Get rows for the last 10 dates - sql

Question

I have a scenario in a Postgres 9.3 database where I have to get the last 10 dates when books were sold. Consider below example:
Store Book
---------- ----------------------
Id Name Id Name Sid Count Date
1 ABC 1 XYZ 1 20 11/11/2015
2 DEF 2 JHG 1 10 11/11/2015
3 UYH 1 10 15/11/2015
4 TRE 1 50 17/11/2015
There is currently no UNIQUE constraint on (name, sid, date) in table book, but we have a service in place that inserts only one count per day.
I have to get results based on store.id. When I pass the ID, the report should be generated with bookname, sold date, and the count of sold copies.
Desired output:
BookName 11/11/2015 15/11/2015 17/11/2015
XYZ 20 -- --
JHG 10 -- --
UYH -- 10 --
TRE -- -- 50

Answer 1

This looks unsuspicious, but it's a hell of a question.
Assumptions
Your counts are integer.
All columns in table book are defined NOT NULL.
The composite (name, sid, date) is unique in table book. You should have a UNIQUE constraint, preferably (for performance) with columns in this order:
UNIQUE(sid, date, name)
This provides the index needed for performance automatically. (Else create one.) See:
Multicolumn index and performance
Is a composite index also good for queries on the first field?
crosstab() queries
To get top performance and short query strings (especially if you run this query often) I suggest the additional module tablefunc providing various crosstab() functions. Basic instructions:
PostgreSQL Crosstab Query
Basic queries
You need to get these right first.
The last 10 days:
SELECT DISTINCT date
FROM book
WHERE sid = 1
ORDER BY date DESC
LIMIT 10;
Numbers for last 10 days using the window function dense_rank():
SELECT *
FROM (
SELECT name
, dense_rank() OVER (ORDER BY date DESC) AS date_rnk
, count
FROM book
WHERE sid = 1
) sub
WHERE date_rnk < 11
ORDER BY name, date_rnk DESC;
(Not including actual dates in this query.)
Column names for output columns (for full solution):
SELECT 'bookname, "' || string_agg(to_char(date, 'DD/MM/YYYY'), '", "' ORDER BY date) || '"'
FROM (
SELECT DISTINCT date
FROM book
WHERE sid = 1
ORDER BY date DESC
LIMIT 10
) sub;
Simple result with static column names
This may be good enough for you - but we don't see actual dates in the result:
SELECT * FROM crosstab(
'SELECT *
FROM (
SELECT name
, dense_rank() OVER (ORDER BY date DESC) AS date_rnk
, count
FROM book
WHERE sid = 1
) sub
WHERE date_rnk < 11
ORDER BY name, date_rnk DESC'
, 'SELECT generate_series(10, 1, -1)'
) AS (bookname text
, date1 int, date2 int, date3 int, date4 int, date5 int
, date6 int, date7 int, date8 int, date9 int, date10 int);
For repeated use I suggest you create this (very fast) generic C function for 10 integer columns once, to simplify things a bit:
CREATE OR REPLACE FUNCTION crosstab_int10(text, text)
RETURNS TABLE (bookname text
, date1 int, date2 int, date3 int, date4 int, date5 int
, date6 int, date7 int, date8 int, date9 int, date10 int)
LANGUAGE C STABLE STRICT AS
'$libdir/tablefunc','crosstab_hash';
Details in this related answer:
Dynamically generate columns for crosstab in PostgreSQL
Then your call becomes:
SELECT * FROM crosstab(
'SELECT *
FROM (
SELECT name
, dense_rank() OVER (ORDER BY date DESC) AS date_rnk
, count
FROM book
WHERE sid = 1
) sub
WHERE date_rnk < 11
ORDER BY name, date_rnk DESC'
, 'SELECT generate_series(10, 1, -1)'
); -- no column definition list required!
Full solution with dynamic column names
Your actual question is more complicated, you also want dynamic column names.
For a given table, the resulting query could look like this then:
SELECT * FROM crosstab_int10(
'SELECT *
FROM (
SELECT name
, dense_rank() OVER (ORDER BY date DESC) AS date_rnk
, count
FROM book
WHERE sid = 1
) sub
WHERE date_rnk < 11
ORDER BY name, date_rnk DESC'
, 'SELECT generate_series(10, 1, -1)'
) AS t(bookname
, "04/11/2015", "05/11/2015", "06/11/2015", "07/11/2015", "08/11/2015"
, "09/11/2015", "10/11/2015", "11/11/2015", "15/11/2015", "17/11/2015");
The difficulty is to distill dynamic column names. Either assemble the query string by hand, or (much rather) let this function do it for you:
CREATE OR REPLACE FUNCTION f_generate_date10_sql(_sid int = 1)
RETURNS text
LANGUAGE sql AS
$func$
SELECT format(
$$SELECT * FROM crosstab_int10(
'SELECT *
FROM (
SELECT name
, dense_rank() OVER (ORDER BY date DESC) AS date_rnk
, count
FROM book
WHERE sid = %1$s
) sub
WHERE date_rnk < 11
ORDER BY name, date_rnk DESC'
, 'SELECT generate_series(10, 1, -1)'
) AS ct(bookname, "$$
|| string_agg(to_char(date, 'DD/MM/YYYY'), '", "' ORDER BY date) || '")'
, _sid)
FROM (
SELECT DISTINCT date
FROM book
WHERE sid = 1
ORDER BY date DESC
LIMIT 10
) sub
$func$;
Call:
SELECT f_generate_date10_sql(1);
This generates the desired query, which you execute in turn.
db<>fiddle here