count-how-many-records-there-are-of-each-type.md

Count How Many Records There Are Of Each Type

Let's say I have a books table full of data. One of the columns on this table is status which represents whether the book is published, in review, or still a draft.

We can find out how many records (books) there are for each status using a group by clause and the count aggregate function.

> select status, count(*)
    from books
    group by status;

  status   | count
-----------+-------
 ø         |   123
 published |   611
 draft     |   364
 review    |   239
(4 rows)

Because we don't have a not null constraint on the status column, there are also some records that have a null value.

We can take this a step further by ordering the output in a consistent way—descending order of the count column.

> select status, count(*)
    from books
    group by status
    order by 2 desc;

  status   | count
-----------+-------
 published |   611
 draft     |   364
 review    |   239
 ø         |   123
(4 rows)

This order by clauses uses a positional index from the select arguments, so the 2 references the count(*) argument.