Oracle SQL re-use subquery "WITH" clause assistance - sql

Question

I have an SQL query in which I need to take the output of a subquery and use it more than once. My existing query works, but only if I repeat the subquery each time I need it. Unfortunately the subquery is complex, and takes time to execute - meaning that multiple iterations really slow the whole thing down.
I have read that you can use the "WITH" statement to assign a subquery output to a variable, in order to re-use that variable. However the problem I'm having is that within the subquery, I need to reference values from the main query. And it appears that if I use WITH - before the main query SELECT - then those references are not recognised. I'll give you a simplified example:
WITH
DateX AS
(
SELECT
MAX(TableSub.Date)
FROM
TableA TableSub
WHERE
TableSub.ID = TableMain.ID
AND TableSub.Event = 'AnotherEvent'
AND TableSub.Date BETWEEN '01-Jan-2015' AND '31-Dec-2015'
)
SELECT
TableMain.ID
FROM
TableA TableMain
WHERE
TableMain.Event = 'MainEvent'
AND TableMain.Date >= DateX
AND (
SELECT
TableSub2.ID
FROM
TableA TableSub2
WHERE
TableSub2.ID = TableMain.ID
TableSub2.Event = 'ThirdEvent'
AND TableSub2.Date <= DateX
) IS NULL
I hope this is clear. It's a simplified version of what I have, but you can see that DateX is used in more than one place: within the main query, and within a subquery. However the problem is that when DateX is defined by WITH, I need to link the ID back to the ID of the main query. And it's not working...
I would be grateful for any advice on this. Am I doing it wrong? Is there a way, or is it just impossible? If so, then should I be using another approach entirely? Thanks.

Answer 1

A better way:
SELECT ID
FROM (
SELECT ID,
"Date",
Event,
LAST_VALUE( CASE Event WHEN 'AnotherEvent' THEN "Date" END IGNORE NULLS )
OVER ( PARTITION BY ID ORDER BY "Date"
ROWS BETWEEN UNBOUNDED PRECEEDING AND UNBOUNDED FOLLOWING
) AS another_date,
FIRST_VALUE( CASE Event WHEN 'ThirdEvent' THEN "Date" END IGNORE NULLS )
OVER ( PARTITION BY ID ORDER BY "Date"
ROWS BETWEEN UNBOUNDED PRECEEDING AND UNBOUNDED FOLLOWING
) AS third_date
FROM TableA
WHERE Event IN ( 'MainEvent', 'ThirdEvent' )
OR ( Event = 'AnotherEvent' AND EXTRACT( YEAR FROM "Date" ) = 2015 )
)
WHERE Event = 'MainEvent'
AND "Date" >= another_date
AND ( third_date IS NULL OR third_date > another_date );

Answer 2

You need to join your DateX CTE on the ID column. Something like:
WITH
DateX AS
(
SELECT
TableSub.ID,
MAX(TableSub.Date) AS MaxDate
FROM
TableA TableSub
WHERE
AND TableSub.Event = 'AnotherEvent'
AND TableSub.Date >= DATE '2015-01-01'
AND TableSub.Date < DATE '2016-01-01'
GROUP BY
TableSub.ID
)
SELECT
TableMain.ID
FROM
TableA TableMain
JOIN
DateX
ON
DateX.ID = TableMain.ID
WHERE
TableMain.Event = 'MainEvent'
AND TableMain.Date >= DateX.MaxDate
AND (
SELECT
TableSub2.ID
FROM
TableA TableSub2
JOIN
DateX
ON
DateX.ID = TableSub2.ID
WHERE
TableSub2.ID = TableMain.ID
TableSub2.Event = 'ThirdEvent'
AND TableSub2.Date <= DateX.MaxDate
) IS NULL
The CTE also needs a column alias for the aggregate; and as you need to join in the ID, you need to include that and group by it.
The last subquery looks odd; you might want NOT EXISTS rather than IS NULL if you're looking for no record. Perhaps your real query is using an aggregate, but even so that might be quicker.
This still may not be the best approach but it's hard to tell from your example. Hitting the same table three times may be unnecessarily expensive.