从表中选择一组动态的列,并获取每个列的总和 [英] Select a dynamic set of columns from a table and get the sum for each
问题描述
是否可以在Postgres中执行以下操作:
Is it possible to do the following in Postgres:
SELECT column_name FROM information_schema WHERE table_name = 'somereport' AND data_type = 'integer';
SELECT SUM(coulmn_name[0]),SUM(coulmn_name[1]) ,SUM(coulmn_name[3]) FROM somereport;
换句话说,我需要根据特定条件从表中选择一组列,然后将表中的每个列求和.
In other words I need to select a group of columns from a table depending on certain criteria, and then sum each of those columns in the table.
我知道我可以循环执行此操作,因此我可以独立地对每一列进行计数,但是显然,这需要对信息模式查询返回的每一列进行查询.例如:
I know I can do this in a loop, so I can count each column independently, but obviously that requires a query for each column returned from the information schema query. Eg:
FOR r IN select column_name from information_schema where report_view_name = 'somereport' and data_type = 'integer';
LOOP
SELECT SUM(r.column_name) FROM somereport;
END
推荐答案
此查询将创建您要使用的完整DML语句:
This query creates the complete DML statement you are after:
WITH x AS (
SELECT 'public'::text AS _schema -- provide schema name ..
,'somereport'::text AS _tbl -- .. and table name once
)
SELECT 'SELECT ' || string_agg('sum(' || quote_ident(column_name)
|| ') AS sum_' || quote_ident(column_name), ', ')
|| E'\nFROM ' || quote_ident(x._schema) || '.' || quote_ident(x._tbl)
FROM x, information_schema.columns
WHERE table_schema = _schema
AND table_name = _tbl
AND data_type = 'integer'
GROUP BY x._schema, x._tbl;
您可以单独执行它,也可以将此查询包装在plpgsql函数中,然后使用EXECUTE
自动运行查询:
You can execute it separately or wrap this query in a plpgsql function and run the query automatically with EXECUTE
:
经过PostgreSQL 9.1.4测试
Tested with PostgreSQL 9.1.4
CREATE OR REPLACE FUNCTION f_get_sums(_schema text, _tbl text)
RETURNS TABLE(names text[], sums bigint[]) AS
$BODY$
BEGIN
RETURN QUERY EXECUTE (
SELECT 'SELECT ''{'
|| string_agg(quote_ident(c.column_name), ', ' ORDER BY c.column_name)
|| '}''::text[],
ARRAY['
|| string_agg('sum(' || quote_ident(c.column_name) || ')'
, ', ' ORDER BY c.column_name)
|| ']
FROM '
|| quote_ident(_schema) || '.' || quote_ident(_tbl)
FROM information_schema.columns c
WHERE table_schema = _schema
AND table_name = _tbl
AND data_type = 'integer'
);
END;
$BODY$
LANGUAGE plpgsql;
致电:
SELECT unnest(names) AS name, unnest (sums) AS col_sum
FROM f_get_sums('public', 'somereport');
返回:
name | col_sum
---------------+---------
int_col1 | 6614
other_int_col | 8364
third_int_col | 2720642
解释
困难在于为函数定义RETURN
类型,而返回的列数和名称将有所不同.一个细节有所帮助:您只需要integer
列.
Explain
The difficulty is to define the RETURN
type for the function, while number and names of columns returned will vary. One detail that helps a little: you only want integer
columns.
我通过形成bigint
的数组解决了这个问题(sum(int_col)
返回bigint
).另外,我返回一个列名数组.两者均按列名按字母顺序排序.
I solved this by forming an array of bigint
(sum(int_col)
returns bigint
). In addition I return an array of column names. Both sorted alphabetically by column name.
In the function call I split up these arrays with unnest()
arriving at the handsome format displayed.
动态创建和执行的查询是高级内容.不要被多层引号弄糊涂了.基本上,您有EXECUTE
,它接受包含SQL查询的文本参数来执行.反过来,此文本由辅助SQL查询提供,该辅助SQL查询构建了主查询的查询字符串.
The dynamically created and executed query is advanced stuff. Don't get confused by multiple layers of quotes. Basically you have EXECUTE
that takes a text argument containing the SQL query to execute. This text, in turn, is provided by secondary SQL query that builds the query string of the primary query.
如果这一次太多了,或者plpgsql
对于您来说还是很新的,请从此相关答案开始,我解释了处理简单得多的功能的基础知识,并提供了主要功能手册的链接.
If this is too much at once or plpgsql
is rather new for you, start with this related answer where I explain the basics dealing with a much simpler function and provide links to the manual for the major features.
如果性能是必不可少的,则直接查询Postgres目录(pg_catalog.pg_attributes
),而不是使用标准化的(但速度较慢)information_schema.columns
.这是一个带有pg_attributes
的简单示例.
If performance is essential query the Postgres catalog directly (pg_catalog.pg_attributes
) instead of using the standardized (but slow) information_schema.columns
. Here is a simple example with pg_attributes
.
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