Question

(This is a follow-up to the answer by “onedaywhen” on a question I posted earlier today.)

Hi everyone. Say I have a table MyTable with two int fields, PrimaryKey and MyNumber. (And other fields not directly relevant to this question). MyNumber has a unique constraint and a check constraint limiting it to BETWEEN 1 AND n. (Let’s say n=5 for now.)

1,2  
2,NULL  
3,5  
4,NULL  
5,NULL  
6,1  
7,NULL

How could an UPDATE be written to change the record where PrimaryKey=2 so that MyNumber has a non-NULL value? I don’t care what value it has, so long as it’s not a NULL and meets it’s two constraints of being unique and within range.

I’m using MS SQL Server, but I’m hoping there’s an answer using standard SQL.

(I’m also hoping there won’t have to be a table with the numbers 1 to n as contents.)

Many thanks.

Answer 1

WITH CTE AS (
    SELECT 1 N
    UNION ALL
    SELECT N + 1 FROM CTE WHERE N < 5
)
UPDATE MyTable
SET MyNumber = (
    SELECT TOP 1 N FROM CTE
    WHERE NOT EXISTS (SELECT * FROM MyTable WHERE MyNumber = N)
)
WHERE PrimaryKey = 2

In plain English:

• Generate integers between 1 and 5 (WITH CTE AS ...).
• Pick the first of these integers that does not already exist in MyNumber (SELECT TOP 1 ...).
• Assign that integer to MyNumber, in the row identified by PrimaryKey = 2 (UPDATE ...).

If this query fails to find a suitable value, it will simply set the MyNumber to NULL.

---

WARNING: This might still viloate UNIQUE constraint on MyNumber in a concurrent environment (i.e. when two concurrently-executing transactions try to run this same query in parallel). So, you’d have to be prepared to retry the query if necessary.