expression

Is an expression of any data type.

bigint if expression is of the varchar(max), nvarchar(max) or varbinary(max) data types; otherwise int.

DATALENGTH is especially useful with varchar, varbinary, text, image, nvarchar, and ntext data types because these data types can store variable-length data.

The DATALENGTH of NULL is NULL.

Note:
Compatibility levels can affect return values. For more information about compatibility levels, see sp_dbcmptlevel (Transact-SQL)

The following example finds the length of the Name column in the Product table.

USE AdventureWorks;
GO
SELECT length = DATALENGTH(Name), Name
FROM Production.Product
ORDER BY Name;
GO

Reference

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