[英]Use query for SQL alias OR join column names to row values?
我正在使用PostgreSQL中的數據,該數據使用數據字典表來提供數據集中其他表的列(變量)名稱的描述。 例如:
表格1:
a00600 | a00900
-------+-------
row 1 | row 1
row 2 | row 2
數據字典(Key)列:
Variable | Description
---------+------------
a00600 | Total population
a00900 | Zipcode
出於報告目的,如何編寫SQL以動態執行以下操作(不指定每個列名稱)?
SELECT 'a00600' AS (SELECT Key.Description
WHERE Key.Variable = 'a00600')
FROM Table 1;
我意識到可能有更好的方法來解析這個問題/問題,並對我需要完成的任何想法持開放態度。
獲取描述而不是實際列名的另一種方法是創建視圖(每個表一個)。 這可以通過自動生成視圖來自動完成。 這看起來相當笨拙,但它具有巨大的優勢,對於“復雜的*查詢,生成的查詢計划將與原始列名稱完全相同。”加入復雜查詢的函數將表現不佳:優化器不能將它們分開,所以結果行為將等同於“一次一行”)示例:
-- tmp schema is only for testing
DROP SCHEMA tmp CASCADE;
CREATE SCHEMA tmp ;
SET search_path=tmp;
CREATE TABLE thedata
( a00600 varchar
, a00900 varchar
);
INSERT INTO thedata(a00600 , a00900) VALUES
('key1', 'data1')
,('key2', 'data2');
CREATE TABLE thedict
( variable varchar
, description varchar
);
INSERT INTO thedict(variable , description) VALUES
('a00600' , 'Total population')
,('a00900' , 'Zipcode' );
CREATE OR REPLACE FUNCTION create_view_definition(zname varchar)
RETURNS varchar AS
$BODY$
DECLARE
thestring varchar;
therecord RECORD;
iter INTEGER ;
thecurs cursor for
SELECT co.attname AS zname, d.description AS zdesc
FROM pg_class ct
JOIN pg_namespace cs ON cs.oid=ct.relnamespace
JOIN pg_attribute co ON co.attrelid = ct.oid AND co.attnum > 0
LEFT JOIN thedict d ON d.variable = co.attname
WHERE ct.relname = 'thedata'
AND cs.nspname = 'tmp'
;
BEGIN
thestring = '' ;
iter = 0;
FOR therecord IN thecurs LOOP
IF (iter = 0) THEN
thestring = 'CREATE VIEW ' || quote_ident('v'||zname) || ' AS ( SELECT ' ;
ELSE
thestring = thestring || ', ';
END IF;
iter=iter+1;
thestring = thestring || quote_ident(therecord.zname);
IF (therecord.zdesc IS NOT NULL) THEN
thestring = thestring || ' AS ' || quote_ident(therecord.zdesc);
END IF;
END LOOP;
IF (iter > 0) THEN
thestring = thestring || ' FROM ' || quote_ident(zname) || ' )' ;
END IF;
RETURN thestring;
END;
$BODY$ LANGUAGE plpgsql;
CREATE OR REPLACE FUNCTION execute_view_definition(zname varchar)
RETURNS INTEGER AS
$BODY$
DECLARE
meat varchar;
BEGIN
meat = create_view_definition(zname);
EXECUTE meat;
RETURN 0;
END;
$BODY$ LANGUAGE plpgsql;
SELECT create_view_definition('thedata');
SELECT execute_view_definition('thedata');
SELECT * FROM vthedata;
結果:
CREATE FUNCTION
CREATE FUNCTION
create_view_definition
---------------------------------------------------------------------------------------------------
CREATE VIEW vthedata AS ( SELECT a00600 AS "Total population", a00900 AS "Zipcode" FROM thedata )
(1 row)
execute_view_definition
-------------------------
0
(1 row)
Total population | Zipcode
------------------+---------
key1 | data1
key2 | data2
(2 rows)
請注意,這只是一個例子。 如果它是真實的,我至少會將生成的視圖放到一個單獨的模式中,以避免名稱沖突和原始模式的污染。
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