SQL：计算表中不同的行值

Question

I have a similar table to this in SQL:

id |tag | entryID
---+----+--------
1  |foo | 0
2  |foo | 0
3  |bar | 3
5  |bar | 3
6  |foo | 3
7  |foo | 3

I want to run a query to count distinct rows in the table (with the id column dropped). The result should look like this (or a transpose of this table):

(tag=foo, entryID=0) | (tag=foo, entryID=3) | (tag=bar, entryID=3)
---------------------+----------------------+---------------------
2                    | 2                    | 2

What should this query look like?

Note : The values in each of the columns are not known beforehand.

Answer 1

You can do this using conditional aggregation:

select sum(tag = 'foo' and entryId = 0) as "tag=foo, entryID=0",
       sum(tag = 'foo' and entryId = 3) as "tag=foo, entryID=3",
       sum(tag = 'bar' and entryId = 3) as "tag=bar, entryID=0"
from t;

However, the normal method is to put the counts in rows, not columns:

select tag, entryId, count(*)
from t
group by tag, entryId;

The rows are much more versatile, because you don't have to list out every combination you might want to match.