Do I need a where clause in a conditional UPDATE? - sql

Question

We imported a lot of data from another table. Now I'm trying to correct some of them.
UPDATE [x10ddata].[dbo].[ResourceTest]
SET [Country] = (CASE
WHEN [Country] IN ('Aezerbaijan', 'AZERBIJAN') THEN 'Azerbaijan'
WHEN [Country] = 'Belgique' THEN 'Belgium'
WHEN [Country] = 'China (RPC)' THEN 'China'
WHEN [Country] = 'Columbia' THEN 'Colombia'
WHEN [Country] = 'Croatia (Local Name: Hrvatska)' THEN 'Croatia'
.....//...
WHEN [Country] IN ('U.S.', 'U.S.A', 'U.S.A.', 'US', 'USA',
'USA - Maryland', 'USAQ') THEN 'United States'
END)
GO
I didn't use ELSE because many rows have valid country. My question is to know whether I need to the WHERE clause to filter the rows that will be affected?
The reason I'm asking this question is that, I've selected into a test table and tried the the script. According to the output, all the rows affected, but when I check closely, not all the rows were affected. It's confusing.
Thanks for helping

Answer 1

The case statement will return null if none of the when clauses are met. You can verify this with this simple sql:
declare #i int
set #i = 2
select case when #i = 1 then 'A' end AS Column1
This will return null since #i is not 1.
To fix this in your case, you can either add the where clause like you said, or the simpler option might be to add ELSE [Country] after all of your WHEN clauses. This would mean "If I don't need to change the country field, then just use the same value that was there before."

Answer 2

You won't need a WHERE clause, but the ELSE clause is needed. Change your statement to:
UPDATE [x10ddata].[dbo].[ResourceTest]
SET [Country] = (CASE
WHEN [Country] IN ('Aezerbaijan', 'AZERBIJAN') THEN 'Azerbaijan'
WHEN [Country] = 'Belgique' THEN 'Belgium'
WHEN [Country] = 'China (RPC)' THEN 'China'
WHEN [Country] = 'Columbia' THEN 'Colombia'
WHEN [Country] = 'Croatia (Local Name: Hrvatska)' THEN 'Croatia'
.....//...
WHEN [Country] IN ('U.S.', 'U.S.A', 'U.S.A.', 'US', 'USA',
'USA - Maryland', 'USAQ') THEN 'United States'
ELSE [Country]
END)

Answer 3

alternatively,
Make a conversion table,
DECLARE #conversion TABLE
(
[Before] NVARCHAR(250) NOT NULL,
[After] NVARCHAR(250) NOT NULL
);
INSERT #conversion
VALUES
('Aezerbaijan', 'Azerbaijan'),
...
('USAQ', 'United States');
Then do,
UPDATE [x10ddata].[dbo].[ResourceTest]
SET [Country] = [C].[After]
FROM
[x10ddata].[dbo].[ResourceTest]
JOIN
#conversion [C]
ON [C].[Before] = [C].[Country];
This has a number of potential performance benefits over the extend CASE approach, among which is only effecting rows that need to change.
Its probably worth using a temporary table instead of a table variable and creating an index on [Before] to optimize the join.

Answer 4

No you don't need a where clause because your CASE statements contain your logic.
Note: If a value doesn't match any of your CASE statements then it will return null. So I recommend adding ELSE [Country] at the end. Here's an example that demonstrates what I'm saying
SELECT * INTO #yourTable
FROM
(
SELECT 1 ID, CAST('OldValue' AS VARCHAR(25)) val
UNION ALL
SELECT 2 , 'OldValue'
UNION ALL
SELECT 3,'Doesnt need to be updated'
) A
SELECT *
FROM #yourTable;
Results:
ID val
----------- -------------------------
1 OldValue
2 OldValue
3 Doesnt need to be updated
Now update:
UPDATE #yourTable
SET val =
CASE
WHEN ID = 1 THEN 'NewValue1'
WHEN ID = 2 THEN 'NewValue2'
--Add this so you leave values alone if they don't match your case statements
ELSE val
END
FROM #yourTable
SELECT *
FROM #yourTable
Results:
ID val
----------- -------------------------
1 NewValue1
2 NewValue2
3 Doesnt need to be updated

Answer 5

No, you don't NEED it. Aside from the performance cost that may be incurred through additional (unnecessary) writes to disk and locking (blocking other sessions), the physical outcome would be the same.
One could argue that you SHOULD use a WHERE clause, not only for performance reasons, but to better capture and convey intentions.