I want to count the number of distinct items in a column subject to a certain condition, for example if the table is like this:
tag | entryID----+---------foo | 0foo | 0bar | 3If I want to count the number of distinct tags as "tag count" and count the number of distinct tags with entry id > 0 as "positive tag count" in the same table, what should I do?
I'm now counting from two different tables where in the second table I've only selected those rows with entryID larger than zero. I think there should be a more compact way to solve this problem.
Best Answer
You can try this:
selectcount(distinct tag) as tag_count,count(distinct (case when entryId > 0 then tag end)) as positive_tag_countfromyour_table_name;The first count(distinct...) is easy.The second one, looks somewhat complex, is actually the same as the first one, except that you use case...when clause. In the case...when clause, you filter only positive values. Zeros or negative values would be evaluated as null and won't be included in count.
One thing to note here is that this can be done by reading the table once. When it seems that you have to read the same table twice or more, it can actually be done by reading once, in most of the time. As a result, it will finish the task a lot faster with less I/O.
This may work:
SELECT Count(tag) AS 'Tag Count'FROM TableGROUP BY tagand
SELECT Count(tag) AS 'Negative Tag Count'FROM TableWHERE entryID > 0GROUP BY tagTry the following statement:
select distinct A.[Tag],count(A.[Tag]) as TAG_COUNT,(SELECT count(*) FROM [TagTbl] AS B WHERE A.[Tag]=B.[Tag] AND B.[ID]>0)from [TagTbl] AS A GROUP BY A.[Tag]The first field will be the tag the second will be the whole count the third will be the positive ones count.
I agree with @ntalbs solution,if you want to count a column's data when the condition of another column's data is valid, you can do this
selectcount(distinct tag) as tag_count,count(distinct tag, case when entryId > 0 then tag end) as positive_tag_countfromyour_table_name;On line 3, I added the column name beside the distinct, so it will count the distinct tags when the entryId is greater than 0
Code counts the unique/distinct combination of Tag & Entry ID when [Entry Id]>0
select count(distinct(concat(tag,entryId)))from customerswhere id>0In the output it will display the count of unique valuesHope this helps
This may also work:
SELECT COUNT(DISTINCT T.tag) as DistinctTag,COUNT(DISTINCT T2.tag) as DistinctPositiveTagFROM Table TLEFT JOIN Table T2 ON T.tag = T2.tag AND T.entryID = T2.entryID AND T2.entryID > 0You need the entryID condition in the left join rather than in a where clause in order to make sure that any items that only have a entryID of 0 get properly counted in the first DISTINCT.