How do you use the TRY_CONVERT function with different data types and styles to convert values with error handling in SQL Server?
AliceWk
Posted by AliceWk
Last Updated: July 14, 2024
SQL TRY_CONVERT
In SQL Server, the TRY_CONVERT function attempts to convert an expression from one data type to another. If the conversion fails, it returns NULL instead of raising an error, which is useful for error handling. The function can also take a style argument, which specifies the format of the data being converted (particularly useful for date and time conversions).
Syntax
TRY_CONVERT(data_type [ (length) ], expression [, style ])
- data_type: The target data type to convert to. - length: Optional. The length of the data type (for types like VARCHAR, CHAR, etc.). - expression: The expression you want to convert. - style: Optional. An integer that specifies the format for the conversion, particularly for date/time formats.
Basic Example
Here's a simple example of using TRY_CONVERT: 
SELECT 
    TRY_CONVERT(INT, '123') AS ConvertedValue,   -- Successfully converts to integer
    TRY_CONVERT(INT, 'ABC') AS ConvertedValue2,  -- Fails and returns NULL
    TRY_CONVERT(DATE, '2023-10-05') AS ConvertedDate,  -- Converts to date
    TRY_CONVERT(DATE, 'invalid_date') AS ConvertedDate2   -- Fails and returns NULL
Using Styles with Dates
When converting date strings, you might want to specify a style. Here’s an example that uses style to convert from string to date: 
SELECT 
    TRY_CONVERT(DATE, '10/05/2023', 101) AS USFormatDate,  -- MM/DD/YYYY
    TRY_CONVERT(DATE, '2023-10-05', 120) AS ISOFormatDate   -- YYYY-MM-DD
Error Handling with Result Checking
You can check the result of TRY_CONVERT and act accordingly. For instance, you might replace NULL values with a default value or log the error: 
SELECT 
    value,
    TRY_CONVERT(INT, value) AS ConvertedValue,
    CASE 
        WHEN TRY_CONVERT(INT, value) IS NULL 
        THEN 'Conversion failed' 
        ELSE 'Conversion successful' 
    END AS ConversionStatus
FROM 
    (VALUES ('123'), ('456'), ('ABC')) AS SampleData(value);
Example with Multiple Data Types
Here's an example that shows how TRY_CONVERT can be used for various data types along with error handling: 
WITH TestData AS (
    SELECT '100' AS StringValue,
           'invalid' AS InvalidValue,
           '2023-10-05' AS ValidDate,
           'InvalidDate' AS InvalidDateValue
)

SELECT 
    StringValue,
    TRY_CONVERT(INT, StringValue) AS ConvertedInt,
    TRY_CONVERT(INT, InvalidValue) AS ConvertedIntInvalid,
    TRY_CONVERT(DATE, ValidDate) AS ConvertedDate,
    TRY_CONVERT(DATE, InvalidDateValue) AS ConvertedDateInvalid
FROM 
    TestData;
Summary
- Use TRY_CONVERT for safe conversion without raising errors. - Provide a style parameter when converting dates to control the format. - Always check for NULL results to handle conversions that fail. - Use CASE statements or similar logic to respond to unsuccessful conversions. This function is very helpful in data cleansing and conversion tasks often encountered in ETL processes, where data may not be consistently formatted.
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