Implicit conversion vs Explicit conversion SQL server

In SQL Server, implicit conversion and explicit conversion refer to how data types are automatically or manually converted when performing operations that involve different data types.

1. Implicit Conversion

Implicit conversion happens automatically when SQL Server needs to convert one data type to another. This occurs when the data types are compatible, and SQL Server can convert the value without needing explicit instructions from the user.

Example of Implicit Conversion:

Suppose you are performing a query where you add a varchar column and an int column. SQL Server will automatically convert the int to varchar to perform the operation.

DECLARE @num INT = 100;
DECLARE @str VARCHAR(50) = 'The number is ';

SELECT @str + CAST(@num AS VARCHAR(50)) AS Result;

In this example, @num (an int) is implicitly converted to a varchar when combined with @str (a varchar).

2. Explicit Conversion

Explicit conversion occurs when you explicitly tell SQL Server how to convert a value from one data type to another using the CAST() or CONVERT() functions.

Example of Explicit Conversion:

DECLARE @num INT = 100;
DECLARE @str VARCHAR(50);

-- Explicitly converting the integer to a string (varchar)
SET @str = CONVERT(VARCHAR(50), @num);

SELECT @str AS Result;

Here, the CONVERT() function is used to explicitly convert the integer value @num into a varchar before assigning it to @str.

Key Differences:

• Implicit Conversion: Automatically handled by SQL Server when the data types are compatible.
• Explicit Conversion: Requires the user to specify the conversion using functions like CAST() or CONVERT().

Note:

Implicit conversions may lead to performance overhead or errors in cases where automatic conversion cannot be done, especially with incompatible data types. In such cases, explicit conversion is preferred.