How to get the biggest id of database in SQL?

Question

How can I get the greatest id between all my database tables?

Intuitively something like this, but doesn't work:

SELECT 
    MAX(n.id_order, s.id_order, pr.id_order, pu.id_order) 
FROM 
    neworders AS n, salesorders AS s, proposalspurchase AS pr, purchaseorders AS pu

I need help with this id_order, is possible get each id easily and compare them to get the max. But is possible do it with just one query?

Answer 1

I need help with this id_order, is possible get each id easily and compare them to get the max. But is possible do it with just one query

Yes, We can get all ids and compare them to get the max using union or union all.

select MAX(id_order) as greatest_id
from (
    select MAX(id_order) as id_order from neworders
    union all
    select MAX(id_order) as id_order from salesorders
    union all
    select MAX(id_order) as id_order from proposalspurchase
    union all
    select MAX(id_order) as id_order from purchaseorders
) as orders;