Кросс-таблица с большим или неопределенным количеством категорий

create table vote (Photo integer, Voter text, Decision text);
insert into vote values
(1, 'Alex', 'Cat'),
(1, 'Bob', 'Dog'),
(1, 'Carol', 'Cat'),
(1, 'Dave', 'Cat'),
(1, 'Ed', 'Cat'),
(2, 'Alex', 'Cat'),
(2, 'Bob', 'Dog'),
(2, 'Carol', 'Cat'),
(2, 'Dave', 'Cat'),
(2, 'Ed', 'Dog'),
(3, 'Alex', 'Horse'),
(3, 'Bob', 'Horse'),
(3, 'Carol', 'Dog'),
(3, 'Dave', 'Horse'),
(3, 'Ed', 'Horse'),
(4, 'Alex', 'Horse'),
(4, 'Bob', 'Horse'),
(4, 'Carol', 'Cat'),
(4, 'Dave', 'Horse'),
(4, 'Ed', 'Horse'),
(5, 'Alex', 'Dog'),
(5, 'Bob', 'Cat'),
(5, 'Carol', 'Cat'),
(5, 'Dave', 'Cat'),
(5, 'Ed', 'Cat')
;

Запрос для кота:

select photo,
    alex + bob + carol + dave + ed as Total,
    alex, bob, carol, dave, ed
from crosstab($$
    select
        photo, voter,
        case decision when 'Cat' then 1 else 0 end
    from vote
    order by photo
    $$,'
    select distinct voter
    from vote
    order by voter
    '
) as (
    photo integer,
    Alex integer,
    Bob integer,
    Carol integer,
    Dave integer,
    Ed integer
);
 photo | total | alex | bob | carol | dave | ed 
-------+-------+------+-----+-------+------+----
     1 |     4 |    1 |   0 |     1 |    1 |  1
     2 |     3 |    1 |   0 |     1 |    1 |  0
     3 |     0 |    0 |   0 |     0 |    0 |  0
     4 |     1 |    0 |   0 |     1 |    0 |  0
     5 |     4 |    0 |   1 |     1 |    1 |  1

Если число избирателей велико или неизвестно, то это можно сделать динамически:

do $do$
declare
voter_list text;
r record;
begin

drop table if exists pivot;

voter_list := (
    select string_agg(distinct voter, ' ' order by voter) from vote
    );

execute(format('
    create table pivot (
        decision text,
        photo integer,
        Total integer,
        %1$s
    )', (replace(voter_list, ' ', ' integer, ') || ' integer')
));

for r in
select distinct decision from vote
loop
    execute (format($f$
        insert into pivot
        select
            %3$L as decision,
            photo,
            %1$s as Total,
            %2$s
        from crosstab($ct$
            select
                photo, voter,
                case decision when %3$L then 1 else 0 end
            from vote
            order by photo
            $ct$,$ct$
            select distinct voter
            from vote
            order by voter
            $ct$
        ) as (
            photo integer,
            %4$s
        );$f$,
        replace(voter_list, ' ', ' + '),
        replace(voter_list, ' ', ', '),
        r.decision,
        replace(voter_list, ' ', ' integer, ') || ' integer'
    ));
end loop;
end; $do$;

Приведенный выше код создал сводную таблицу со всеми решениями:

select * from pivot where decision = 'Cat';

Ваше пожелание подразумевает перенос части данных (имен) в заголовки столбцов, т.е. схему результирующей таблицы. Поскольку это где-то между неудобным и невозможным, я бы рекомендовал просто сортировать и суммировать данные в sql, а все остальное делать вне базы данных.

SELECT Photo, Voter
FROM data
WHERE Decision = '...'
ORDER BY Photo, Voter

и

SELECT Photo, COUNT(*) AS Total
FROM data
WHERE Decision = '...'
GROUP BY Photo
ORDER BY Photo;

Используя тот же образец данных, что и Clodoaldo («создать голосование по таблице...») и используя функцию plpythonu make_pivot_table (ниже), вы можете запустить:

create temp table pivot_data on commit drop as 
    select * from vote where decision = 'Cat' union select photo, null, null from vote;

select * from make_pivot_table('{photo}', 'voter',  'decision', 'count', 'pivot_data',
  'pivot_result', false);

select * from pivot_result order by photo;

Определение функции make_pivot_table:

-- make_pivot_table
-- python version 0.9
-- last edited 2015-08-11 

create or replace function
 make_pivot_table(row_headers text[], category_field text, value_field text,
  value_action text, input_table text, output_table text, keep_result boolean)
returns void as
$$
# imports
from collections import defaultdict
import operator
import string

# constants
BATCH_SIZE = 100
VALID_ACTIONS = ('count', 'sum', 'min', 'max')
NULL_CATEGORY_NAME = 'NULL_CATEGORY'
TOTAL_COL = 'total'

