In SQL, the UNION and UNION ALL operators are used to combine the result sets of multiple SELECT statements into a single result set. They are crucial for tasks like merging data from different tables or views. The key difference lies in how they handle duplicate rows. UNION, by default, eliminates duplicate rows from the combined result set. This is useful when you want a unique list of values. Think of it as finding the unique values across multiple tables. UNION ALL, on the other hand, returns all rows from the combined result sets, including duplicates. This is more efficient if you need all the data, even if some values appear multiple times in the different SELECT statements. Imagine you need to aggregate data from multiple sources, and you want to keep all the records, even if some have identical values.Understanding the difference is critical for writing efficient and accurate queries. Using UNION when you don't need to eliminate duplicates can lead to unnecessary processing and potentially incorrect results. Conversely, using UNION ALL when you need unique results will result in redundant data, which can be inefficient.
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