How do you use the TRY_CONVERT function to handle data conversion errors gracefully?
HenryPk
Posted by HenryPk
Last Updated: July 21, 2024
TRY_CONVERT errors
The TRY_CONVERT function in SQL Server is designed to attempt to convert an expression from one data type to another. If the conversion is successful, it returns the converted value. If the conversion fails—typically due to type incompatibilities—it returns NULL instead of throwing an error. This allows you to handle potential conversion errors gracefully.
Syntax
TRY_CONVERT(data_type, expression [, style])
- data_type: The target data type to which you want to convert the expression. - expression: The value you want to convert. - style: An optional integer that specifies the format for the conversion, relevant for date and time data types.
Example of Usage
Suppose you have a table that contains a column of text data representing numeric values, and you want to convert these values to integers. Some of these values may not be convertible, such as 'abc', so you'd like to handle this situation without causing an error.
Sample Table Creation
CREATE TABLE SampleData (
    Id INT PRIMARY KEY,
    NumberText NVARCHAR(50)
);

INSERT INTO SampleData (Id, NumberText) VALUES 
(1, '10'),
(2, '20'),
(3, 'abc'),  -- This will cause a conversion error
(4, '30');
Using TRY_CONVERT
To convert the NumberText column to integers and handle potential errors gracefully: 
SELECT 
    Id,
    NumberText,
    TRY_CONVERT(INT, NumberText) AS ConvertedNumber
FROM 
    SampleData;
Result Set
This query will produce a result as follows: | Id | NumberText | ConvertedNumber | |----|------------|------------------| | 1 | 10 | 10 | | 2 | 20 | 20 | | 3 | abc | NULL | | 4 | 30 | 30 |
Key Points
- Graceful Handling: Instead of generating an error when encountering 'abc', the TRY_CONVERT function simply returns NULL, allowing the rest of the operation to proceed. - NULL Handling: You can further handle the NULL values in your queries using conditions (e.g., WHERE, CASE statements) to filter or replace them.
Example with Handling NULLs
You might want to replace NULL with a default value (like 0) in your results: 
SELECT 
    Id,
    NumberText,
    ISNULL(TRY_CONVERT(INT, NumberText), 0) AS ConvertedNumber
FROM 
    SampleData;
Conclusion
Using TRY_CONVERT is a helpful technique for enhancing the robustness of SQL queries by preventing runtime errors during data type conversions, allowing you to manage conversion failures gracefully.
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