EverSQL Database Performance Knowledge Base
Dynamically generate columns in PostgreSQL
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I have seen that there are quit a few similar questions like this one, but I havent understood how to code it myself. Please have in mind that I am just a beginner in this field.
Basically I want to pivot the table like this:
zoom | day | point zoom | 2015-10-01 | 2015-10-02 | ......
------+-----------+------- ---> ------+------------+-------------+
1 | 2015-10-01 | 201 1 | 201 | 685 |
2 | 2015-10-01 | 43 2 | 43 | 346 |
3 | 2015-10-01 | 80 3 | 80 | 534 |
4 | 2015-10-01 | 324 4 | 324 | 786 |
5 | 2015-10-01 | 25 5 | 25 | 685 |
1 | 2015-10-02 | 685
2 | 2015-10-02 | 346
3 | 2015-10-02 | 534
4 | 2015-10-02 | 555
5 | 2015-10-02 | 786
:
:
:
Time can vary.
Results on left I get with:
SELECT
zoom,
to_char(date_trunc('day', time), 'YYYY-MM-DD') AS day,
count(*) as point
FROM province
WHERE time >= '2015-05-01' AND time < '2015-06-01'
GROUP BY to_char(date_trunc('day', time), 'YYYY-MM-DD'), zoom;
I have read that there are some issues if I use count and also that it would be better if I use CASE and GROUP BY, however I have no idea how to CASE this.
Crosstab itself doesnt support dynamic creation of column names, but that can be achieved with crosstab_hash, if I understood it correctly.
This might be probably nice solution: http://okbob.blogspot.ca/2008/08/using-cursors-for-generating-cross.html however I am stucked with it trying to program it myself.
I have to use this kind of pivoting quite often, so I would appriciate any kind of help and additional explanation behind it.
Edit1
I am trying to figure out how crosstab works with dates, currently without returning dynamic names of columns. Later on I will explain why. It is realted to the main question. For this example I am using only period of 2 dates.
Based on @Erwin Brandstetter answer:
SELECT * FROM crosstab(
'SELECT zoom, day, point
FROM province
ORDER BY 1, 2'
, $$VALUES ('2015-10-01'::date), ('2015-10-02')$$)
AS ct (zoom text, day1 int, day2 int);
returned results are:
zoom | day1 | day2 |
-----+------------+-------------+
1 | 201 | 685 |
2 | 43 | 346 |
3 | 80 | 534 |
4 | 324 | 786 |
I am trying to get this
zoom | 2015-10-01 | 2015-10-02 |
-----+------------+-------------+
1 | 201 | 685 |
2 | 43 | 346 |
3 | 80 | 534 |
4 | 324 | 786 |
but my query doesnt work:
SELECT *
FROM crosstab(
'SELECT *
FROM province
ORDER BY 1,2')
AS ct (zoom text, "2015-10-01" date, "2015-10-02" date);
ERROR: return and sql tuple descriptions are incompatible
Edit1, Q1. Why does this doesnt work and how can I return results like that?
I have read links that @Erwin Brandstetter provided me, especially this one: Execute a dynamic crosstab query. I have copied/pasted his function:
CREATE OR REPLACE FUNCTION pivottab(_tbl regclass,
_row text, _cat text,
_expr text,
_type regtype)
RETURNS text AS
$func$
DECLARE
_cat_list text;
_col_list text;
BEGIN
-- generate categories for xtab param and col definition list
EXECUTE format(
$$SELECT string_agg(quote_literal(x.cat), '), (')
, string_agg(quote_ident (x.cat), %L)
FROM (SELECT DISTINCT %I AS cat FROM %s ORDER BY 1) x$$
, ' ' || _type || ', ', _cat, _tbl)
INTO _cat_list, _col_list;
-- generate query string
RETURN format(
'SELECT * FROM crosstab(
$q$SELECT %I, %I, %s
FROM %I
GROUP BY 1, 2
ORDER BY 1, 2$q$
, $c$VALUES (%5$s)$c$
) ct(%1$I text, %6$s %7$s)'
, _row, _cat, _expr, _tbl, _cat_list, _col_list, _type
);
END
$func$ LANGUAGE plpgsql;
and call it with query
SELECT pivottab('province','zoom','day','point','date');
Function returned me:
pivottab
----------------------------------------------------------
SELECT * FROM crosstab( +
$q$SELECT zoom, day, point +
FROM province +
GROUP BY 1, 2 +
ORDER BY 1, 2$q$ +
, $c$VALUES ('2015-10-01'), ('2015-10-02')$c$ +
) ct(zoom text, "2015-10-01" date, "2015-10-02" date)
(1 row)
So when I edited the query and added ; (it would be nice that ; is already there) I got:
ERROR: column "province.point" must appear in the GROUP BY clause or be used in an aggregate function
Edit1, Q2. Any ideas how to solove this?
Edit1, Q3. I guess next question will be how to execute function automaticlly, which is also mentioned on the same link, but got stucked on previous steps.