Question

I have a table which contains unique indexes;lets say

A B
1 1
1 2
1 3
1 4

And i want to update the B column,since sql updates them one by one if it tries to update the first column to 2 for ex i get::

A B
1 2
1 2
1 3
1 4

And as a result i would get two duplicate values at the 1st and 2nd row and of course an error message.

And i should update the table like that:

A B
1 2
1 1
1 3
1 4

So whats the course of action i should follow in case of this scenario?

Regards.

Maybe i should update the question abit:
What if i wanted to change the b column completely; such as:

A B
1 4
1 2
1 3
1 1

Answer 1

The solution is to do the swap in a single statement:

UPDATE YOUR_TABLE
SET B = (CASE B WHEN 1 THEN 2 ELSE 1 END)
WHERE A = 1 AND B IN (1, 2)

— EDIT —

To update several rows at once, you can do a JOIN-update from the temporary table:

CREATE TABLE #ROWS_TO_UPDATE (
    A int,
    B int,
    NEW_B int,
    PRIMARY KEY (A, B)
);

INSERT INTO #ROWS_TO_UPDATE (A, B, NEW_B) VALUES (1, 1, 4);
INSERT INTO #ROWS_TO_UPDATE (A, B, NEW_B) VALUES (1, 2, 3);
INSERT INTO #ROWS_TO_UPDATE (A, B, NEW_B) VALUES (1, 3, 2);
INSERT INTO #ROWS_TO_UPDATE (A, B, NEW_B) VALUES (1, 4, 1);

UPDATE YOUR_TABLE
SET B = NEW_B
FROM
    YOUR_TABLE JOIN #ROWS_TO_UPDATE
        ON YOUR_TABLE.A = #ROWS_TO_UPDATE.A AND YOUR_TABLE.B = #ROWS_TO_UPDATE.B;

DROP TABLE #ROWS_TO_UPDATE;

The above code transforms the following data…

A B
1 1
1 2
1 3
1 4

…to this:

A B
1 4
1 2
1 3
1 1