Difference Between JOIN and UNION in SQL

Both of these are types of clauses in SQL. We use them both for combining data from two or more than two relations.

There is a major difference between JOIN and UNION in SQL. Using the JOIN clause, we combine the attributes of two given relations and, as a result, form tuples. Whereas we use the UNION clause when we want to combine the results obtained from two queries. They both combine data differently. The format of the result that they obtain also varies. In this article, we will dig deeper into the difference between JOIN and UNION in SQL. But let us first understand the individual functionalities of both clauses.

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What is JOIN in SQL?

This clause combines tuples in SQL from two tables or relations. Thus, the JOIN clause results in the formation of a longer size of tuples. This resultant tuple has attributes of both the concerning relations. The JOIN clause combines both of these on the basis of the common attributes between them. This clause comes in various types- RIGHT JOIN, LEFT JOIN, FULL OUTER JOIN, and INNER JOIN.

Type #1 INNER JOIN – It helps in combining tuples from two tables when they have a common attribute.
Type #2 FULL OUTER JOIN – It combines tuples from both of the tables. The presence of a common attribute isn’t a prerequisite in this case.
Type #3 LEFT JOIN – This clause results in the tuples from the left table and only the matching ones from the right table.
Type #4 RIGHT JOIN – This clause results in the tuples from the right table and only the matching ones from the left table.

Here, the INNER JOIN acts just like the JOIN clause. Thus, for using this clause, you can drop the keyword INNER and directly use the keyword JOIN for performing the INNER JOIN.

What is UNION in SQL?

It is a clause that we use for set operations in SQL. The UNION clause combines the result obtained from two of the queries. The result generated from UNION involves tuples from both the relations that are present in the query. For applying the UNION clause on two of the relations, they must satisfy both these conditions:

Both the relations must possess a similar number of attributes.
The corresponding attributes must have the same domain.

UNION is of two major types: the UNION and the UNION ALL. The result that we obtain using the UNION clause has no duplicates. But the results that we obtain using the UNION ALL always retain all of their duplicates.