Data Type Conversion

Implicit and Explicit Conversion

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Data types can be converted either implicitly or explicitly.

Implicit conversions are not visible to the user. SQL Server automatically converts the data from one data type to another. For example, when a smallint is compared to an int, the smallint is implicitly converted to int before the comparison proceeds.

GETDATE() implicitly converts to date style 0. SYSDATETIME() implicitly converts to date style 21.
Explicit conversions use the CAST or CONVERT functions.

The CAST and CONVERT functions convert a value (a local variable, a column, or another expression) from one data type to another. For example, the following CAST function converts the numeric value of $157.27 into a character string of '157.27':

CAST ( $157.27 AS VARCHAR(10) )

Use CAST instead of CONVERT if you want Transact-SQL program code to comply with ISO. Use CONVERT instead of CAST to take advantage of the style functionality in CONVERT.
The following illustration shows all explicit and implicit data type conversions that are allowed for SQL Server system-supplied data types. These include xml, bigint, and sql_variant. There is no implicit conversion on assignment from the sql_variant data type, but there is implicit conversion to sql_variant.