How do I get first unused ID in the table?

Question

I have to write a query wherein i need to allocate a ID (unique key) for a particular record which is not being used / is not being generated / does not exist i

Answer 1

Should work under MySql.

SELECT TOP 100
    T1.ID + 1 AS FREE_ID 
FROM TABLE1 T1
LEFT JOIN TABLE2 T2 ON T2.ID = T1.ID + 1
WHERE T2.ID IS NULL