Using CASE Statements In A SQL UPDATE Query
Posted August 30, 2007 at 7:35 AM
Nothing revolutionary here, I just recently tried to use a SQL CASE statement as part of an SQL UPDATE statement. I had never tried this before and I am quite happy that it works. I am working on a transactional system that allows for the voiding of account credits. As part of the Void process, I need to make sure that I never let an account balance drop below zero dollars (business requirement - these aren't "real" dollars). In something like ColdFusion, I would use the Max() function:
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- Max( 0, (balance - void_credit) )
But, in SQL, the MAX() function performs aggregate calculations on groups, not "max of two numbers" calculations. As such, I tried the following CASE statement:
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- <cfquery name="qUpdate" datasource="xxx">
- UPDATE
- [account]
- SET
- balance =
- (
- CASE
- WHEN
- ((balance - 10.00) < 0)
- THEN
- 0
- ELSE
- (balance - 10.00)
- END
- )
- WHERE
- id = 1
- </cfquery>
I am hard coding the variables here, but you get the point. I think it's kind of cool that this works. I guess there's nothing about it that should have made me think that it wouldn't work; I just never tried this technique before. I love the fact that you can reference values in the database row that you are going to update. Very cool!
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Reader Comments
Nice work Ben,
I always like to see people leaning on thier database a little to do this kind of work for them, many people would simply break that in to two queries, the first being a SELECT and then use CF conditionals to determine the update type.
I'm a big fan of having SQL do this stuff for you, as CF developers its very easy to forget that SQL is an entire language all of its own and is MASSIVLY powerfull for this data manipulation stuff, it took me quite a while to earn full respect for SQL as its own language when I first started developing, but once I did you start thinking about things in a very different way.
Rob
MySQL has a Greatest() function:
http://dev.mysql.com/doc/refman/5.1/en/comparison-operators.html#function_greatest
I'm sure the other servers have something equivalent. Otherwise, you'd probably want to use tsql or a stored proc.
@Rob,
I know what you mean. SQL is awesomely powerful. I am still learning new things all the time. I still have yet to fully understand the whole CONVERT() function and I would love to learn more about Cursors and that sort of stuff.
@Todd,
It looks like MS SQL Server doesn't have the Greatest() equivalent, at least nothing that I could find on Google.
@Ben: Yup, tsql it is. Anyway, your case statement works.
Another nice technique that I have found invaluable on occasion is using a join in an update statement. I don't know if something like this works on MySQL but on SQL Server you can do something like the following
UPDATE p
SET
p.col1 = p.col1 + q.otherCol
FROM table1 AS p join table2 AS q
ON p.fk_table2_pk = q.pk
WHERE
q.filterColumn = 'something'
@John,
That is awesome! I knew you could do something like that on a View, but I had no idea you could just update with join usage in a standard statement. Crazy!
@Ben/John,
I had just learned you could do joins in an UPDATE SQL statement early last month. I should have guessed you could since the FROM statement is there and joining would making sense if you need to bring data from other locations. I learned how to use it because I needed to move data from one database to another (QA to DEV). It's easy to get lost in SQL but it can really do some neat stuff.
It's amazing what SQL can do once you start digging into it.
A little gotcha about CASE statements is that you can only return simple values in your THEN clauses. It's a little less flexible than using dynamic CF in your queries, but as long as you are only looking to return simple values the CASE statement is very powerful.
Also check out IF and IFNULL. IFNULL is nice if for example, your column value is null but you want to return 0:
IFNULL(mycol,0) as col
This will return the column value if it is not null, or 0 if it is null. This can save a lot of extra CF coding if you happen to be in that situation.
When I say simple values I mean you can't do something like this:
CASE WHEN 1
THEN 'Where colval = 1'
ELSE
'Where colval = 2'
END
In other words you can't change the structure of the query the way you can using dynamic CF.
SQL can do many magical stuff.
