Código:SELECT id + 1 FROM mytable mo WHERE NOT EXISTS ( SELECT NULL FROM mytable mi WHERE mi.id = mo.id + 1 ) ORDER BY id LIMIT 1
SELECT MIN(idcliente)+1
FROM
(
SELECT 10000 'idCliente'
UNION ALL
SELECT idCliente FROM Cliente
UNION ALL
SELECT 99999 'idCliente'
) AS TMP
WHERE TMP.idCliente + 1 NOT IN (SELECT idCliente FROM Cliente)