Can't set column name equal to variable which is determined by the user - sql

Question

I am creating a result set where I want the column name to be equal to a variable name that is et during run time. Is that possible ? How do I do that?
In the example below the user choses the date (myDate) before running the query (e.g 2015-06-11). The I want the column name to be that date (2015-06-11). How do I do that? FYI: I'm using Teradata.
SELECT
table_A.Cnt as ?myDate
/* I can't write ?myDate like that. I also tried to convert it to a string */
FROM
(
SELECT COUNT(*) AS Cnt FROM A
WHERE theDate=?myDate
) AS table_A

Answer 1

What you are trying to do is parameterize an object (or the name of an object) rather than parameterize a value, which seems straight forward when you think up the idea, but it's a bit more difficult to pull off.
First off, only an SP allows you to write and execute SQL dynamically, which is what you are doing here. Second, it's a little verbose. Third, it opens you up to SQL injection issues since you are slipping a parameter from a user into SQL then executing it, so proceed cautiously and do what you can to prevent a-holes from mucking up your system.
CREATE PROCEDURE paramMyField
(
IN myDate Date,
--This has to be less than 30 otherwise Teradata will be angry.
--I would set it low just to keep injection possibilities to minimum
IN fieldName VARCHAR(10)
)
--Tell it how many result sets this thing is going to return:
DYNAMIC RESULT SETS 1
--Set the security (using the security of the bloke that sets this thing off, if you don't trust them, neither do I)
SQL SECURITY INVOKER
BEGIN
--We'll need a variable to hold the dynamically generated sql statement
DECLARE dynSQL VARCHAR(5000);
--And we'll need a cursor and a statement
DECLARE dynCursor CURSOR WITH RETURN ONLY FOR dynStatement;
SET dynSQL = '
SELECT
table_A.Cnt as ' || fieldName || '
FROM
(
SELECT COUNT(*) AS Cnt FROM A
WHERE theDate = DATE ''' || myDate || '''
) AS table_A;';
--Now to prep the statement
PREPARE dynStatement FROM dynSQL;
--And open the cursor (we will open and not close it so it's sent back as a resultset
OPEN dynCursor;
END;
There's a lot happening there, but basically it's a stored procedure that takes in two parameters (the date and the name of the field) and spits back a record set that is the results of the SQL statement with a dynamically named field. It does this by using a dynamic SQL statement.
This is executed by running something like:
CALL paramMyField(DATE '2015-06-15', 'Whatever');