UNION vs. UNION ALL: When Should You Use Each for Data Merging?

SQL data merging: the difference and application of UNION and UNION ALL

SQL provides two operators with different purposes when merging data from multiple tables or queries: UNION and UNION ALL. The main difference between these two operators is how they handle duplicate rows in the resulting dataset.

UNION operator

The

UNION operator eliminates duplicate rows, meaning rows with all column values ​​the same will only appear once in the result set. This feature is particularly useful when duplicate data is not required or duplicate data should be prevented from skewing results. For example, if you want to aggregate data from two tables that contain records with overlapping primary keys, UNION will automatically remove any duplicate entries.

UNION ALL operator

In contrast, the UNION ALL operator retains all rows in the result set, including duplicate rows. This means that if the same row appears in both input datasets, it will appear twice in the merged result. This behavior can be useful when you need to preserve the original number of rows or display all possible permutations of data. For example, when creating a cross join of two tables, UNION ALL will maintain the Cartesian product even though certain row combinations may result in duplicate values.

Performance Considerations

It is important to note that UNION may cause performance degradation compared to UNION ALL. This is because UNION requires additional computation to identify and remove duplicate rows. The impact will depend on the size of the data set and the complexity of the query. Generally speaking, if data uniqueness is not required, UNION ALL can be preferred to improve performance.

Summary

Selecting UNION or UNION ALL depends on the specific database operation task. UNION is a suitable choice if you need to eliminate duplicates and ensure a concise result set. On the other hand, if you want to keep all rows regardless of duplicates and performance is a factor, UNION ALL is the preferred operator.

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