Single query to return 'count of all rows', 'count of distinct value in row' and 'count of rows having value'

Posted on 2004-11-03
Last Modified: 2008-01-09
If I have a table 'TABLE' like the following:
-----  ----- ----
A      Y      Z
B      Y      Z
B      Y      Q
C      Y      Q
C      Y      Q
I would like to write a single select statement that returns three values in one row:

count(distinct foo) (count of all distinct values in FOO) (Would return 3 in this example)
count() (count of all rows) (Would return 5 in this example)
count() (count of all BAZ containing 'Q') (group by baz having baz = 'Q' / Would return 3 in this example)
Make sense? Any single statement do that or must this be split into two seperate queries?
(IE: select count(distinct foo), count(1) from TABLE;  select count(1) from TABLE group by BAZ having BAZ = 'Q';)
Question by:gripe
    LVL 3

    Accepted Solution

    Use SUM(CASE baz WHEN 'Q' THEN 1 ELSE 0 END).

    --- begin postgres code ---
    create table table1 (
          foo char(1) not null,
          bar char(1) not null,
          baz char(1) not null

    insert into table1 (foo, bar, baz) values ('A', 'Y', 'Z');
    insert into table1 (foo, bar, baz) values ('B', 'Y', 'Z');
    insert into table1 (foo, bar, baz) values ('B', 'Y', 'Q');
    insert into table1 (foo, bar, baz) values ('C', 'Y', 'Q');
    insert into table1 (foo, bar, baz) values ('C', 'Y', 'Q');

    select count(distinct foo), count(*), sum(case baz when 'Q' then 1 else 0 end)
      from table1;
    LVL 13

    Author Comment

    Thanks a lot.. didn't know about tthose case blocks, but that will definitely come in handy.

    Is this a postgres thing? Do you know where it's documented?

    LVL 3

    Expert Comment

    CASE is part of the ANSI SQL standard and fully supported in Postgres.  You should be able to use it anywhere you would use an expression, such as in a function call (demonstrated here), on the right side of an assignment (SET field = CASE ...), or in a WHERE clause (WHERE x = CASE ...).  There are too many situations to list.  Wherever the docs say "expression", you should be able to use CASE.

    A similar use to your question is for cross-tab queries:
    SUM(CASE foo WHEN 'A' THEN 1 ELSE 0 END) AS total_a,
    SUM(CASE foo WHEN 'B' THEN 1 ELSE 0 END) AS total_b,
    SUM(CASE foo WHEN 'C' THEN 1 ELSE 0 END) AS total_c
    COUNT(*) AS total_all
    FROM table1

    Here is a link to the docs:

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