How do you use the TRY_PARSE and TRY_CONVERT functions to handle data conversion errors gracefully?
Posted by TinaGrn
Last Updated: July 15, 2024
In SQL Server, the TRY_PARSE and TRY_CONVERT functions provide ways to attempt to convert data types and handle any potential conversion errors without raising an error that would stop your query execution. These functions return a NULL value when the conversion fails instead of throwing an error, which allows you to handle potential issues more gracefully.
Using TRY_PARSE
TRY_PARSE is used to convert a string representation of a value to a specific data type (like datetime, int, etc.) based on the provided culture info. It will return NULL if the conversion fails. Syntax:
TRY_PARSE(expression AS data_type [ USING culture ])
Example:
SELECT 
    TRY_PARSE(column_name AS int USING 'en-US') AS ParsedValue
FROM your_table;
In this example, column_name is converted to an integer. If the conversion fails (for example, if column_name contains non-numeric characters), it will return NULL instead of throwing an error.
Using TRY_CONVERT
TRY_CONVERT is more general and can be used to convert data types in a straightforward manner without the need for culture settings. It also returns NULL if the conversion fails. Syntax:
TRY_CONVERT(data_type, expression)
Example:
SELECT 
    TRY_CONVERT(datetime, column_name) AS ConvertedDate
FROM your_table;
In this example, column_name is attempted to be converted to a datetime. If the conversion fails (e.g., if column_name contains an invalid date), it will return NULL.
Example Scenario
Here is how you might use both TRY_PARSE and TRY_CONVERT in practice, assuming you have a table with mixed data types in a column:
CREATE TABLE SampleData (
    TextValue NVARCHAR(50)
);

INSERT INTO SampleData (TextValue) VALUES 
('123'), 
('abc'), 
('2021-10-31'), 
('not a date');

SELECT 
    TextValue,
    TRY_PARSE(TextValue AS int USING 'en-US') AS ParsedInt,       -- Will output NULL for 'abc'
    TRY_CONVERT(datetime, TextValue) AS ConvertedDate            -- Will output NULL for 'abc' and 'not a date'
FROM SampleData;
Handling NULLs
After using these functions, you can also handle the resulting NULLs in your query using ISNULL, COALESCE, or similar functions to provide a default value:
SELECT 
    TextValue,
    ISNULL(TRY_PARSE(TextValue AS int USING 'en-US'), 0) AS SafeParsedInt,      -- Replace NULL with 0
    ISNULL(TRY_CONVERT(datetime, TextValue), '1900-01-01') AS SafeConvertedDate -- Replace NULL with a default date
FROM SampleData;
Summary
- TRY_PARSE is useful for parsing strings to specific data types, taking culture into account. - TRY_CONVERT is more straightforward for type conversion that does not require culture settings. - Both functions return NULL upon conversion failure, preventing errors and allowing for cleaner error handling in SQL queries.
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