The Difference Between Join and Union in SQL
Introduction
In SQL, JOIN and UNION are two important operations used to combine data from multiple tables. While both are used to combine data, they are not interchangeable and serve different purposes. In this article, we will explore the key differences between JOIN and UNION in SQL.
JOIN
JOIN is an operation in SQL that combines data from two or more tables into a single result set based on a common column between the tables. JOIN uses the JOIN keyword and can be of different types, including INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN.
JOIN is useful when you want to retrieve data from related tables. For instance, if you have two tables, one for customers’ information and another for their orders, JOIN can help you merge these tables based on a common customer ID column. With JOIN, you can extract data from one or more tables based on common values.
UNION
UNION, on the other hand, is used to combine data from two or more tables with the same structure into a single result set. Unlike JOIN, UNION does not require a common column between the tables.
When using UNION, the columns in both tables must have the same data type, and the number of columns should be the same. This is because the UNION operation appends the rows from one table to another. If the columns are not matched, UNION will fail.
UNION is useful when you want to combine data from tables that have the same structure. For instance, you may have two tables, one with customer information and another with order information, but they have different IDs. In this case, UNION is helpful in merging the two tables and selecting data based on specific criteria.
Differences
The main difference between JOIN and UNION is that JOIN combines data based on a common column between the tables while UNION combines data from tables with the same structure. JOIN requires the tables to have a common column, and the operation can be of different types, while UNION does not require a common column, but columns in both tables must be matched.
JOIN results in a larger result set than either of the original tables since it combines rows from both tables. UNION, on the other hand, can only return distinct rows from both tables.
Conclusion
JOIN and UNION are two essential operations in SQL used for combining data from multiple tables. Both have different functionalities, and while they can be used together, they are not interchangeable. Understanding the difference between JOIN and UNION can help you use them effectively and efficiently in your SQL queries.
Table difference between join and union
| Join | Union |
|---|---|
| Joins combine rows from two or more tables based on a related column between them. | Unions combine rows from two or more tables with the same structure, adding them together as a single result set. |
| Join operation retrieves only the matching rows from two or more tables. | Union operation retrieves all the rows from two or more tables including duplicates. |
| Join operation requires a common column between the tables being joined. | Union operation doesn’t require a common column between the tables being united, but the column number and data types should be the same. |
| Join types include inner join, left join, right join, and full outer join. | Union operation does not have any join types. |
| Join operation results in a table, which is a subset of the joined tables. | Union operation results in a table, which contains all the rows from the combined tables. |