Since PostgreSQL 9.4 there's the FILTER clause, which allows for a very concise query to count the true values:

select count(*) filter (where myCol)
from tbl;

The above query is a bad example in that a simple WHERE clause would suffice, and is for demonstrating the syntax only. Where the FILTER clause shines is that it is easy to combine with other aggregates:

select count(*), -- all
       count(myCol), -- non null
       count(*) filter (where myCol) -- true
from tbl;

The clause is especially handy for aggregates on a column that uses another column as the predicate, while allowing to fetch differently filtered aggregates in a single query:

select count(*),
       sum(otherCol) filter (where myCol)
from tbl;

Cast the Boolean to an integer and sum.

SELECT count(*),sum(myCol::int);

You get 6,3.

probably, the best approach is to use nullif function.

in general

select
    count(nullif(myCol = false, true)),  -- count true values
    count(nullif(myCol = true, true)),   -- count false values
    count(myCol);

or in short

select
    count(nullif(myCol, true)),  -- count false values
    count(nullif(myCol, false)), -- count true values
    count(myCol);

http://www.postgresql.org/docs/9.0/static/functions-conditional.html

The shortest and laziest (without casting) solution would be to use the formula:

SELECT COUNT(myCol OR NULL) FROM myTable;

Try it yourself:

SELECT COUNT(x < 7 OR NULL)
   FROM GENERATE_SERIES(0,10) t(x);

gives the same result than

SELECT SUM(CASE WHEN x < 7 THEN 1 ELSE 0 END)
   FROM GENERATE_SERIES(0,10) t(x);

Simply convert boolean field to integer and do a sum. This will work on postgresql :

select sum(myCol::int) from <table name>

Hope that helps!

select f1,
       CASE WHEN f1 = 't' THEN COUNT(*) 
            WHEN f1 = 'f' THEN COUNT(*) 
            END AS counts,
       (SELECT COUNT(*) FROM mytable) AS total_counts
from mytable
group by f1

Or Maybe this

SELECT SUM(CASE WHEN f1 = 't' THEN 1 END) AS t,
       SUM(CASE WHEN f1 = 'f' THEN 1 END) AS f,
       SUM(CASE WHEN f1 NOT IN ('t','f') OR f1 IS NULL THEN 1 END) AS others,
       SUM(CASE WHEN f1 IS NOT NULL OR f1 IS NULL THEN 1 ELSE 0 END) AS total_count
FROM mytable;

In MySQL, you can do this as well:

SELECT count(*) AS total
     , coalesce(sum(myCol), 0) AS countTrue --yes, you can add TRUEs as TRUE=1 and FALSE=0 !!
FROM yourTable
;

I think that in Postgres, this works:

SELECT count(*) AS total
     , coalesce(sum(myCol::int), 0) AS countTrue --convert Boolean to Integer
FROM yourTable
;

or better (to avoid :: and use standard SQL syntax):

SELECT count(*) AS total
     , coalesce(sum(CAST(myCol AS int)), 0) AS countTrue --convert Boolean to Integer
FROM yourTable
;

Note: The COALESCE() function is used to avoid a problem with how SUM() behaves when all the values are NULL. We want it to return 0 (and not NULL).

Benchmark

TL;DR: take the solution you like. There's no significant difference.

Utility scripts

before(){
    psql <<-SQL
        create table bench (
                id         serial
            , thebool    boolean
        );

        insert into bench (thebool)
        select (random() > 0.5)
        from generate_series(1, 1e6) g;


        analyze bench;
    SQL
}
after(){
    psql -c 'drop table bench'
}
test(){
    echo $(tput bold)$1$(tput sgr0)
    psql -c "explain analyze select $1 from bench" | tail -4 | head -2
}

Actual benchmark

Made on a 1.4GHz i5 MacBookPro, psql and pg 12.4 (pg in a linux docker container):

before  
test 'count(*) filter (where thebool)'
# Planning Time: 0.138 ms
# Execution Time: 4424.042 ms
test 'count(case when thebool then 1 end)'
# Planning Time: 0.156 ms
# Execution Time: 4638.861 ms
test 'count(nullif(thebool, false))'
# Planning Time: 0.201 ms
# Execution Time: 5267.631 ms
test 'count(thebool or null)'
# Planning Time: 0.202 ms
# Execution Time: 4672.700 ms
test 'sum(thebool::integer)'
# Planning Time: 0.155 ms
# Execution Time: 4602.406 ms
test 'coalesce(sum(case when thebool THEN 1 ELSE 0 END), 0)'
# Planning Time: 0.167 ms
# Execution Time: 4416.503 ms
after

SELECT count(*)         -- or count(myCol)
FROM   <table name>     -- replace <table name> with your table
WHERE  myCol = true;

Here's a way with Windowing Function:

SELECT DISTINCT *, count(*) over(partition by myCol)
FROM   <table name>;

-- Outputs:
-- --------------
-- myCol | count
-- ------+-------
--  f    |  2
--  t    |  3
--       |  1

select count(myCol)
from mytable
group by myCol
;

will group the 3 possible states of bool (false, true, 0) in three rows especially handy when grouping together with another column like day