How to do a count on a union query

问题

I have the following query:

select distinct profile_id from userprofile_...

union

select distinct profile_id from productions_...

How would I get the count of the total number of results?

回答1:

If you want a total count for all records, then you would do this:

SELECT COUNT(*)
FROM
(
    select distinct profile_id 
    from userprofile_...

    union all

    select distinct profile_id 
    from productions_...
) x

回答2:

you should use Union All if there are equals rows in both tables, because Union makes a distinct

select count(*) from 
(select distinct profile_id from userprofile_...

union ALL

select distinct profile_id from productions_...) x

In this case, if you got a same Profile_Id in both tables (id is probably a number, so it's possible), then if you use Union, if you got Id = 1 in both tables, you will lost one row (it will appear one time instead of two)

回答3:

This will perform pretty well:

select count(*) from (
    select profile_id
    from userprofile_...
    union
    select profile_id
    from productions_...
) x

The use of union guarantees distinct values - union removes duplicates, union all preserves them. This means you don't need the distinct keyword (the other answers don't exploit this fact and end up doing more work).

Edited:

If you want to total number of different profile_id in each, where given values that appear in both table are considered different values, use this:

select sum(count) from (
    select count(distinct profile_id) as count
    from userprofile_...
    union all
    select count(distinct profile_id)
    from productions_...
) x

This query will out-perform all other answers, because the database can efficiently count distinct values within a table much faster than from the unioned list. The sum() simply adds the two counts together.

回答4:

As omg ponies has already pointed out that there is no use of using distinct with UNION, you can use UNION ALL in your case.....

SELECT COUNT(*) 
FROM 
( 
select distinct profile_id from userprofile_...
union all
select distinct profile_id from productions_...
) AS t1

回答5:

These will not work if in one of the COUNT(*) the result is equals to 0.

This will be better:

SELECT SUM(total)
FROM
(
    select COUNT(distinct profile_id) AS total
    from userprofile_...

    union all

    select COUNT(distinct profile_id) AS total
    from productions_...
) x

回答6:

Best solution is to add count of two query results. It will not be a problem if the table contains large number of records. And you don't need to use union query. Ex:

SELECT (select COUNT(distinct profile_id) from userprofile_...) + 
(select COUNT(distinct profile_id) from productions_...) AS total

来源：https://stackoverflow.com/questions/11731655/how-to-do-a-count-on-a-union-query

标签

mysql

sql

union

aggregate-functions