When working with SQL Server, you may come across the UNION and UNION ALL operators. These operators are used to combine datasets, but they have some key differences that you should be aware of.

The UNION operator combines datasets while removing any duplicate rows, whereas the UNION ALL operator combines datasets without removing duplicates. This means that UNION ALL will retain all the data from both datasets, while UNION will only keep distinct data.

Let’s take a look at an example to understand why UNION ALL works, but UNION produces an error:

USE TempDB
GO

-- Create the first table
CREATE TABLE text_union(comment TEXT)
INSERT INTO text_union
SELECT 'this is just a test comment'

-- Create the second table
CREATE TABLE text_union_all(comment TEXT)
INSERT INTO text_union_all
SELECT 'Other test comment'
GO

-- Query using UNION ALL (works fine)
USE tempdb;
SELECT comment FROM text_union
UNION ALL
SELECT comment FROM text_union_all

-- Query using UNION (produces error)
USE tempdb;
SELECT comment FROM text_union
UNION
SELECT comment FROM text_union_all

The error message clearly states that the TEXT datatype cannot be used in the UNION operation. But why is that?

The reason is that the TEXT datatype is considered a large object (LOB) data type, and LOB data types have some limitations when it comes to certain operations, including UNION. The UNION operator requires both datasets to have compatible data types, and LOB data types like TEXT are not compatible with other data types.

So, if you need to use UNION with LOB data types, you will need to convert them to a compatible data type, such as VARCHAR(MAX) or NVARCHAR(MAX).

In conclusion, understanding the difference between UNION and UNION ALL in SQL Server is crucial when working with datasets. While UNION removes duplicates, UNION ALL retains all the data. However, it’s important to note that LOB data types like TEXT cannot be used with the UNION operator due to compatibility issues.

If you have any further questions or insights on this topic, please leave a comment below. I will be happy to address them.