Merging Data from Multiple Tables with Same Structure Using SQL UNION and UNION ALL

In SQL, when you need to fetch data from multiple tables that have the same structure, there are two common methods: UNION and UNION ALL. These operators are powerful tools for data consolidation. Let's explore how to use them effectively.

Understanding SQL UNION and UNION ALL

Both UNION and UNION ALL are used to combine the results of multiple SELECT statements. However, they differ in how they handle duplicate rows, which is a key consideration when optimizing performance.

Using UNUNION

The UNION operator combines the results of two or more SELECT statements into one result set. Importantly, UNION excludes any duplicate rows from the final output. This can be useful when you want to avoid redundancy in your data, but it also means that the database must perform an additional step to identify and remove duplicates.

Here is the syntax for using UNION:

SELECT column1, column2, column3
FROM table1
UNION
SELECT column1, column2, column3
FROM table2
UNION
SELECT column1, column2, column3
FROM table3

Using UNION ALL

If you want to include all rows from multiple tables in your result set, including duplicates, you should use UNION ALL. By using UNION ALL, you avoid the overhead of duplicate checking, which can make your queries more efficient, especially when working with large datasets.

The syntax for UNION ALL is similar to UNION:

SELECT column1, column2, column3
FROM table1
UNION ALL
SELECT column1, column2, column3
FROM table2
UNION ALL
SELECT column1, column2, column3
FROM table3

Practical Example

Suppose you have three tables named sales_2021, sales_2022, and sales_2023. Each table has the same structure and contains the columns id, amount, date. Here is how you can combine these tables using UNION ALL without duplicates:

SELECT id, amount, date
FROM sales_2021
UNION ALL
SELECT id, amount, date
FROM sales_2022
UNION ALL
SELECT id, amount, date
FROM sales_2023

Important Considerations

Column Alignment

It is crucial to ensure that the order and data types of the columns in each SELECT statement are the same. This alignment ensures that the UNION or UNION ALL operations work correctly and produce the expected results.

Performance Implications

UNION ALL is generally faster than UNION because it does not need to check for and remove duplicate rows. However, the difference in performance can be negligible for smaller datasets. For larger datasets, the additional overhead of UNION can become significant.

Limitations and Caveats

While UNION and UNION ALL are powerful methods, they do have some limitations. Some databases have restrictions on the number of SELECT statements that can be combined with UNION. Additionally, using UNION or UNION ALL without the ALL keyword can lead to unexpected results if there are duplicate rows that need to be excluded.

Furthermore, UNUnions do not preserve the order of the result set by default. If you need to sort the data, you can apply the ORDER BY clause at the end of your query or within individual SELECT statements using the LIMIT clause.

Conclusion

Using SQL UNION or UNION ALL is a highly effective way to consolidate data from multiple tables that share the same structure. By carefully considering the use of these operators and implementing best practices, you can efficiently manage complex datasets and ensure that your data is both accurate and useful.