How to Sort an SQL Union Query with a Custom Order

When working with SQL databases, it's often necessary to combine data from different tables into a single result set using a UNION query. However, sometimes the default sorting of the combined result set is not sufficient, and you need to sort the result in a specific way. This article will guide you through sorting an SQL union query with a custom order using various techniques, including the CASE statement and the ORDER BY clause.

Introduction to UNION Queries

A UNION query combines the results of two or more SELECT statements into a single result set. When using UNION, the number and data types of the columns must be the same, and the number of columns must match. If you want to sort the combined result set, you can use the ORDER BY clause at the end of the query.

Sorting with ORDER BY Clause

The most straightforward way to sort a SQL union query is by using the ORDER BY clause at the end of the statement. However, if the default order is not the desired order, you can use the CASE statement within the ORDER BY clause to achieve a custom order.

Using CASE Statement in ORDER BY

The CASE statement allows you to specify conditions to sort the rows. Here's an example:

SELECT * FROM table1 UNION SELECT * FROM table2 ORDER BY CASE WHEN 'value' THEN 1 WHEN 'another_value' THEN 2 ELSE 3 END

In this example, the result set will be sorted first by rows from table1 where column1 equals 'value', and then by rows from table2 where column2 equals 'another_value'. If neither condition is met, the rows will be sorted with a third order.

Using Column in SELECT List

If you need to know which part of the union produced the row, you should include an additional column in the SELECT list that acts as an identifier. This can be done using a combination of CASE statement and an identifier column in the union query. Here's an example:

SELECT , CASE WHEN IS NOT NULL THEN 1 ELSE 2 END as id, FROM table1 UNION SELECT , CASE WHEN IS NOT NULL THEN 1 ELSE 2 END as id, FROM table2 ORDER BY id, column1

In this example, the id column is used to distinguish between rows from table1 and table2, ensuring the correct sorting order.

Including OVER and CTE

Another advanced technique for sorting an SQL union query is by using the OVER clause or WITH CTE (Common Table Expression). The OVER clause is used in window functions to create a window of rows over which a function is calculated. The WITH CTE allows you to define a temporary result set that can be referenced in subsequent queries.

Using OVER Clause

Here's an example of how you can use the OVER clause with a window function to sort a union query:

SELECT *, ROW_NUMBER() OVER (PARTITION BY source ORDER BY column1) as row_num FROM ( SELECT , , 1 as source FROM table1 UNION SELECT , , 2 as source FROM table2 ) as subquery ORDER BY source, row_num

In this example, the ROW_NUMBER function is used in the OVER clause to generate a row number for each row in the union result set, partitioned by the source column. The final query orders the result set by the source column and the generated row numbers.

Using WITH CTE

A WITH CTE can be used to improve readability and performance of the query. Here's an example:

WITH source1 AS ( SELECT , FROM table1 ), source2 AS ( SELECT , FROM table2 ) SELECT *, ROW_NUMBER() OVER (ORDER BY column1) as row_num FROM ( SELECT column1, column2, 1 as source FROM source1 UNION SELECT column1, column2, 2 as source FROM source2 ) as subquery ORDER BY source, row_num

In this example, the CTEs source1 and source2 are defined to contain the sub-queries for table1 and table2, respectively. The final query uses the ROW_NUMBER function and the OVER clause to generate row numbers and sort the combined result set.

Conclusion

Sorting an SQL union query with a custom order can be achieved using various techniques, such as the CASE statement, an identifier column in the select list, and advanced features like the OVER clause or WITH CTE. By following the examples provided in this article, you can ensure that your union query is sorted in the desired order, making your data more accessible and easier to analyze.