looping by pairs in SQL - sql

Question

I've looked at the few topics here that I thought would answer this, but they don't seem to be exactly what I'm looking for.
Basically, I have a database that, among other things, has a user table. Due to those very same users, we often end up with duplicates. In other words, one person has two separate accounts and uses both of them until we catch on and merge them into one account, tell them which it is and to stop trying to use the second account. Unfortunately, the decision of which account to keep and which to axe isn't always something that follows a formula, but rather an admin's knowledge of the situation and I'm informed on a line-by-line basis of which stays and goes.
Part of the merge involves modifying several tables by replacing any occurrences of the discarded ID with the ID we're keeping. The last guy to have this job had a script to do this, but it simply assumed that the most recent login was the one to keep and pulled those pairs of IDs and made the mods. I, however, can't rely on that being the case. I have a spreadsheet with pairs of IDs that I would love to be able to run through and process. Until now, I've been doing this all by hand, one at a time.
So, what I'm looking for is something to the effect of:
foreach (x,y) in (oldID1, newID1, oldID2, newID2, ...){
go through tables and change all instances of x to y;
}
Hopefully, that was clear enough.
Thanks!

Answer 1

For simple one-off jobs like this, I like to use LINQPad to whip up a quick script. You could just use an anonymous-typed array for your id-pairs, and execute the script for each pair (assuming SQL Server):
var idpairs = new [] {
new { OldId = 5, NewId = 10 },
new { OldId = 23, NewId = 45 },
new { OldId = 443, NewId = 299 }
};
using (var con = new SqlConnection(connectionString)) {
foreach (var idpair in idpairs) {
var oldId = idpair.OldId;
var newId = idpair.NewId;
// prepare script with old/new id's inserted
var sql = string.Format("blah blah blah {0} blah blah {1} blah",
oldId, newId);
// execute the command
var cmd = con.CreateCommand();
cmd.CommandText = sql;
cmd.ExecuteNonQuery();
}
}

Answer 2

I am going to make a number of assumptions since the information is not completely supplied. One assumption is that the spreadsheet is separated into the fields described above (oldID,newID)
Import the spreadsheet into a table on SQL.
Then, use the following code (first for MS SQL):
DECLARE #oldID int, #newID int
DECLARE CURSOR idCursor STATIC FOR
SELECT oldID,newID FROM idSpreadsheetTable
OPEN idCursor
FETCH NEXT FROM idCursor INTO #oldID,#newID
WHILE (##FETCH_STATUS = 0)
BEGIN
UPDATE Table1 SET idField = #newID WHERE idField = #oldID
... More update table commands
FETCH NEXT FROM idCursor INTO #oldID,#newID
END
CLOSE idCursor
DEALLOCATE idCursor
and here is mySQL syntax:
DECLARE done INT DEFAULT FALSE;
DECLARE oldID,newID int;
DECLARE idCursor CURSOR FOR SELECT oldID,newID FROM idSpreadsheetTable
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
OPEN idCursor;
read_loop: LOOP
FETCH idCursor INTO oldID, newID;
IF done THEN
LEAVE read_loop;
END IF;
UPDATE Table1 SET idField = newID WHERE idField = oldID;
... More update table commands
END LOOP;
CLOSE idCursor;