SQL`GROUP BY`和`MAX（）`

Question

Language    Date
----------------------
HTML        2017-03-15
HTML        2017-03-15
HTML        2017-03-19
JavaScript  2017-03-12
JavaScript  2017-03-17
PHP         2017-03-15
PHP         2017-03-17
PHP         2017-03-18
PHP         2017-03-18

I need a result grouped by language with the second column a COUNT() of each language row from the latest date entry. Not all languages have entries on the same dates.

For the table above, results should be:

HTML        1
JavaScript  1
PHP         2

Answer 1

Here is one method:

select language, count(*)
from t
where t.date = (select max(t2.date)
                from t t2
                where t2.language = t.language
               )
group by language;

This uses a correlated subquery go match the most recent date for each language. There are other methods, such as using a join or:

select language, count(*)
from t
where (t.language, t.date) in (select t2.language, max(t2.date)
                               from t t2
                               group by t2.language
                             )
group by language;