我有一个包含n列的表，我需要创建一个视图，其中包含每列中每个唯一值的频率。 n是未知的，因为我需要在具有不同列数的多个表上应用解决方案。
例如我有桌子：
column1 column2 column3
value1 value2 value3
value2 value2 value1
value1 value2 value2
视图应该是这样的：
columnname value frequency
column1 value1 2
column1 value2 1
column2 value2 3
...
由于我对sql的经验很少，所以非常感谢任何帮助。
非常感谢提前！
到目前为止，我已经提出了这个问题，但现在却有点像石头了。
CREATE or REPLACE FUNCTION create_view () RETURNS setof record AS $$
DECLARE
col RECORD;
BEGIN
for col in execute 'select column_name from information_schema.columns
where table_name = ''table123''' LOOP
???
END LOOP;
return;
END;
$$
LANGUAGE 'plpgsql';
答案 0 :(得分：0)
SELECT 'col1' AS col, col1 AS val, count(*) AS ct FROM tbl GROUP BY col1
UNION ALL
SELECT 'col2', col2, count(*) FROM tbl GROUP BY col2
UNION ALL
SELECT 'col3', col3, count(*) FROM tbl GROUP BY col3
CREATE OR REPLACE FUNCTION f_demo(_schema text, _tbl text)
RETURNS TABLE(col text, val text, ct bigint) AS
$xx$
DECLARE
_fld text;
_sql text := '';
BEGIN
FOR _fld IN
SELECT a.attname -- use quote_ident to safeguard against SQLi
FROM pg_catalog.pg_attribute a
WHERE a.attrelid = (COALESCE(_schema || '.', '') || _tbl)::regclass
AND a.attnum > 0
AND NOT a.attisdropped
-- AND a.attname ~~ '%col%' -- if you want to pick specific columns
LOOP
RETURN QUERY EXECUTE
'SELECT $1, ' || quote_ident(_fld) || '::text, count(*)
FROM ' || COALESCE(quote_ident(_schema) || '.', '') || quote_ident(_tbl) || '
GROUP BY 2'
USING _fld;
END LOOP;
END;
$xx$
LANGUAGE 'plpgsql';
呼叫：
SELECT * FROM f_demo('public', 'mytable');
或者，如果您想使用search_path提供的架构：
SELECT * FROM f_demo(NULL, 'mytable');