character_expression

Is an expression of character or binary data. character_expression can be a constant, variable, or column. character_expression can be of any data type, except text or ntext, that can be implicitly converted to varchar or nvarchar. Otherwise, use the CAST function to explicitly convert character_expression.

integer_expression

Is a positive integer that specifies how many characters of the character_expression will be returned. If integer_expression is negative, an error is returned. If integer_expression is type bigint and contains a large value, character_expression must be of a large data type such as varchar(max).

Returns varchar when character_expression is a non-Unicode character data type.

Returns nvarchar when character_expression is a Unicode character data type.

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

A. Using LEFT with a column

USE AdventureWorks;
GO
SELECT LEFT(Name, 5) 
FROM Production.Product
ORDER BY ProductID;
GO

B. Using LEFT with a character string

SELECT LEFT('abcdefg',2)
GO

-- 
ab 

(1 row(s) affected)

Reference

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