilising table-named aliases to assign a unique name to each version of the table. It’s ideal for extracting data structures or comparing rows in the same database.

  • CROSS JOIN:

The SQL CROSS JOIN statement, widely renowned as a cartesian join, retrieves all row combinations from each table. Assume you need to locate all size and colour combinations. A CROSS JOIN would be useful in this scenario. one should note that this join doesn’t require any conditions to join two tables. CROSS JOIN combines every row from the initial table with every row from the second table, and the result includes all combinations of data from the two tables.

Differences Between JOIN and UNION In SQL

SNO.JOINUNION
1.In JOIN, data is combined to create new columns.IN UNION, data is combined to create new rows.
2.Data from several tables are combined using JOIN depending on matching criteria.In UNION, the results of two or even more SELECT queries are combined using SQL.
3.In JOIN, Each table’s related columns can have multiple data types.IN UNION, the datatypes of the relevant columns chosen from each table must match.
4. There may not be an equal number of columns chosen from each table.There should be a consistent number of columns chosen from each table.
5. It might not produce separate columns.IN UNION, separate rows are returned.
6.The LEFT, RIGHT, FULL OUTER, and INNER JOIN clauses are the four main varieties of JOIN clauses.The UNION clause has two major types: the UNION and UNION ALL.

To integrate information into a data result, utilise both joins & unions. Both of them approach this in different ways. A join is used to integrate the columns of several tables. The union merges the rows of many tables.