Cursor and Update rows based on value/date

Question

SQL Server 2012
Microsoft SQL Server Management Studio
11.0.3128.0
Microsoft Analysis Services Client Tools
11.0.3128.0
Microsoft Data Access Components (MDAC)
6.1.7601.17514
Microsoft MSXML 3.0 4.0 5.0 6.0 
Microsoft Internet Explorer
9.11.9600.16518
Microsoft .NET Framework
4.0.30319.18408
Operating System
6.1.7601
The objective of this is to test the Cursor and use it on a production environment after this is fixed. What I would like to do is update rows in a column i duplicated originally called 'HiredDate' from AdventureWorks2012 HumanResources.Employee table. I
made a duplicate column called 'DateToChange' and would like to change it based on a date I have picked, which returns normally 2 results (i.e. date is '04/07/2003'). The code runs but will not change both dates. It did run however with an error but changed
only 1 of the 2 rows because it said ['nothing available in next fetch'].
The code to add the columns and perform the query to get the results I am running this against:
-- ADD column 'DateToChange'
ALTER TABLE [HumanResources].[Employee] ADD DateToChange Date NOT NULL;
-- Copy 'HireDate' data to 'DateToChange'
UPDATE HumanResources.Employee SET DateToChange = HireDate;
-- Change 'DateToChange' to NOT NULL
ALTER TABLE [HumanResources].[Employee] ALTER COLUMN DateToChange Date NOT NULL;
SELECT BusinessEntityID,HireDate, CONVERT( char(10),[DateToChange],101) AS [Formatted Hire Date]
FROM HumanResources.Employee
WHERE [DateToChange] = '04/07/2003';
Code:
USE AdventureWorks2012;
GO
-- Holds output of the CURSOR
DECLARE @EmployeeID INT
DECLARE @HiredDate DATETIME
DECLARE @HiredModified DATETIME
DECLARE @ChangeDateTo DATETIME
--Declare cursor
-- SCROLL CURSOR ALLOWS "for extra options" to pul multiple records: i.e. PRIOR, ABSOLUTE ##, RELATIVE ##
DECLARE TestCursor CURSOR SCROLL FOR
-- SELECT statement of what records going to be used by CURSOR
-- Assign the query to the cursor.
SELECT /*HumanResources.Employee.BusinessEntityID, HumanResources.Employee.HireDate,*/ CONVERT( char(10),[DateToChange],101) AS [Formatted Hire Date]
FROM HumanResources.Employee
WHERE DateToChange = '01/01/1901'
/*ORDER BY HireDate DESC*/ FOR UPDATE OF [DateToChange];
-- Initiate CURSOR and load records
OPEN TestCursor
-- Get first row from query
FETCH NEXT FROM TestCursor
INTO @HiredModified
-- Logic to tell the Cursor while "@@FETCH_STATUS" 0 the cursor has successfully fetched the next record.
WHILE (@@FETCH_STATUS = 0 AND @@CURSOR_ROWS = -1)
BEGIN
FETCH NEXT FROM TestCursor
IF (@HiredModified = '04/07/2003')/*05/18/2006*/
-- Sets @HiredModifiedDate data to use for the change
SELECT @ChangeDateTo = '01/01/1901'
UPDATE HumanResources.Employee
SET [DateToChange] = @ChangeDateTo --'01/01/1901'
FROM HumanResources.Employee
WHERE CURRENT OF TestCursor;
END
-- CLOSE CURSOR
CLOSE TestCursor;
-- Remove any references held by cursor
DEALLOCATE TestCursor;
GO
This query is run successfully but it does not produce the desired results to change the dates
04/07/2003 to 01/01/1901.
I would like the query to essentially be able to run the initial select statement, and then update and iterate through the returned results while replacing the necessary column in each row.
I am also open to changes or a different design all together. 
For this query I need:
1. To narrow the initial set of information
2. Check if the information returned, in particular a date, is before [i.e. this current month minus 12 months or
12 months before current month]
3. Next replace the dates with the needed date
[Haven't written this out yet but it will need to be done]
4. After all this is done I will then need to update a column on each row:
if the 'date' is within 12 months to 12 months from the date checked
NOTE: I am new to TSQL and have only been doing this for a few days, but I will understand or read up on what is explained if given enough information. Thank you in advance for anyone who may be able to help.

Answer 1

The first thing you need to do is forget about cursors.  Those are rarely needed.  Instead you need to learn the basics of the tsql language and how to work with data in sets.  For starters, your looping logic is incorrect.  You open
the cursur and immediately fetch the first row.  You enter the loop and the first thing in the loop does what?  Fetches another row.  That means you have "lost" the values from the first row fetched.  You also do not test the success of
that fetch but immediately try to use the fetched value.  In addition, your cursor includes the condition "DateToChange = '01/01/1901' " - by extension you only select rows where HireDate is Jan 1 1901.  So the value fetched into @HiredModified will
never be anything different - it will always be Jan 1 1901.  The IF logic inside your loop will always evaluate to FALSE.  
But forget all that.  In words, tell us what you are trying to do.  It seems that you intend to add a new column to a table - one that is not null (ultimately) and is set to a particular value based on some criteria.  Since you intend the
column to be not null, it is simpler to just add the column as not null with a default.  Because you are adding the column, the assumption is that you need to set the appropriate value for EVERY row in the table so the actual default value can be anything.
 Given the bogosity of the 1/1/1901 value, why not use this as your default and then set the column based on the Hiredate afterwards.  Simply follow the alter table statement with an update statement.  I don't really understand what your logic
or goal is, but perhaps that will come with a better description.  In short: 
alter table xxx add DateToChange date default '19010101'
update xxx set DateToChange = HireDate where [some unclear condition]
Lastly, you should consider wrapping everything you do in a transaction so that you recover from any errors.  In a production system, you should consider making a backup immediately before you do anything - strongly consider and have a good reason not
to do so if that is your choice (and have a recovery plan just in case).