LÍMITE agrupado en PostgreSQL: ¿muestra las primeras N filas para cada grupo?
Necesito tomar las primeras N filas para cada grupo, ordenadas por columna personalizada.
Dado el siguiente cuadro:
db=# SELECT * FROM xxx;
id | section_id | name
----+------------+------
1 | 1 | A
2 | 1 | B
3 | 1 | C
4 | 1 | D
5 | 2 | E
6 | 2 | F
7 | 3 | G
8 | 2 | H
(8 rows)
Necesito las primeras 2 filas (ordenadas por name ) para cada section_id , es decir, un resultado similar a:
id | section_id | name
----+------------+------
1 | 1 | A
2 | 1 | B
5 | 2 | E
6 | 2 | F
7 | 3 | G
(5 rows)
Estoy usando PostgreSQL 8.3.5.
125
Author: Kouber Saparev, 2009-07-14
5 answers
Nueva solución (PostgreSQL 8.4)
SELECT
*
FROM (
SELECT
ROW_NUMBER() OVER (PARTITION BY section_id ORDER BY name) AS r,
t.*
FROM
xxx t) x
WHERE
x.r <= 2;
200
Author: Dave,
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2016-08-11 09:08:48
Warning: date(): Invalid date.timezone value 'Europe/Kyiv', we selected the timezone 'UTC' for now. in /var/www/agent_stack/data/www/ajaxhispano.com/template/agent.layouts/content.php on line 61
2016-08-11 09:08:48
Desde v9. 3 se puede hacer una unión lateral
select distinct t_outer.section_id, t_top.id, t_top.name from t t_outer
join lateral (
select * from t t_inner
where t_inner.section_id = t_outer.section_id
order by t_inner.name
limit 2
) t_top on true
order by t_outer.section_id;
Podría ser más rápido pero, por supuesto, debe probar el rendimiento específicamente en sus datos y caso de uso.
13
Author: poshest,
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2016-06-16 14:25:29
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2016-06-16 14:25:29
Aquí hay otra solución (PostgreSQL
SELECT
*
FROM
xxx a
WHERE (
SELECT
COUNT(*)
FROM
xxx
WHERE
section_id = a.section_id
AND
name <= a.name
) <= 2
11
Author: Kouber Saparev,
Warning: date(): Invalid date.timezone value 'Europe/Kyiv', we selected the timezone 'UTC' for now. in /var/www/agent_stack/data/www/ajaxhispano.com/template/agent.layouts/content.php on line 61
2016-02-15 11:26:01
Warning: date(): Invalid date.timezone value 'Europe/Kyiv', we selected the timezone 'UTC' for now. in /var/www/agent_stack/data/www/ajaxhispano.com/template/agent.layouts/content.php on line 61
2016-02-15 11:26:01
SELECT x.*
FROM (
SELECT section_id,
COALESCE
(
(
SELECT xi
FROM xxx xi
WHERE xi.section_id = xo.section_id
ORDER BY
name, id
OFFSET 1 LIMIT 1
),
(
SELECT xi
FROM xxx xi
WHERE xi.section_id = xo.section_id
ORDER BY
name DESC, id DESC
LIMIT 1
)
) AS mlast
FROM (
SELECT DISTINCT section_id
FROM xxx
) xo
) xoo
JOIN xxx x
ON x.section_id = xoo.section_id
AND (x.name, x.id) <= ((mlast).name, (mlast).id)
2
Author: Quassnoi,
Warning: date(): Invalid date.timezone value 'Europe/Kyiv', we selected the timezone 'UTC' for now. in /var/www/agent_stack/data/www/ajaxhispano.com/template/agent.layouts/content.php on line 61
2009-07-14 15:35:15
Warning: date(): Invalid date.timezone value 'Europe/Kyiv', we selected the timezone 'UTC' for now. in /var/www/agent_stack/data/www/ajaxhispano.com/template/agent.layouts/content.php on line 61
2009-07-14 15:35:15
-- ranking without WINDOW functions
-- EXPLAIN ANALYZE
WITH rnk AS (
SELECT x1.id
, COUNT(x2.id) AS rnk
FROM xxx x1
LEFT JOIN xxx x2 ON x1.section_id = x2.section_id AND x2.name <= x1.name
GROUP BY x1.id
)
SELECT this.*
FROM xxx this
JOIN rnk ON rnk.id = this.id
WHERE rnk.rnk <=2
ORDER BY this.section_id, rnk.rnk
;
-- The same without using a CTE
-- EXPLAIN ANALYZE
SELECT this.*
FROM xxx this
JOIN ( SELECT x1.id
, COUNT(x2.id) AS rnk
FROM xxx x1
LEFT JOIN xxx x2 ON x1.section_id = x2.section_id AND x2.name <= x1.name
GROUP BY x1.id
) rnk
ON rnk.id = this.id
WHERE rnk.rnk <=2
ORDER BY this.section_id, rnk.rnk
;
2
Author: wildplasser,
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2012-12-07 20:53:01
Warning: date(): Invalid date.timezone value 'Europe/Kyiv', we selected the timezone 'UTC' for now. in /var/www/agent_stack/data/www/ajaxhispano.com/template/agent.layouts/content.php on line 61
2012-12-07 20:53:01