SQL Examples
SQL Union Query
SQL Union Query
Union query with UNION ALL combines results, managing deduplication.
Understanding SQL Union Queries
The SQL UNION operator is used to combine the result-set of two or more SELECT statements. Each SELECT statement within the UNION must have the same number of columns in the same order, with similar data types.
UNION effectively removes duplicate rows from the result set, whereas UNION ALL includes duplicate rows. This makes UNION ALL faster, as it does not perform the deduplication step.
Basic SQL UNION Example
Consider two tables, employees_2022 and employees_2023, where both contain employee records for different years. To combine these records into a single list without duplicates, you can use the UNION operator:
Using UNION ALL for Complete Results
If you want to include all records, even duplicates, you can use the UNION ALL operator. This is useful when you need to see all entries, including those that appear in both tables:
Performance Considerations
When deciding between UNION and UNION ALL, consider that UNION ALL is generally more performant because it skips the deduplication process. Use UNION ALL when you are certain that there are no duplicates or when duplicates are acceptable in your results.
For large datasets, the performance difference can be significant, making UNION ALL a preferred choice in such cases.
Conclusion
The UNION and UNION ALL operators are powerful tools in SQL for combining query results. By choosing the appropriate operator, you can optimize your queries for either deduplication or performance. Understanding the differences allows for better data management and retrieval.
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