What is the primary difference between UNION and UNION ALL in SQL?

The primary distinction between UNION and UNION ALL lies in how they handle duplicate rows. When using UNION, the SQL operation combines the results of two or more queries and automatically removes any duplicate records from the final result set. This means that if the same row appears in both datasets being combined, it will only appear once in the output.

On the other hand, UNION ALL retains all rows from the combined queries, including duplicates. It simply stacks the results together without performing any deduplication. This makes UNION ALL potentially more efficient than UNION, as it does not require the additional processing step of checking for and removing duplicates.

Understanding this difference is important when deciding which operation to use in SQL, depending on whether preserving duplicates or removing them is necessary for a given query.