Datatypes are a fundamental concept in SQL Server and often there is a need to convert them from one datatype to another. However, developers can sometimes get confused when it comes to datatype conversion. In SQL Server, there are two important concepts related to datatype conversion: Implicit Conversion and Explicit Conversion.

Implicit Conversion

Implicit conversions are those conversions that occur without specifying either the CAST or CONVERT function. SQL Server automatically performs these conversions for you. For example, if you are trying to convert a value from datetime2 to time or from tinyint to int, SQL Server will handle the conversion implicitly.

Explicit Conversion

Explicit conversions, on the other hand, require the CAST or CONVERT function to be specified. These conversions need to be explicitly defined by the developer. For example, if you are attempting to convert a timestamp to smalldatetime or datetime to int, you will need to use either the CAST or CONVERT function with appropriate parameters.

Example:

Let’s take a look at a quick example to understand both types of conversions:

-- Implicit Conversion
DECLARE @implicitConversion INT
SET @implicitConversion = 10.5
SELECT @implicitConversion

-- Explicit Conversion
DECLARE @explicitConversion VARCHAR(10)
SET @explicitConversion = CAST(123 AS VARCHAR(10))
SELECT @explicitConversion

In the above example, we can see how we need both types of conversions in different situations. It is important to note that there are many different datatypes in SQL Server, and it is humanly impossible to know which datatype requires implicit conversion and which requires explicit conversion. Additionally, there are cases where the conversion is not possible at all.

To help developers with datatype conversion, Microsoft has published a SQL Server Data Type Conversion Chart. This chart provides a grid that displays various conversion possibilities as well as a quick guide to help you understand the conversion process.

Understanding SQL Server data type conversion is crucial for developers working with databases. By knowing when to use implicit or explicit conversion, developers can ensure accurate and efficient data manipulation in their SQL Server applications.

Download the SQL Server Data Type Conversion Chart here.