力扣第619题

select coalesce(
    (
        select num
        from mynumbers
        group by num
        having count(num)=1
        order by num desc
        limit 1
    ),null) num 

这是我本来的思路：

COALESCE 函数会返回参数列表里第一个非 NULL 的值。若所有参数皆为 NULL，则返回 NULL。

但是运行速度太慢了，可以尝试：

with t as
    (
        select num
        from mynumbers
        group by num
        having count(num)=1
        order by num desc
        limit 1
    )
select max(num) as num from t;