A company uses a SQL Server database to store information about its customers and its business transactions. A new area code has been introduced for your city. The area code 111 remains the same for telephone numbers with prefixes that are less than 500. The numbers with prefixes that are 500 and greater will be assigned an area code of 222. All telephone numbers in the Phone column in the Customers table are stored as char(12) strings of the following format, ‘999-999-9999’. i must make the appropriate changes to the Customers table
as quickly as possible using the least administrative effort. Which one should I use ?
a.
UPDATE Customers SET Phone = ‘222-‘ + SUBSTRING(Phone,5,8)
FROM Customers WHERE SUBSTRING(Phone,1,3) = ‘111’
AND SUBSTRING(Phone,5,3) >= 500
b.
DECLARE PhoneCursor CURSOR FOR
SELECT Phone FROM Customers
WHERE SUBSTRING(Phone,1,3) = 111
AND SUBSTRING(Phone,5,3) >= 500
OPEN PhoneCursor
FETCH NEXT FROM PhoneCursor
WHILE @@FETCH_STATUS = 0
BEGIN
UPDATE Customers
SET Phone = ‘222’ + SUBSTRING(Phone,5,8)
WHERE CURRENT OF PhoneCursor
FETCH NEXT FROM PhoneCursor
END
CLOSE PhoneCursor
DEALLOCATE PhoneCursor
The big update will hold a transaction against the database for, potentially, a long time... locking things up and causing all kinds of havoc.
For this, I would recommend a cursor to spread that load out over a period of time.
I've also done a 'chunked' update... something like this:
DECLARE @Done bit = 0
WHILE @Done = 0
BEGIN
UPDATE TOP(10000)
Customers SET Phone = ‘222-‘ + SUBSTRING(Phone,5,8)
FROM Customers WHERE SUBSTRING(Phone,1,3) = ‘111’
AND SUBSTRING(Phone,5,3) >= 500
IF @@ROWCOUNT = 0
BEGIN
SET @Done = 1
END
END