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Inside Microsoft SQL Server 2008: T-SQL Querying
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Such support would allow for simple standard solutions and naturally also lend itself to better optimization. Another option to handle the task at hand is to use a CTE. By using a CTE, you can come up with a simple solution that is easy to troubleshoot and maintain, yielding an ef cient plan very similar to the one that uses a join UPDATE. Simply create a CTE out of a join SELECT and then UPDATE the target table through the CTE, like so:
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BEGIN TRAN; WITH UPD_CTE AS ( SELECT O.shipcountry AS set_country, C.country AS get_country, O.shipregion AS set_region, C.region AS get_region, O.shipcity AS set_city, C.city AS get_city FROM Sales.Orders AS O JOIN Sales.Customers AS C ON O.custid = C.custid WHERE C.country = 'USA' ) UPDATE UPD_CTE SET set_country = get_country, set_region = get_region, set_city = get_city; ROLLBACK TRAN
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Note Even though CTEs are de ned by ANSI SQL, the DELETE and UPDATE syntax against CTEs
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implemented in SQL Server is not standard.
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This UPDATE generates an identical plan to the one generated for the UPDATE based on a join. In SQL Server 2008 you can handle such tasks using a MERGE statement that you also express using join semantics. I ll discuss MERGE and provide examples later in this chapter. You should be aware of another issue when using the join-based UPDATE. When you modify the table on the one side of a one-to-many join, you might end up with a nondeterministic update. To demonstrate the problem, run the following code, which creates the tables Customers and Orders and populates them with sample data:
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USE tempdb; GO IF OBJECT_ID('dbo.Orders') IS NOT NULL DROP TABLE dbo.Orders; IF OBJECT_ID('dbo.Customers') IS NOT NULL DROP TABLE dbo.Customers; GO
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CREATE TABLE dbo.Customers ( custid VARCHAR(5) NOT NULL PRIMARY KEY, qty INT NULL ); INSERT INTO dbo.Customers(custid) VALUES('A'),('B'); CREATE TABLE dbo.Orders ( orderid INT NOT NULL PRIMARY KEY, custid VARCHAR(5) NOT NULL REFERENCES dbo.Customers, qty INT NOT NULL ); INSERT INTO dbo.Orders(orderid, custid, qty) VALUES (1, 'A', 20), (2, 'A', 10), (3, 'A', 30), (4, 'B', 35), (5, 'B', 45), (6, 'B', 15);
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Data Modi cation
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Customers and Orders have a one-to-many relationship. Notice that each row in Customers currently has three related rows in Orders. Now, examine the following UPDATE and see if you can guess how Customers would look after the UPDATE:
UPDATE C SET qty = O.qty FROM dbo.Customers AS C JOIN dbo.Orders AS O ON C.custid = O.custid;
The truth is that the UPDATE is nondeterministic. You can t guarantee which of the values from the related Orders rows is used to update the qty value in Customers. Remember that you cannot assume or rely on any physical order of the data. For example, run the following query against Customers after running the preceding UPDATE:
SELECT custid, qty FROM dbo.Customers;
You might get the following output:
custid -----A B qty ----------20 35
But you might just as easily get the following output:
custid -----A B qty ----------10 45
Inside Microsoft SQL Server 2008: T-SQL Querying
It is interesting to note that if you attempt such an update with the MERGE statement, where the same target row is modi ed more than once, SQL Server raises an error, and the statement fails to execute. The MERGE statement doesn t allow such a nondeterministic update like the join UPDATE allows. When you re done experimenting with nondeterministic UPDATEs, run the following code to drop Orders and Customers:
IF OBJECT_ID('dbo.Orders') IS NOT NULL DROP TABLE dbo.Orders; IF OBJECT_ID('dbo.Customers') IS NOT NULL DROP TABLE dbo.Customers;
Updating Large Value Types
This section covers updates of large value types (VARCHAR(MAX), NVARCHAR(MAX), and VARBINARY(MAX)) using the WRITE method. In my examples I ll use the CustomerData table that you create by running the code provided earlier in the chapter in Listing 10-1. Then run the following code to insert a row into the table:
INSERT INTO dbo.CustomerData(custid, txt_data) VALUES(102, 'Customer 102 text data');
To update a column of a large value type, you can use a regular UPDATE statement setting the column to a result of an expression. For example, if you want to modify a certain section within, such as the txt_data column value for customer 102, you could set the column to the result of an expression using the STUFF function. However, an update using regular data manipulation would result in overwriting the entire string using full logging, which is inef cient with large values. Instead, the UPDATE statement supports a WRITE method for large value types. The WRITE method allows you to modify only a section within the string and not overwrite the whole thing. Plus, when the database recovery model is not set to FULL, some of the updates using the WRITE method can bene t from minimal logging. Those include inserting or appending new data. Logically, the WRITE method is similar to the STUFF function. It accepts three arguments: @expression, @offset, and @length. The @expression argument replaces @length units (characters/bytes) starting from @offset position in the target value. Note The @offset argument is zero based. For example, the following code operates on the txt_data column value for customer 102. It replaces the string 102 located at offset 9 (zero based) with the string one hundred and two , resulting in the string Customer one hundred and two text data :
UPDATE dbo.CustomerData SET txt_data.WRITE('one hundred and two', 9, 3) WHERE custid = 102;
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