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

I named your table unused.

SELECT  id
FROM    (
        SELECT  1 AS id
        ) q1
WHERE   NOT EXISTS
        (
        SELECT  1
        FROM    unused
        WHERE   id = 1
        )
UNION ALL
SELECT  *
FROM    (
        SELECT  id + 1
        FROM    unused t
        WHERE   NOT EXISTS
                (
                SELECT  1
                FROM    unused ti
                WHERE   ti.id = t.id + 1
                )
        ORDER BY
                id
        LIMIT 1
        ) q2
ORDER BY
        id
LIMIT 1

This query consists of two parts.

The first part:

SELECT  *
FROM    (
        SELECT  1 AS id
        ) q
WHERE   NOT EXISTS
        (
        SELECT  1
        FROM    unused
        WHERE   id = 1
        )

selects a 1 is there is no entry in the table with this id.

The second part:

SELECT  *
FROM    (
        SELECT  id + 1
        FROM    unused t
        WHERE   NOT EXISTS
                (
                SELECT  1
                FROM    unused ti
                WHERE   ti.id = t.id + 1
                )
        ORDER BY
                id
        LIMIT 1
        ) q2

selects a first id in the table for which there is no next id.

The resulting query selects the least of these two values.