t-sql reordering rows - tsql

Question

I have a table with a column called "Priority". No two records should have the same priority value.
If I add a new entry with the same priority value as an existing record, it should increment the
priority of the other records that follow IF the previous row's increment causes a duplicate priority.
For example:
We want to insert a record with a priority of 2.
(BEFORE)
priority
1
2
3
5
(AFTER)
Priority
1
2
3
4
5
Another example:
Insert a record with a priority of 2
(BEFORE)
priority
1
2
3
5
7
(AFTER)
Priority
1
2
3
4
5
7
I am doing this with the following code and it works as long as there are no gaps in sequence:
UPDATE MyTable SET Priority = Priority + 1
WHERE LocationId = #LocationId AND Priority >= #priorityToInsert
The problem is that this update statement increments ALL priority values. Example #2 above fails
because the record with priority 7 gets incremented to 8 when it shouldn't.
Please help!

Answer 1

You could use something like this:
UPDATE MyTable t1
SET t1.Priority = t1.Priority + 1
WHERE t1.LocationId = #LocationId
AND t1.Priority >= #priorityToInsert
AND NOT EXISTS (
SELECT *
FROM MyTable t2
WHERE t2.LocationId = #LocationId
AND t2.Priority = t1.Priority - 1
)
The problem is that if you also had an 8 in your case, it WOULD advance the 8 to 9 because it sees the 7, even though the 7 wouldn't advance.
Because it's recursive like this, you would probably have to use a recursive or iterative technique.
I would think about a different design or perhaps something with triggers.