How to Use UNION ALL with Implicit Data Type Conversions in SQL Server

Understanding Implicit Data Type Conversion in SQL Server

When working with multiple columns of different data types in a single query, it can be challenging to ensure that the final result set is consistent in terms of data type. In this article, we will explore the concept of implicit data type conversion in SQL Server and how to use it effectively.

Introduction to Implicit Data Type Conversion

Implicit data type conversion refers to the process of automatically converting data from one data type to another when necessary. This can occur during a calculation or operation involving multiple columns with different data types.

In SQL Server, there are specific rules governing implicit data type conversions, which are outlined in the Data-Type Precedence document. According to these rules, some data types can be implicitly converted to others.

The Challenge of Downcasting Columns

The problem arises when we want to explicitly convert all columns in a result set to a specific data type, such as nvarchar(MAX). In the context of your question, you are trying to create a CSV-style table with implicit conversions using a UNION ALL operator.

However, SQL Server has a precedence rule that states “backwards” conversion, meaning it converts from more general to more specific data types. This means that if you have a column of type float and another column of type nvarchar(MAX), the database engine will implicitly convert the float value to nvarchar(MAX) during the calculation.

Using UNION ALL with Implicit Conversions

In your original query, you were using UNION ALL to combine two queries: one that selects only the column titles (#CSV_Column_Titles) and another that retrieves data from a temporary table or query results (#QueryResults).

However, as you pointed out, this approach does not work because SQL Server will implicitly convert the float values in #QueryResults to nvarchar(MAX) during the UNION ALL operation.

Creating a Temporary Table with Explicit Conversions

To solve your problem, one possible solution is to create a temporary table (#QueryResultsCast) that has the same schema as your desired output table (#CSV_Output). Then, you can insert data from #QueryResults into this temporary table using explicit conversions.

Here’s an example query:

-- This will create a table like #CSV_Output, to store intermediate results
SELECT * INTO #QueryResultsCast FROM #CSV_Output WHERE 0 = 1

-- cast all via INSERT
INSERT INTO #QueryResultsCast
SELECT CAST(*) AS nvarchar(MAX) FROM #QueryResults

-- Original desired output (retrieving from #QueryResultsCast)
INSERT INTO #CSV_Output          
SELECT * from #CSV_Column_Titles   --  "n" columns, all columns are nvarchar( MAX ) )
UNION ALL
SELECT * FROM #QueryResultsCast       --  "n" columns, all must be implicitly converted to nvarchar( MAX )

In this example, we first create a temporary table #QueryResultsCast with the same schema as our desired output table. We then insert data from #QueryResults into this temporary table using explicit conversions.

Conclusion

When working with multiple columns of different data types in SQL Server, it’s essential to understand how implicit data type conversion works and when to use explicit conversions. By creating a temporary table with the same schema as your desired output table and inserting data from another query using explicit conversions, you can achieve the desired result without relying on dynamic SQL.

Note that this approach may require additional I/O operations compared to other methods, but it provides more control over the conversion process and ensures consistency in terms of data type.

Last modified on 2024-10-20