What is the purpose of a union operation in SQL?

The purpose of a union operation in SQL is to combine the results of two or more SELECT statements into a single result set.

In SQL, the UNION operator is used to combine the result sets of 2 or more SELECT statements. It removes duplicate rows from the results. Each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order.

The main purpose of the UNION operator is to efficiently combine data from multiple tables into a single, unified view. This can be particularly useful when you need to pull related data from several tables for analysis. For example, if you have a database of books and you want to create a list of all authors and publishers, you could use a UNION operation to combine the author names from the 'authors' table with the publisher names from the 'publishers' table.

It's important to note that UNION only selects distinct values. If you want to select all values (including duplicates), you would use UNION ALL. This is a key difference between UNION and UNION ALL and can be a crucial consideration depending on what you want to achieve with your data.

In summary, the UNION operation in SQL is a powerful tool for combining and analysing data across multiple tables. It allows for the creation of comprehensive and diverse data sets, making it easier to extract meaningful insights from your database.