PostgreSQL-使用函数命名选择查询中的列

Question

I'm trying to set the name of a column in a 'select .. from' query dinamically. So I created a function to return the name of the column and tried to use it in a select query.

The function is this one:

CREATE OR REPLACE FUNCTION get_column_name(iddoc int, campo text) RETURNS text
AS $$
#print_strict_params on
DECLARE
campoid text;
BEGIN

    SELECT campo_id INTO STRICT campoid
    FROM campos_td c
    JOIN (SELECT field4, field3 
        FROM frm7 
        WHERE iddocumento = iddoc) f ON c.tema_id = f.field3

    RETURN 'field10' || campoid;
END
$$ LANGUAGE plpgsql;

And I've tried things like:

SELECT get_column_name(2, 'Size') FROM frm7
SELECT f.get_column_name(2, 'Size') FROM frm7 as f

But none of them works.

Answer 1

pgsql EXECUTE 命令使您可以运行任意SQL字符串，特别是使用在运行时传递的列名构建的SQL字符串。

Answer 2

I am not well versed in sql but you may try this:

DECLARE @query NVARCHAR(MAX);

set @query = 'SELECT ' + get_column_name(2, 'Size') + ' FROM frm7';

execute @query;

Example:

DECLARE @query NVARCHAR(MAX);
DECLARE @colName NVARCHAR(MAX) = 'col1'; 
set @query = 'SELECT ' + @colName + ' FROM frm7';

execute @query;