"The SQL command ran, there's no error message but nothing changed! What's going on? Do you think it's a virus? Maybe there's a bug in .NET!"
The junior programmer was almost in tears when he came to me for help. Proud and confident in his abilities and faced with failure on what should have been an easy task, the poor little fellow's puffed up ego collapsed like a house of cards. I patted him gently on the shoulder and said soothingly, "It's because you're an idiot."
OK, I didn't really say that.
Actually, having an SQL update statement not change any records and not produce an error is expected and desirable behavior:
If you run an update to add a late fee to all outstanding accounts over 2 weeks late and there are no late accounts….the update will succeed, no records will be modified and there will be no error message. This is as it should be.
When an update does not produce an error and no records are modified, it's because the WHERE clause did not locate any records to update. Sometimes this is OK, other times it is a mistake.
Here are three ways this can happen by mistake:
Case Sensitivity Misses:
If the collating sequence of the database or column is case sensitive, the text in the column may not match the text in the WHERE clause: Z123 does NOT equal z123. In SQL Server the default is case insensitive.
Space Padded Data:
The field type in the database may space pad the data. In SQL Server, if a column type is char(6) or nchar(6) and 'ABC' is saved, what is actually put in the table is 'ABC ' (note the 3 spaces after ABC). A search for 'ABC' will fail because 'ABC' does not equal 'ABC '.
Dates with Extraneous Times Stamps
DateTime columns have two components (Date and Time…duh). If you are searching a DateTime column either the data or the search criteria may include a timestamp that causes the search to fail:
12-31-2000 12:23:45 AM does not equal 12-31-2000 12:00:00 AM
When storing dates, it is a best practice to truncate the time of DateTime fields so the times are zero (or 12:00:00 AM).
Here is an SQL statement to scrub existing data:
UPDATE MyTable SET MyDate = dateadd(dd, datediff(dd, 0, MyDate), 0)
When searching for records you can set the search criteria to include only the date by using the Date property of the DateTime structure:
DateTime SearchDate = DateTime.Now.Date;
Note: SQL Server 2008 has new data types of just DATE and TIME.
Bonus: Floating Point Numbers:
This is extremely rare but if you have floating point numbers in the data and expect to use an exact WHERE clause to retrieve and update data, good luck. As everyone knows who's taken Numerical Analysis 1001:
1.50000000000 does not equal 1.49999999999
Final:
If you are unsure of what is going to be updated in a statement, it is easy to replace the UPDATE with a SELECT and check what is actually being retrieved.
I hope this helps someone.
[Edit]
One more thing that can cause a WHERE clause to fail….
A field containing NULL will not be found if the WHERE clause is looking for an empty string (and vice versa).
下面來自微軟文檔，使用from語句進行更新注意的情況：
Use caution when specifying the FROM clause to provide the criteria for the update operation. The results of an UPDATE statement are undefined if the statement includes a FROM clause that is not specified in such a way that only one value is available for each column occurrence that is updated, that is if the UPDATE statement is not deterministic. For example, in the UPDATE statement in the following script, both rows in Table1 meet the qualifications of the FROM clause in the UPDATE statement; but it is undefined which row from Table1 is used to update the row in Table2.
大概意思就是：在使用帶有from語句的update時，如果from語句中的表的記錄不是一對一的關係，而是一對多，或者是多對多，那麼對於update來說，沒有辦法決定使用哪個記錄進行更新的，如下例中的table1中有兩條記錄和table2中的一條記錄對應，那麼使用table1中的記錄更新table2中的記錄的最終結果是不確定的（not deterministic）。
USE AdventureWorks2012;
GO
IF OBJECT_ID ('dbo.Table1', 'U') IS NOT NULL
DROP TABLE dbo.Table1;
GO
IF OBJECT_ID ('dbo.Table2', 'U') IS NOT NULL
DROP TABLE dbo.Table2;
GO
CREATE TABLE dbo.Table1
(ColA int NOT NULL, ColB decimal(10,3) NOT NULL);
GO
CREATE TABLE dbo.Table2
(ColA int PRIMARY KEY NOT NULL, ColB decimal(10,3) NOT NULL);
GO
INSERT INTO dbo.Table1 VALUES(1, 10.0), (1, 20.0);
INSERT INTO dbo.Table2 VALUES(1, 0.0);
GO
UPDATE dbo.Table2
SET dbo.Table2.ColB = dbo.Table2.ColB + dbo.Table1.ColB
FROM dbo.Table2
INNER JOIN dbo.Table1
ON (dbo.Table2.ColA = dbo.Table1.ColA);
GO
SELECT ColA, ColB
FROM dbo.Table2;