expression

Is any valid expression of any one of the data types in the character and binary data type category, except the image, ntext, or text data types. Both expressions must be of the same data type, or one expression must be able to be implicitly converted to the data type of the other expression.

An explicit conversion to character data must be used when concatenating binary strings and any characters between the binary strings. The following example shows when CONVERT, or CAST, must be used with binary concatenation and when CONVERT, or CAST, does not have to be used.

DECLARE @mybin1 varbinary(5), @mybin2 varbinary(5)
SET @mybin1 = 0xFF
SET @mybin2 = 0xA5
-- No CONVERT or CAST function is required because this example 
-- concatenates two binary strings.
SELECT @mybin1 + @mybin2
-- A CONVERT or CAST function is required because this example
-- concatenates two binary strings plus a space.
SELECT CONVERT(varchar(5), @mybin1) + ' ' 
   + CONVERT(varchar(5), @mybin2)
-- Here is the same conversion using CAST.
SELECT CAST(@mybin1 AS varchar(5)) + ' ' 
   + CAST(@mybin2 AS varchar(5))

The + (String Concatenation) operator behaves differently when it works with an empty, zero-length string than when it works with NULL, or unknown values. A zero-length character string can be specified as two single quotation marks without any characters inside the quotation marks. A zero-length binary string can be specified as 0x without any byte values specified in the hexadecimal constant. Concatenating a zero-length string always concatenates the two specified strings. When you work with strings with a null value, the result of the concatenation depends on the session settings. Just like arithmetic operations that are performed on null values, when a null value is added to a known value the result is typically an unknown value, a string concatenation operation that is performed with a null value should also produce a null result. However, you can change this behavior by changing the setting of CONCAT_NULL_YIELDS_NULL for the current session. For more information, see SET CONCAT_NULL_YIELDS_NULL (Transact-SQL).

If the result of the concatenation of strings exceeds the limit of 8,000 bytes, the result is truncated. However, if at least one of the strings concatenated is a large value type, truncation does not occur.

Returns the data type of the argument with the highest precedence. For more information, see Data Type Precedence (Transact-SQL).

A. Using string concatenation

USE AdventureWorks;
GO
SELECT (LastName + ', ' + FirstName) AS Name
FROM Person.Contact
ORDER BY LastName ASC, FirstName ASC;

B. Combining numeric and date data types

USE AdventureWorks;
GO
SELECT 'The order is due on ' + CONVERT(varchar(12), DueDate, 101)
FROM Sales.SalesOrderHeader
WHERE SalesOrderID = 50001;
GO

------------------------------------------------ 
The order is due on 04/23/2003            

(1 row(s) affected)

C. Using multiple string concatenation

USE AdventureWorks;
GO
SELECT (LastName + ',' + SPACE(1) + SUBSTRING(FirstName, 1, 1) + '.') AS Name, e.Title
FROM Person.Contact AS c
    JOIN HumanResources.Employee AS e
    ON c.ContactID = e.ContactID
WHERE e.Title LIKE 'Vice%'
ORDER BY LastName ASC;
GO

Name               Title                                         
-------------      ---------------
Duffy, T.         Vice President of Engineering
Hamilton, J.      Vice President of Production
Welcker, B.       Vice President of Sales
(3 row(s) affected)

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