一个SQL查询可对不同条件下的记录进行计数

Question

How can I count the total number of records, the number of unique of users and number of records on which the status is 2 from the table participants in one query?

I know how to accomplish this using 3 separate queries: SELECT COUNT( ) FROM participants SELECT COUNT( ) FROM participants GROUP BY user SELECT COUNT(*) FROM participants WHERE status = 2

But this doesn't really seem efficient?

Table participants

id         user           status 
10     john@example.com      1       
11     john@example.com      1     
12     john@example.com      1     
13    sally@mailing.com      1     
14    sally@mailing.com      2     
15   edward@halloworld.com   1     
16   edward@halloworld.com   1     
17   edward@halloworld.com   2 
18     mike@bestmail.net     2      
19     mike@bestmail.net     1 
29     nat@worldcom.com      0

Answer 1

Since you want just one result (per requirement), you don't need a group by clause at all, and all of these requirements can be created as arguments for the count function:

SELECT COUNT(*) AS total_records,
       COUNT(DISTINCT user) AS distinct_users_count,
       COUNT(CASE status WHEN 2 ELSE NULL END) AS status_2_count
FROM   participants

Answer 2

Just use conditional aggregation:

select count(*) as numrecords, count(distinct user) as numusers,
       sum(status = 2) as numstatus_2
from participants p;