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If only one employee quali es, you get the employee ID in the @empid variable. If no employee quali es, the subquery sets @empid to NULL. When you get a NULL, you know that you had no matches. If multiple employees qualify, you get an error saying that the subquery returned more than one value. In such a case, you will realize that something is wrong with your assumptions or with the design of your code. But the problem will surface as opposed to eluding you. When you understand how an assignment SELECT works, you can use it to your advantage. For example, a SET statement can assign only one variable at a time. An assignment SELECT can assign values to multiple variables within the same statement. With well-designed code, this capability can give you performance bene ts. For example, the following code assigns the rst name and last name of a given employee to variables:
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DECLARE @firstname AS NVARCHAR(10), @lastname AS NVARCHAR(20); SELECT @firstname = NULL, @lastname = NULL; SELECT @firstname = firstname, @lastname = lastname FROM HR.Employees WHERE empid = 3; SELECT @firstname, @lastname;
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Notice that this code uses the primary key to lter an employee, meaning that you cannot get more than one row back. The code also initializes the @ rstname and @lastname variables with NULLs. If no employee quali es, the variables simply retain the NULLs. This type of assignment is especially useful in triggers when you want to read attributes from the special tables inserted and deleted into your own variables, after you verify that only one row was affected. Technically, you could rely on the fact that an assignment SELECT performs multiple assignments when multiple rows qualify. For example, you could do aggregate calculations, such as concatenating all order IDs for a given customer:
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DECLARE @Orders AS VARCHAR(8000), @custid AS INT; SET @custid = 1; SET @Orders = ''; SELECT @Orders = @Orders + CAST(orderid AS VARCHAR(10)) + ';' FROM Sales.Orders WHERE custid = @custid; SELECT @Orders;
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However, this code is far from being standard, and the ability to apply such an assignment SELECT with multiple rows is not of cially documented. This type of assignment is also often used with an ORDER BY clause, assuming that the order of concatenation is guaranteed, like so:
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DECLARE @Orders AS VARCHAR(8000), @custid AS INT; SET @custid = 1; SET @Orders = '';
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SELECT @Orders = @Orders + CAST(orderid AS VARCHAR(10)) + ';' FROM Sales.Orders WHERE custid = @custid ORDER BY orderdate, orderid; SELECT @Orders;
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But again, no of cial documentation de nes the behavior of such multirow assignment SELECT statements, let alone ones that include an ORDER BY clause. I did stumble across the following blog by Microsoft s Conor Cunningham, in which he indicates that this undocumented technique does guarantee concatenation order: http://blogs.msdn.com/ sqltips/archive/2005/07/20/441053.aspx. However, I have to stress that I feel very awkward about this technique, and I m reluctant to trust it to always work, including in future versions of the product. You have enough supported and guaranteed techniques to choose from for such calculations, many of which I covered in 8, Aggregating and Pivoting Data.
Assignment UPDATE
T-SQL also supports a nonstandard UPDATE syntax that can assign values to variables in addition to modifying data. To demonstrate the technique, rst run the following code, which creates the table T1 and populates it with sample data:
USE tempdb; IF OBJECT_ID('dbo.T1') IS NOT NULL DROP TABLE dbo.T1; CREATE TABLE dbo.T1 ( col1 INT NOT NULL, col2 VARCHAR(5) NOT NULL ); GO INSERT INTO dbo.T1(col1, col2) VALUES (0, 'A'), (0, 'B'), (0, 'C'), (0, 'C'), (0, 'C'), (0, 'B'), (0, 'A'), (0, 'A'), (0, 'C'), (0, 'C');
Currently, the T1 table has no primary key, and there s no way to uniquely identify the rows. Suppose that you wanted to assign unique integers to col1 and then make it the primary key. You can use the following assignment UPDATE to achieve this task:
DECLARE @i AS INT; SET @i = 0; UPDATE dbo.T1 SET @i = col1 = @i + 1; SELECT * FROM dbo.T1;
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