SQL合并可防止对轮询进行双重投票

Question

我正在开发一个民意调查应用程序。SQL模式为：

polls -> * options (poll_id) -> * answers (option_id)

或者："民意调查有很多选项，选项可以有答案（又称投票）"

每个用户每次投票我只能允许一次投票。这是merge子句（显然不起作用）：

  merge into answers
  using (select count(*)
         from answers, options, polls
         where answers.option_id = options.id
         and options.poll_id = polls.id
         and polls.id = {poll_id}
         and answers.owner_id = {owner_id}) votes
  on (votes = 0)
  when matched then
  insert into answers values (NULL, {option_id}, {owner_id}, NOW())

Answer 1

如果你从不想更新，那么一个简单的插入就可以了。我认为根本不需要合并：

insert into answers (some_value, option_id, owner_id, last_modified)
select null, {option_id}, {owner_id}, current_timestamp
from dual
where not exists (select 1 
                  from answers a
                    join options o on o.id = a.option_id
                    join polls p on p.id = o.poll_id
                  where a.owner_id = {owner_id}
                    and p.id = {polls_id}

我明确列出了insert子句的列，因为不这样做的编码风格不好。当然，我只是猜测了你的列名，因为你没有向我们展示你的表定义。

Answer 2

试试这个：

MERGE INTO answers
USING 
(SELECT options.id 
             FROM options, polls
             WHERE options.poll_id = polls.id
             AND polls.id = {poll_id}
) options
ON (answers.option_id = options.id AND answers.owner_id = {owner_id})
WHEN NOT MATCHED THEN
    INSERT INTO answers VALUES (NULL, {option_id}, {owner_id}, SYSDATE)
WHEN MATCHED THEN
    -- Do what you need to do for an update