Difference between JOIN and UNION in SQL

By Priyanshu Vaish|Updated : September 28th, 2022

A significant Difference between JOIN and UNION in SQL is that using the JOIN clause, the tuples form by combining the attributes of two given relations, whereas the UNION clause combines the output of two queries. The result from JOIN and UNION may not be the same.

Difference between JOIN and UNION in SQL PDF

JOIN and UNION has widely used clauses. In this article, we have discussed the difference between JOIN and UNION in SQL on various factors along with that, we have seen the functionality of the JOIN and UNION individually.

Difference Between JOIN and UNION in SQL

The simple difference between JOIN and UNION in SQL is that JOIN combines the two tables into new columns, whereas UNION adds the data in two tables into new rows.

Key Differences Between JOIN and UNION in SQL

JOIN

UNION

When performing a JOIN, both tables do not require the same number of columns.

When performing a UNION, select statements must have the same number of columns.

The datatypes of the columns do not have to be the same when performing a JOIN.

When performing a UNION, the datatypes of the columns should be the same.

JOIN connects tables based on their logical relationship to one another.

UNION is used to combine multiple select queries into a single result set.

The JOIN clause is classified into four types: LEFT, RIGHT, FULL OUTER, and INNER JOIN.

There are two major types of the UNION clause- the UNION and the UNION ALL.

JOIN is commonly used to combine data from multiple tables.

UNION is commonly used to combine data from multiple select queries.

JOIN and UNION in SQL

JOIN and UNION are two important clauses in SQL. Both are used to combine data from two or more tables. The usage of each clause differs according to the usage of the clause. Let us understand both JOIN and UNION clauses in SQL in detail.

What is JOIN in SQL?

You can retrieve data from two or more tables using JOIN based on logical relationships between the tables. JOIN specifies how SQL Server should use data from one table to select rows from another. A JOIN condition specifies how two tables in a query are related by:

  • Choosing which column from each table will be used for the JOIN. A common JOIN condition specifies a foreign key from one table and its corresponding key in the other table.
  • Specifying a logical operator (such as = or >) to be used when comparing column values.

What is UNION in SQL?

The UNION operator joins the results of two or more SELECT command queries into a single distinct result set. This operator removes any duplicates from the combined results. The syntax used for the UNION in SQL is as follows.

SELECT column_1, column_2,...column_n

FROM table_1

UNION

SELECT column_1, column_2,...column_n

FROM table_2;

The UNION operator does not permit duplicates, but we can use the UNION ALL clause in SQL if we want duplicates to be present in two or more combinations of SELECT statements.

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FAQs on Difference between JOIN and UNION in SQL

  • The major difference between JOIN and UNION in SQL is that we form tuples by combining the attributes of two given relations using the JOIN clause. However, when we want to combine the results of two queries, we use the UNION clause.

  • The difference between JOIN and UNION in SQL regarding the types is that the JOIN clause is classified into four types: LEFT, RIGHT, FULL OUTER, and INNER JOIN. On the other hand, there are two major types of the UNION clause- the UNION and the UNION ALL.

  • The difference between JOIN and UNION regarding the datatypes is that in JOIN, the datatypes of the columns do not have to be the same, whereas in UNION, the datatypes of the columns should be the same.

  • The difference between full JOIN and UNION all is that full Join combines data into new columns. When two tables are joined, the data from the first table is displayed in one set of columns alongside the data from the second table in the same row, whereas the Unions ALL combine data to create new rows.

  • In SQL, there are four types of JOINs:

    • LEFT JOIN
    • RIGHT JOIN
    • FULL OUTER JOIN
    • INNER JOIN

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