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SQL Server: Error converting data type nvarchar to numeric

Have you been running into the following error message?

SQL Server: Error converting data type nvarchar to numeric

According to our Experts, this error arises when SQL Server tries to convert a string (nvarchar) into a numeric data type (e.g., int, float, or decimal) but fails due to invalid characters or improper formatting in the string.

Today, we will look at some of the causes behind the error and how to resolve it.

What Causes the Error?

1. The nvarchar column or variable contains characters that cannot be converted to a number (e.g., letters, symbols, or whitespace).
2. Attempting to convert an empty string (”) or NULL to a numeric value leads to conversion failures.
3. Strings containing numeric-looking data but formatted incorrectly for the target data type, such as:
   • A string like “123.45” failing if the target type is int.
   • A string like “1,234” failing if the `,` is not supported by the locale or conversion settings.
4. SQL Server may implicitly try to convert an nvarchar value to a numeric type during operations like WHERE, JOIN, or calculations.
5. Columns with mixed data types (e.g., numbers and text) will fail when attempting to cast or convert non-numeric entries.

Example Scenarios

1. Direct Conversion Fails

   SELECT CAST('abc' AS int);

   This query throws an error because ‘abc’ is not a valid integer.
2. Implicit Conversion in Queries

   
SELECT *
FROM Orders
WHERE OrderAmount > '500';


   Here, SQL Server tries to convert ‘500’ to match the OrderAmount column’s numeric type. If OrderAmount contains non-numeric strings, the query will fail.
3. Mixed Data in a Column

   Suppose a column `Amount` has these values: `’100′, ‘200’, ‘abc’, ‘300’`

   A query like:

   SELECT CAST(Amount AS int) FROM Table;

   It will fail because ‘abc’ cannot be converted to an integer.

How to Fix the Error

1. Use `TRY_CAST` or `TRY_CONVERT` to safely test conversions. These functions return NULL if conversion fails instead of throwing an error.

   SELECT TRY_CAST('abc' AS int); -- Returns NULL
SELECT TRY_CAST('123' AS int); -- Returns 123

2. Ensure only numeric data is selected for conversion using `ISNUMERIC` or custom logic:

   SELECT Amount
FROM Table
WHERE ISNUMERIC(Amount) = 1;


   Since `ISNUMERIC` has limitations, we can use pattern matching for better precision:

   
SELECT Amount
FROM Table
WHERE Amount NOT LIKE '%[^0-9]%'; -- Filters out non-numeric characters

3. If the column has mixed data, clean it up by updating invalid rows to NULL or a default value:

   
UPDATE Table
SET Amount = NULL
WHERE TRY_CAST(Amount AS int) IS NULL;

4. If the strings use specific formatting, clean them up before conversion. For instance:

   
SELECT REPLACE('1,234', ',', '') AS CleanedNumber;


   Then cast:

   SELECT CAST(REPLACE('1,234', ',', '') AS int);
5. Ensure the locale settings in SQL Server match the numeric formatting of your strings (e.g., decimals and commas).

Best Practices to Avoid the Error

1. Store numeric data in numeric columns (int, float, etc.) rather than nvarchar.
2. Ensure data being inserted into nvarchar columns is clean and adheres to expected formats.
3. Use scripts or tools to preprocess and sanitize data before running queries.
4. Explicitly cast or convert data where needed instead of relying on SQL Server to infer the conversion.

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Conclusion

Following these guidelines and troubleshooting steps, we can easily manage and prevent errors when converting nvarchar to numeric data types in SQL Server.

In brief, our Support Experts demonstrated how to fix SQL Server: Error converting data type nvarchar to numeric.