创建另一个具有足够id值的查找表bar并使用query：

select min(b.id) 
from bar b
 left join foo f on f.id=b.id 
where b.id between 50 and 59 and f.id is null;

如果不能使用其他表，则可以使用递归CTE来模拟一个表：

with recursive bar (id) AS
(
  select 1
  union all
  select id + 1 from bar where id < 100
)
select min(b.id) 
from bar b
 left join foo f on f.id=b.id 
where b.id between 50 and 59 and f.id is null;

如果你想获得下一个免费的id，你可以用途：

SELECT COALESCE(MIN(id), 0) + 1
FROM `foo` 
WHERE NOT EXISTS (SELECT id FROM `foo` t2 WHERE t2.id = `foo`.id + 1);

如果id有一个起始值（例如50）：

SELECT COALESCE(MIN(id), 0) + 1
FROM `foo` 
WHERE NOT EXISTS (SELECT id FROM `foo` t2 WHERE t2.id = `foo`.id + 1);
AND id > 50

如果id应该在某个范围内（例如在50和59之间）：

SELECT COALESCE(MIN(id), 0) + 1
FROM `foo` 
WHERE NOT EXISTS (SELECT id FROM `foo` t2 WHERE t2.id = `foo`.id + 1);
AND id > 50
AND id < 59

注意：id和50必须存在，否则上面的查询不会得到51。