Choosing the Right Tool: CAST or CONVERT for Your T-SQL Needs

Standardization:

• CAST: This is part of the ANSI-SQL standard, meaning it's widely supported across different database systems.
• CONVERT: This is specific to T-SQL and won't work in other database languages.

Flexibility:

• CAST: Simpler and more straightforward. It offers basic data type conversion.
• CONVERT: More powerful and flexible. It allows you to format the output, especially for dates, times, and currency values.

Here's a table summarizing the key points:

Choosing Between CAST and CONVERT:

• Portability: If you need code to work across different databases, use CAST for its wider support.
• Formatting: If you require specific formatting for output, especially with dates, times, or currency, CONVERT is the way to go.
• Simplicity: For basic conversions, CAST is easier to use and understand.

Here are some resources for further reading:

Simple Type Conversion (CAST):

This example converts an integer (123) to a string (VARCHAR).

SELECT CAST(123 AS VARCHAR(10));

This achieves the same result as the previous example but uses CONVERT.

SELECT CONVERT(VARCHAR(10), 123);

Date Conversion with Formatting (CONVERT):

This converts a string in US date format ("1/2/2024") to a SQL Server DATE data type.

SELECT CONVERT(DATE, '1/2/2024', 101);  -- 101 represents US date format style

Currency Conversion with CAST:

This converts a decimal value (10.3496847) to a MONEY data type, rounding the value.

SELECT CAST(10.3496847 AS MONEY);

Implicit Conversion:

T-SQL can sometimes perform implicit conversions automatically. For example, if you add an integer (10) to a string containing a number ("20"), it will convert the string to a number before performing the addition. However, this can be risky as unexpected behavior might occur if the conversion fails. It's generally better to be explicit with CAST or CONVERT.

String Manipulation Functions (For Simple Conversions):

For converting between strings and numbers, you can use string manipulation functions along with basic math operators. Here's an example:

DECLARE @myString VARCHAR(10) = '123.45';
SELECT CAST(@myString AS FLOAT) AS ConvertedValue;  -- This might throw an error if the string isn't a valid number

-- Safer approach using string manipulation
SELECT CONVERT(FLOAT, REPLACE(@myString, '.', '')) / 100 AS ConvertedValue;  -- Replace '.' with '' and divide by 100

TRY_CAST or TRY_CONVERT (For Error Handling):

These functions work similarly to CAST and CONVERT, but they return NULL instead of throwing an error if the conversion fails. This can be useful for handling potential invalid data gracefully.

CASE Expressions (For Conditional Conversions):

If you need to convert data based on specific conditions, you can use a CASE expression. This allows you to define different conversion logic depending on the input value.

Remember:

• Implicit conversion should be used cautiously due to potential risks.
• String manipulation functions can be used for simple conversions but might be less readable for complex scenarios.
• TRY_CAST and TRY_CONVERT are useful for error handling.
• CASE expressions offer flexibility for conditional conversions.