显示基于计数的SQL查询结果

Question

I am looking for a way to only show results of a query based upon a count. I have the following:

# Returns expected number
SELECT systems.hostname AS Hostname, systems.date AS "Last Checked In", count(systems2updates.2package) AS "Needed Updates" 
FROM systems2updates RIGHT JOIN systems on systems2updates.2hostname=systems.hostname 
GROUP BY systems.hostname;

# Shows all entires, including those with 0
SELECT systems.hostname AS Hostname, systems.date AS "Last Checked In", count(systems2updates.2package) AS "Needed Updates" FROM systems2updates 
RIGHT JOIN systems on systems2updates.2hostname=systems.hostname 
WHERE "Needed Updates" > 0 
GROUP BY systems.hostname;

# Returns nothing
SELECT systems.hostname AS Hostname, systems.date AS "Last Checked In", count(systems2updates.2package) AS "Needed Updates" 
FROM systems2updates RIGHT JOIN systems on systems2updates.2hostname=systems.hostname WHERE "Needed Updates" = 0 
GROUP BY systems.hostname;

Any assistance greatly appreciated. Thank you.

Answer 1

WHERE limits the records you are using. The aggregation result "Needed Updates" does not exist at that moment. If you want to filter on aggregation results, use HAVING :

...
GROUP BY systems.hostname
HAVING "Needed Updates" = 0;

BTW: MySQL allows column aliases in HAVING . This is not allowed in standard SQL however, where you would have to GROUP BY count(systems2updates.2package) .

UPDATE: MySQL is not always standard-compliant when it comes to quotes. Alias names in double quotes can be taken for a string by MySQL. Better avoid such alias names.

select 
  s.hostname as hostname, 
  s.date as last_checked_in, 
  count(su.2package) as needed_updates 
from systems s
left join systems2updates su on su.2hostname = s.hostname
group by s.hostname
having count(su.2package) = 0;

Answer 2

I assume you are looking to only return rows where the COUNT is greater than a certain value? If that's the case, the HAVING clause is the easiest way. For example:

SELECT sampleValue, COUNT(*)
FROM dbo.SomeTable
GROUP BY sampleValue
HAVING COUNT(*) > 5

Note that HAVING does not perform well, and you may want to look at an alternative solution using the ROW_NUMBER() function.