Useful SQL Server functions: TRY_CAST & TRY_CONVERT

You've probably used the CAST and CONVERT functions to switch between data types, but did you know there's a couple of handy counterparts to these? The TRY_CAST and TRY_CONVERT functions.

This post will look at these two functions, how to use them, and what makes them useful.

TRY_CAST & TRY_CONVERT

TRY_CAST & TRY_CONVERT follow the same syntax as CAST and CONVERT but with one key difference: TRY_CAST and TRY_CONVERT return NULL if the conversion fails.

The functions are laid out like this:

TRY_CAST
TRY_CAST (column_or_value AS data_type)
TRY_CONVERT
TRY_CONVERT (data_type, column_or_value, optional_style)

Let's look at a couple of simple examples.

SELECT
    TRY_CAST('42' AS INTEGER) as result
    ,TRY_CAST('42' AS INTEGER) * TRY_CAST('42' AS INTEGER) as result_sq
SELECT
    TRY_CAST('To be, or not to be' AS DATE) as result

Notice when the conversion failed, the column has a NULL value, and the query successfully ran.

There is one catch to be aware of, however. TRY_CAST and TRY_CONVERT will still error if the conversion is not permitted.

This example returns an error because an INTEGER to DATE conversion is not allowed.

SELECT
    TRY_CAST(42 AS DATE) as result

For more details on permitted conversions, see the table here.

Why is this useful?

Firstly because it won't break a query if conversion fails (some exceptions, see above), the returned value will be NULL instead of an error.

Secondly, TRY_CAST and TRY_CONVERT are useful because they allow you to try alternatives if conversion fails.

When used in a CASE statement, this lets you test if a conversion is possible and respond if not. You can keep trying other conversions until you've exhausted all options and return NULL.

An example

Let's pretend for a moment that the data in this example represents a table of imported Microsoft Excel data. We need to convert the messy_date column, a VARCHAR, to a DATE, but (typically) it's full of mixed formats.

We can use TRY_CONVERT and TRY_CAST here to test three different conversions and then return NULL.

-- Setup example data
DROP TABLE IF EXISTS #excel_data
CREATE TABLE #excel_data (
    messy_date VARCHAR(100)
)
INSERT INTO #excel_data(messy_date)
VALUES
    ('30/08/2020')
    ,('02/25/2020')
    ,('44081')
    ,(NULL)
    ,('#N/A')


DECLARE @excel_serial_start DATE = '1900-01-01'

SELECT
    CASE
        WHEN TRY_CONVERT(DATE, #excel_data.messy_date, 103) IS NOT NULL THEN
            CONVERT(DATE, #excel_data.messy_date, 103)
        WHEN TRY_CONVERT(DATE, #excel_data.messy_date, 101) IS NOT NULL THEN
            CONVERT(DATE, #excel_data.messy_date, 101)
        WHEN TRY_CAST(#excel_data.messy_date AS INT) IS NOT NULL THEN
            DATEADD(DAY, CAST(#excel_data.messy_date AS INT), @excel_serial_start)
        ELSE
            NULL
    END as result
FROM
    #excel_data

First, we test for the British date format (103). If that fails, we try the US date format (101).

Our example data includes dates represented as the Excel serial number. To handle this, we test if we can convert it to an INTEGER and if we can use the DATEADD function to return the date. If all of these are unsuccessful, then we return NULL.

Pretty handy, right? We've managed to convert three different date formats without using convoluted string manipulation.

Conclusion

In this article, we've taken a look at two useful SQL Server conversion functions, TRY_CAST and TRY_CONVERT.

Next time you've got any messy Data Type conversions, give TRY_CAST or TRY_CONVERT a try (pun intended).

Further reading