"如何在 SQL 数据库中按 UUID 进行循环？"

Question

I have a list of agents that I want to assign tasks to using round robin.

id <\/th><\/tr><\/thead>
uuid1 <\/td><\/tr>
uuid2 <\/td><\/tr>
uuid3 <\/td><\/tr>
uuid4 <\/td><\/tr><\/tbody><\/table> How to get rows of the table above in a round robin fashion?

Answer 1

select nxt
from (
  select
    -- for every id, find the next item, if not exists, use the earliest (min) 
    id, coalesce(lead(id) over (order by id),(select min(id) from Tbl)) as nxt 
  from Tbl
  ) as Agents
where id = 'uuid4'

Answer 2

I don't have PostgreSQL handy right now, but this works under SQL Server and I don't see an obvious reason why it shouldn't under any other DBMS:

COALESCE(
    (SELECT TOP 1 id FROM agents WHERE @current_id < id ORDER BY id),
    (SELECT TOP 1 id FROM agents ORDER BY id)
)

Answer 3

You can combine two queries where the second is only run if the first one didn't return anything:

with next_agent as (
  SELECT id
  FROM agents 
  WHERE id > $1 -- the last ID retrieved or NULL if it's the first
  ORDER BY id
  limit 1
)
select *
from next_agent
union all
select *
from agents
where not exists (select * from next_agent)
order by id
limit 1