Обсуждение: Queyring for columns which are exist in table.

Hi All,

I want to fire a query such that if the particular column does not exist then query should return some default value.

For that I have tried following experiment.

CREATE TABLE tbl (

    c1 integer,

    c2 integer,

    c3 integer

);

INSERT INTO tbl VALUES (1, 2, 3);

INSERT INTO tbl VALUES (2, 3, 4);

INSERT INTO tbl VALUES (3, 4, 5);

INSERT INTO tbl VALUES (4, 5, 6);

INSERT INTO tbl VALUES (5, 6, 7);

INSERT INTO tbl VALUES (6, 7, 8);

INSERT INTO tbl VALUES (7, 8, 9);

INSERT INTO tbl VALUES (8, 9, 10);

CREATE OR REPLACE FUNCTION ColumnAlreadyExists(name, name) RETURNS INTEGER AS E'

DECLARE columnCount INTEGER;

BEGIN

SELECT COUNT (pg_attribute.attname) into columnCount FROM pg_attribute,pg_class, pg_type WHERE ((pg_attribute.attrelid=pg_class.oid) AND (pg_attribute.atttypid=pg_type.oid) AND (pg_class.relname = $1) AND (pg_attribute.attname = $2));

IF columnCount = 0 THEN

RETURN 0;

END IF;

RETURN 1;

END;

' LANGUAGE 'plpgsql';

DROP FUNCTION checkColumn(name,name,name);

CREATE OR REPLACE FUNCTION checkColumn(name, name, name) RETURNS name AS E'

DECLARE isColumnExist INTEGER;

BEGIN

SELECT ColumnAlreadyExists ($1,$2) into isColumnExist;

IF isColumnExist = 0 THEN

RETURN name($3);

ELSE

RETURN name($2);

END IF;

END;

' LANGUAGE 'plpgsql';

Function checkColumn should return proper column name (second parameter) if column exist and third parameter if column not exist.

NOW when I try to execute following command it returns improper result.

I expect proper column values as a output of query.

SELECT(checkColumn('tbl','c2','0'))::name FROM tbl;

mydb=# SELECT (checkColumn('tbl','c2','0'))::name FROM tbl;

 checkcolumn

-------------

 c2

 c2

 c2

 c2

 c2

 c2

 c2

 c2

(8 rows)

mydb=#

Above query should return actual values present for c2 column in tbl.

But it’s not working as desired.

Please help me in this.

Thanks in advance,

Santosh.