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

are you allowed to have a utility table? if so i would create a table like so:

CREATE TABLE number_helper (
    n INT NOT NULL
   ,PRIMARY KEY(n)
);

Fill it with all positive 32 bit integers (assuming the id you need to generate is a positive 32 bit integer)

Then you can select like so:

SELECT MIN(h.n) as nextID
FROM my_table t
LEFT JOIN number_helper h ON h.n = t.ID
WHERE t.ID IS NULL

Haven't actually tested this but it should work.