How to convert Nvarchar column to INT

3 min read 08-10-2024

Converting an NVARCHAR column to INT can be a common task in SQL Server when dealing with databases. The need to convert data types arises frequently, especially when the NVARCHAR data type is used to store numerical values. This article will guide you through the process of converting an NVARCHAR column to INT, showcasing the original code and providing insights to ensure a smooth transition.

Understanding the Problem

Often, you may find that numeric data has been stored as NVARCHAR for various reasons, such as legacy system constraints or data import processes. However, performing calculations or comparisons on these values requires them to be in an INT format. Failing to convert these values properly can lead to errors, incorrect results, or inefficient queries. Thus, we need an effective method for converting these columns without losing data integrity.

Original Code Example

Let's consider a simple example where you have a table named Employees with a column EmployeeID defined as NVARCHAR:

CREATE TABLE Employees (
    EmployeeID NVARCHAR(10),
    EmployeeName NVARCHAR(50)
);

If you try to perform a numeric operation, like a sum or a filter, directly on EmployeeID, you will encounter issues. Therefore, the first step is to convert this NVARCHAR column to INT.

Conversion Method

You can use the CAST or CONVERT function in SQL Server to change the data type. Here’s how you can perform this conversion:

SELECT CAST(EmployeeID AS INT) AS ConvertedEmployeeID
FROM Employees
WHERE ISNUMERIC(EmployeeID) = 1;

In this example, we use ISNUMERIC() to filter out any values that cannot be converted to an INT. This is crucial because trying to convert a non-numeric string will result in an error.

Analysis and Insights

Data Quality: Before converting, it’s essential to ensure that the data in the NVARCHAR column is indeed numeric. As shown in the query, using ISNUMERIC() helps prevent runtime errors. However, be mindful that ISNUMERIC() may return TRUE for some non-integer values (like decimals), so additional validation may be necessary.
Error Handling: In case your data has some invalid entries, you may want to address these beforehand. For instance, you can use a query to identify non-numeric values:
```
SELECT EmployeeID 
FROM Employees 
WHERE ISNUMERIC(EmployeeID) = 0;
```
Performance Considerations: Performing conversion on large datasets can lead to performance overhead. If possible, consider creating a new INT column, populating it with the converted values, and then dropping the old NVARCHAR column.

Example of Populating a New INT Column

Here’s how you could safely implement the conversion into a new column:

ALTER TABLE Employees ADD ConvertedEmployeeID INT;

UPDATE Employees 
SET ConvertedEmployeeID = CAST(EmployeeID AS INT) 
WHERE ISNUMERIC(EmployeeID) = 1;

After ensuring your new column is populated correctly, you can then decide whether to drop the old NVARCHAR column.

Conclusion

Converting an NVARCHAR column to INT in SQL Server is a straightforward process if executed with care. By understanding the data type requirements and taking necessary precautions, you can prevent errors and ensure data integrity throughout the conversion process. Always validate your data before performing type conversion, and consider the implications on performance with larger datasets.

Additional Resources

By following the guidelines and insights shared in this article, you can confidently handle NVARCHAR to INT conversions in your SQL Server databases.