I use SQL for all the business logic. I use ColdFusion for presentation.
CASE statement can be effectively used in ORDER BY :
http://blog.sqlauthority.com/2007/07/17/sql-server-case-statement-in-order-by-clause-order-by-using-variable/
People do create whole another SP (as another comment earlier) instead of using CASE.
I use other technologies with SQL but ColdFusion has been my favorite since day one.
Regards,
Pinal
@John Eric,
I just tried your technique (UPDATE + JOIN) and it works like a charm! Very cool stuff:
http://www.bennadel.com/index.cfm?dax=blog:938.view
Thanks for the hot tip.
Hi,
What if I want to "switch" the field name? Your example is
Update xxx
Set balance = (Case
.....
End)
Where ......
But I want to do like this:
Update xxx
Set (Case
.....
End) = 1000
Where ......
I have tried this syntax, but it doesn't work. What should I do?
Thanks.
@Kenny,
Are you trying to dynamically select the actual column to update? Could this be something that would be done more effectively using the parent language (ColdFusion, ASP, etc):
UPDATE xxx
<cfif A>
SET a = 1000
<cfelse>
SET b = 1000
</cfif>
WHERE....
Is that what you're trying to do?
Yes, that's what I am trying to do. I have many stored procedures update the same table but different fields, and I am thinking to merge them into one sproc. By doing so, the front end developer can just call the same sproc and do several update processes by varying the keywords.
@Kenny,
You can probably build a dynamic SQL statement and then EXEC() it or something. I've never done this personally, but I think this is how a lot of people handle pagination.
I have the following SQL statement that meets all the requirements of what I am trying to do…
SELECT Diff_Adj,
Case
When Diff_Adj < 0
Then '-'+right ('00000000'+convert (varchar(10), convert (int, -Diff_Adj*100)), 9)
Else right ('000000000'+convert (varchar(10), convert (int, Diff_Adj*100)), 10)
End
FROM dbo.Tran405_CC1
…and these are the results:
Diff_Adj
0000000100
0000000000
0000000000
-000000100
-008638200
Now, instead of running a SELECT statement, I need to run an UPDATE statement to modify data in my table, here is the Update statement I am using to do so…
UPDATE dbo.Tran405_CC1
SET Diff_Adj =
CASE
WHEN Diff_Adj < 0
THEN '-'+right ('00000000'+convert (varchar(10), convert (int, -Diff_Adj*100)), 9)
ELSE right ('000000000'+convert (varchar(10), convert (int, Diff_Adj*100)), 10)
END
…and these are my results:
Diff_Adj
1.00
.00
.00
-1.00
-86382.00
FYI…the data type for Diff_Adj column is Decimal (10, 2)
Thanks in advance
CORRECTED CODE
I have the following SQL statement that meets all the requirements of what I am trying to do…
SELECT Diff_Adj,
Case
When Diff_Adj < 0
Then '-'+right ('00000000'+convert (varchar(10), convert (int, -Diff_Adj*100)), 9)
Else right ('000000000'+convert (varchar(10), convert (int, Diff_Adj*100)), 10)
End
FROM dbo.Tran405_CC1
…and these are the results:
Diff_Adj
0000000001
0000000000
0000000000
-000000001
-000086382
Now, instead of running a SELECT statement, I need to run an UPDATE statement to modify data in my table, here is the Update statement I am using to do so…
UPDATE dbo.Tran405_CC1
SET Diff_Adj =
CASE
WHEN Diff_Adj < 0
THEN '-'+right ('00000000'+convert (varchar(10), convert (int, -Diff_Adj*100)), 9)
ELSE right ('000000000'+convert (varchar(10), convert (int, Diff_Adj*100)), 10)
END
…and these are my results:
Diff_Adj
1.00
.00
.00
-1.00
-86382.00
FYI…the data type for Diff_Adj column is Decimal (10, 2)
Thanks in advance
