如何获得不对特定民意调查投票的选民的数量(How to get the count of voters who don't vote on a specific poll)

 我能够在民意调查中得到总票数。 但我需要得到选民没有投票的总数。 例如，选民＃1仅投票给总统类别和参议员类别，选民＃2仅投票给副主席和参议员。 最后，选民＃3没有投票，我仍然将选民投票＃3投票并留下NULL。 因此，结果应该是没有在特定类别上投票的选民数量。 
 
PRESIDENT      : 2
VICE PRESIDENT : 2
SENATORS       : 1
 
 在选民＃1没有投票支持vp的情况下，选民＃2没有投票支持pres，选民＃3则投了空票。 
 
 这是我的表格。 
 
Polls
id | poll_title 
 1 | presidential election

Poll_categories
id | poll_id | category_name
 1 | 1       | President
 2 | 2       | Vice-President
 3 | 3       | Senator

Poll_items
id | poll_category_id | item_name
 1 | 1                | Obama
 2 | 1                | Bush
 3 | 1                | Clinton
 4 | 2                | Biden
 5 | 3                | Shelby
 6 | 3                | Murkowski

Poll_votes
id | voters_id | poll_item_id
 1 | 1         |  1
 2 | 1         |  5
 3 | 1         |  6
 4 | 2         |  4
 5 | 2         |  6
 6 | 3         |  NULL
 
 到目前为止，这是我的查询，但我不知道接下来该做什么。 我也无法显示elections_id 3，因为它是NULL 
 
select c.id, c.category_name, v.voters_id from poll_category c
LEFT JOIN (select v.user_id, v.poll_item_id, i.poll_category_id as cat_id, i.item_name 
           from poll_votes v 
           LEFT JOIN poll_items i on i.id = v.poll_item_id) v
    ON v.cat_id = c.id
WHERE c.poll_id = 1

I was able to get the total votes on my polls. But I need to get the total count where the voters didn't vote. For example voter#1 voted on president category and senator category only and voter#2 voted on vice pres and senator only. And last, voter#3 doesn't vote which i still put voter#3 on the poll votes and leave it NULL. So result should be NUMBERS OF VOTERS WHO DIDN'T VOTE ON A SPECIFIC CATEGORY.
 
PRESIDENT      : 2
VICE PRESIDENT : 2
SENATORS       : 1
 
Where in voter#1 didn't vote for vp, voter#2 didn't vote for pres and voter#3 sent an empty votes.
 
Here are my tables.
 
Polls
id | poll_title 
 1 | presidential election

Poll_categories
id | poll_id | category_name
 1 | 1       | President
 2 | 2       | Vice-President
 3 | 3       | Senator

Poll_items
id | poll_category_id | item_name
 1 | 1                | Obama
 2 | 1                | Bush
 3 | 1                | Clinton
 4 | 2                | Biden
 5 | 3                | Shelby
 6 | 3                | Murkowski

Poll_votes
id | voters_id | poll_item_id
 1 | 1         |  1
 2 | 1         |  5
 3 | 1         |  6
 4 | 2         |  4
 5 | 2         |  6
 6 | 3         |  NULL
 
So far here is my query, but i don't know what to do next. I also can't show the voters_id 3 because it is NULL
 
select c.id, c.category_name, v.voters_id from poll_category c
LEFT JOIN (select v.user_id, v.poll_item_id, i.poll_category_id as cat_id, i.item_name 
           from poll_votes v 
           LEFT JOIN poll_items i on i.id = v.poll_item_id) v
    ON v.cat_id = c.id
WHERE c.poll_id = 1

原文：https://stackoverflow.com/questions/35933893