+= (String Concatenation Assignment) (Transact-SQL)

מאמר
09/03/2024

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW) SQL analytics endpoint in Microsoft Fabric Warehouse in Microsoft Fabric

Concatenates two strings and sets the string to the result of the operation. For example, if a variable @x equals 'Adventure', then @x += 'Works' takes the original value of @x, adds 'Works' to the string, and sets @x to that new value 'AdventureWorks'.

Transact-SQL syntax conventions

Syntax

expression += expression

Arguments

expression
Is any valid expression of any of the character data types.

Result Types

Returns the data type that is defined for the variable.

Remarks

SET @v1 += 'expression' is equivalent to SET @v1 = @v1 + ('expression'). Also, SET @v1 = @v2 + @v3 + @v4 is equivalent to SET @v1 = (@v2 + @v3) + @v4.

The += operator cannot be used without a variable. For example, the following code will cause an error:

SELECT 'Adventure' += 'Works'

Examples

A. Concatenation using += operator

The following example concatenates using the += operator.

DECLARE @v1 VARCHAR(40);  
SET @v1 = 'This is the original.';  
SET @v1 += ' More text.';  
PRINT @v1;

Here's the result set.

This is the original. More text.

B. Order of evaluation while concatenating using += operator

The following example concatenates multiple strings to form one long string and then tries to compute the length of the final string. This example demonstrates the evaluation order and truncation rules, while using the concatenation operator.

DECLARE @x VARCHAR(4000) = REPLICATE('x', 4000)
DECLARE @z VARCHAR(8000) = REPLICATE('z',8000)
DECLARE @y VARCHAR(max);
 
SET @y = '';
SET @y += @x + @z;
SELECT LEN(@y) AS Y; -- 8000
 
SET @y = '';
SET @y = @y + @x + @z;
SELECT LEN(@y) AS Y; -- 12000
 
SET @y = '';
SET @y = @y +(@x + @z);
SELECT LEN(@y) AS Y; -- 8000
-- or
SET @y = '';
SET @y = @x + @z + @y;
SELECT LEN(@y) AS Y; -- 8000
GO

Here's the result set.

Y       
------- 
8000 
 
(1 row(s) affected) 
 
   
Y       
------- 
12000 
 
(1 row(s) affected) 

Y       
------- 
8000 
 
(1 row(s) affected) 
 
Y       
------- 
8000 
 
(1 row(s) affected)

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