Last week, we explored an interesting puzzle related to SQL Server. The puzzle revolved around the difference between the UNION and UNION ALL operators and why the use of the TEXT datatype is not allowed in the UNION operation. In this blog post, we will delve into the answer to this puzzle and gain a deeper understanding of the limitations of the UNION operator in SQL Server.
During the puzzle, we posed a simple question: Why is the TEXT datatype not allowed to be used in the UNION operation? Let’s explore the answer together.
Several valid answers were provided by our readers, and we will highlight a few of them here:
- Tim Monfries: The UNION operator must perform a comparison operation to determine uniqueness, whereas UNION ALL does not since it returns all records. The TEXT datatype is not comparable, which means that UNION’s comparison-for-uniqueness operation cannot be performed, resulting in a query failure.
- Ruslan Aleksandrovic: The UNION operator needs to select all values from the second SELECT statement that are not present in the first SELECT statement and compare the values. However, the TEXT datatype is challenging to compare. On the other hand, UNION ALL does not perform any comparison and simply selects all values from all statements.
- Sakthi Balaji: The UNION operator throws an error when used with the TEXT datatype because it uses the SORT physical operator in the background to identify distinct data between datasets. Since larger datatypes like TEXT and NTEXT cannot be used for SORT operations, an error is thrown when using UNION and ORDER BY on these datatypes. This information can be observed in the Query Execution plan.
These answers shed light on the limitations of the UNION operator when dealing with the TEXT datatype. The need for comparison and sorting operations in UNION makes it incompatible with the TEXT datatype due to its unique characteristics.
Understanding these limitations can help SQL Server developers and administrators make informed decisions when working with the UNION operator and choosing appropriate datatypes for their queries.
Thank you to all our readers who participated in the puzzle and provided insightful answers. We hope this blog post has deepened your understanding of the UNION operator and its limitations in SQL Server.