Access sql to retrieve counts of values meeting a condition - sql

Question

I'm trying to write a query in Access that will return a count of values for each site in a table where the value exceeds a specified level, but also, for sites that have no values exceeding that level, return a specified value, such as "NA".
I've tried Iif, Switch, Union, sub queries, querying a different query, but no luck. I can get all the counts exceeding the level, or all sites with "NA" correct but showing total count for the rest, not just count above the level.
For example, in the table below, assuming level > 10, Houston = "NA", Detroit = 2, Pittsburgh PA = 3. I just can't get both sides of the query to work.
Apologize in advance for poor formatting.
+-----------------+-------+
| 1. Site | Value |
+-----------------+-------+
| 2. Houston | 10 |
| 3. Houston | 3 |
| 4. Houston | 0 |
| 5. Detroit | 15 |
| 6. Detroit | 7 |
| 7. Detroit | 4 |
| 8. Detroit | 12 |
| 9. Pittsburgh | 23 |
| 10. Pittsburgh | 2 |
| 11. Pittsburgh | 18 |
| 12. Pittsburgh | 12 |
+-----------------+-------+

Answer 1

Another solution is to use conditional aggregation, as follows :
SELECT site, SUM(IIf(value > 10, 1, 0)) AS value
FROM mytable
GROUP BY site
This approach should be more efficient than self-joining the table, since it requires to scan the table only once.
The SUM(IIf ...) is a handy construct to count how many records satisfy a given condition.
NB : it is generally not a good idea to return two different data types in the same column (in your use case, either a number or string 'NA'). Most RDBMS do not allow that. So I provided a query that will return 0 when there are not matches, instead of NA. If you really want 'NA', you can try :
IIF(
SUM(IIf(value > 10, 1, 0)) = 0,
'NA',
STR(SUM(IIf(value > 10, 1, 0)))
) AS value
This demo on DB Fiddle, with your sample data returns :
site | value
:--------- | ----:
Detroit | 2
Houston | 0
Pittsburgh | 3

Answer 2

Get a list of all sites independant of the counts (SiteList derived table below)
LEFT Join this back to your base table (SiteValues) to get the counts for each site where it's meeting threshold. --note should join on key which I'm not sure what is for this table. site alone isn't enough
Count the values from the siteValues dataset as NULL's will get counted as 0.
WORKING DEMO:
.
SELECT SiteList.Site, Count(Sitevalues.Site)
FROM (SELECT site, value
FROM TableName) SiteList
LEFT JOIN TableName SiteValues
on SiteList.Site = SiteValues.Site
and SiteValues.Value > 10
and SiteValues.Value = SiteList.value
GROUP BY SiteList.Site
GIVING US:
+----+------------+------------------+
| | Site | (No column name) |
+----+------------+------------------+
| 1 | Detroit | 2 |
| 2 | Houston | 0 |
| 3 | Pittsburgh | 3 |
+----+------------+------------------+
Or if you need the NA you have to cast the count to a varchar
SELECT SiteList.Site, case when Count(Sitevalues.Site) = 0 then 'NA' else cast(count(Sitevalues.site) as varchar(10)) end as SitesMeetingThreshold
FROM (SELECT site, value
FROM TableName) SiteList
LEFT JOIN TableName SiteValues
on SiteList.Site = SiteValues.Site
and SiteValues.Value > 10
and SiteValues.Value = SiteList.value
GROUP BY SiteList.Site

Answer 3

Just use conditional aggregation:
select site,
max(iif(value > 10, 1, 0)) as cnt_11plus
from t
group by site;
I think 0 is better than N/A. But if you want that you'll need to convert the results to a string.
select site,
iif(max(iif(value > 10, 1, 0)) > 0,
str(max(iif(value > 10, 1, 0))),
"N/A"
) as cnt_11plus
from t
group by site;

Answer 4

You can use UNION like this:
SELECT site, count(value) AS counter
FROM sites
WHERE value > 10
GROUP BY site
UNION
SELECT s.site, 'NA' AS counter
FROM sites AS s
WHERE value <= 10
AND NOT EXISTS (
SELECT 1 FROM sites WHERE site = s.site AND value > 10
)
GROUP BY site
Results:
site counter
Detroit 2
Houston NA
Pittsburgh 3
There is no need to convert the integer counter to Text, because Access does this implicitly for you.