Re: updation order of fields in a table

[Rick James]

The SETs are probably done in order, thereby creating the "wrong" answer.

If you are doing just one row, this might be best:

BEGIN;   -- Assuming you are using InnoDB.
SELECT @adjustment := LEAST(AmtCr, AmtDr);
    FROM tbl1
    WHERE ...   -- assuming it hits exactly one row
    FOR UPDATE;
UPDATE tbl1 SET
        AmtDr = AmtDr - @adjustment,
        AmtCr = AmtCr + @adjustment
    WHERE ...;
COMMIT;

Note that I rewrote your confusing IFs into something more logical. (I am not sure I got the business logic correct.)

To do the entire table,
ALTER TABLE tbl1 ADD COLUMN `Adj` ...; -- done once
1. BEGIN
2. UPDATE SET Adj = ...
3. UPDATE SET AmtDr = ..., AmtCr = ...;
4. UPDATE SET Adj = 0;
5. COMMIT;
Keep this new column if you expect to repeat this process.

But why do it? You can always get the data you need without actually adjusting the values, can't you?