# functions
def table_exists(tablename):
    plan = plpy.prepare("""select table_schema, table_name from
        information_schema.Tables where table_schema not in ('information_schema',
        'pg_catalog') and table_name = $1""", ["text"])
    rows = plpy.execute(plan, [input_table], 2)
    return bool(rows)

def make_rowkey(row):
    return tuple([row[header] for header in row_headers])

def quote_if_needed(value):
    return plpy.quote_literal(value) if isinstance(value, basestring) else str(value)

# assumes None is never a value in the dct
def update_if(dct, key, new_value, op, result=True):
    current_value = dct.get(key)
    if current_value is None or op(value, current_value) == result:
        dct[key] = new_value

def update_output_table(output_table, row_headers, colname, value):
    pg_value = plpy.quote_literal(value) if isinstance(value, basestring) else value
    sql = 'update %s set %s = %s where ' % (output_table, plpy.quote_ident(colname), 
                                            pg_value)
    conditions = []
    for index, row_header in enumerate(row_headers):
        conditions.append('%s = %s' % (plpy.quote_ident(row_header),
                                       quote_if_needed(rowkey[index])))
    sql += ' and '.join(conditions)
    plpy.execute(sql)


# -----------------

if not table_exists(input_table):
    plpy.error('input_table %s dones not exist' % input_table)

if value_action not in VALID_ACTIONS:
    plpy.error('%s is not a recognised action' % value_action)

# load the data into a dict
count_dict = defaultdict(int)
sum_dict = defaultdict(float)
total_dict = defaultdict(float)
min_dict = dict()
max_dict = dict()
categories_seen = set()
rowkeys_seen = set()
do_total = value_action in ('count', 'sum')

cursor = plpy.cursor('select * from %s' % plpy.quote_ident(input_table))
while True:
    rows = cursor.fetch(BATCH_SIZE)
    if not rows:
        break
    for row in rows:
        rowkey = make_rowkey(row)
        rowkeys_seen.add(rowkey)
        category = row[category_field]           
        value = row[value_field]
        dctkey = (rowkey, category)

        # skip if value field is null
        if value is None:
            continue

        categories_seen.add(category)

        if value_action == 'count':
        count_dict[dctkey] += 1
        total_dict[rowkey] += 1
    if value_action == 'sum':
            sum_dict[dctkey] += value
            total_dict[rowkey] += value
        if value_action == 'min':
            update_if(min_dict, dctkey, value, operator.lt)
        if value_action == 'max':
            update_if(max_dict, dctkey, value, operator.gt)

plpy.notice('seen %s summary rows and %s categories' % (len(rowkeys_seen),
                                                        len(categories_seen)))

# get the columns types
coltype_dict = dict()
input_type_sql = 'select * from %s where false' % plpy.quote_ident(input_table)
input_type_result = plpy.execute(input_type_sql)
for index, colname in enumerate(input_type_result.colnames()):
    coltype_num = input_type_result.coltypes()[index]
    coltype_sql = 'select typname from pg_type where oid = %s' % coltype_num
    coltype = list(plpy.cursor(coltype_sql))[0]
    plpy.notice('%s: %s' % (colname, coltype['typname']))
    coltype_dict[colname] = coltype['typname']

plpy.execute('drop table if exists %s' % plpy.quote_ident(output_table))
sql_parts = []
if keep_result:
    sql_parts.append('create table %s (' % plpy.quote_ident(output_table))
else:
    sql_parts.append('create temp table %s (' % plpy.quote_ident(output_table))

cols = []
for row_header in row_headers:
    cols.append('%s %s' % (plpy.quote_ident(row_header), coltype_dict[row_header]))

cat_type = 'bigint' if value_action == 'count' else coltype_dict[value_field]

for col in sorted(categories_seen):
    if col is None:
        cols.append('%s %s' % (plpy.quote_ident(NULL_CATEGORY_NAME), cat_type))
    else:
        cols.append('%s %s' % (plpy.quote_ident(col), cat_type))

if do_total:
    cols.append('%s %s' % (TOTAL_COL, cat_type))

sql_parts.append(',\n'.join(cols))
if keep_result:
    sql_parts.append(')')
else:
    sql_parts.append(') on commit drop')
plpy.execute('\n'.join(sql_parts))

dict_map = {'count': count_dict, 'sum': sum_dict, 'min': min_dict, 'max': max_dict }
value_dict = dict_map[value_action]
for rowkey in rowkeys_seen:
    sql = 'insert into %s values (' % plpy.quote_ident(output_table)
    sql += ', '.join([quote_if_needed(part) for part in rowkey])
    sql += ')'
    plpy.execute(sql)

if do_total:
    for rowkey, value in total_dict.iteritems():
        update_output_table(output_table, row_headers, TOTAL_COL, value)

for (rowkey, category), value in value_dict.iteritems():
    # put in cateogry value
    colname = NULL_CATEGORY_NAME if category is None else category
    update_output_table(output_table, row_headers, colname, value)

$$ language plpythonu