UNION vs UNION ALL in SQL Explained

When diving into relational databases, understanding the nuances between UNION and UNION ALL is crucial for data manipulation and management. Both commands are used to combine results from two or more SELECT statements, but they serve distinct purposes that can significantly impact query performance and output accuracy. UNION is often favored for retrieving a unique set of records, as it eliminates duplicate rows from the result set. This can be particularly useful during data aggregation tasks when the goal is to gather distinct entries from various tables or queries.

In contrast, UNION ALL includes all records, even duplicates, making it a more efficient choice in scenarios where every occurrence of a record is essential. For scenarios where performance matters, utilizing UNION ALL can lead to faster query execution because it bypasses the overhead of duplicate checking. Candidates preparing for technical interviews should be familiar not only with these commands but also with their differences in execution plans and performance implications in SQL databases. Understanding how to effectively use these operations can set candidates apart when discussing data retrieval strategies.

When considering database optimization, knowing when to apply UNION versus UNION ALL can greatly affect results; hence, related implications on indexes and databases schema should also be explored. Furthermore, grasping the implications of set operations helps in designing better queries that involve joins and subqueries, enhancing overall database performance. Aside from their direct differences, it's also worth studying related SQL concepts like JOINs and filtering techniques which can further refine data selection processes. Having a comprehensive grasp of how UNION and UNION ALL operate within different SQL environments such as MySQL, SQL Server, and PostgreSQL can bolster a candidate's proficiency in managing and querying databases effectively..