Oracle SQL, return unique (max) row in subquery in SELECT header (before FROM, WHERE)

Question

I am running SQL through Business Objects on an Oracle server. I have the following code (simplified), which sometimes runs:

SELECT
  a.col1,
  a.col2,
  a.date1,
    (
    SELECT
      b.date2
    FROM
      tbl b
    WHERE
      b.ID = a.ID
      AND
      b.date2 >= a.date1-60
    ) AS SUBDATE
FROM
  tbl a
WHERE
  a.col1 = 'foo'
  AND
  a.col2 = 'bar'
  AND
  a.date1 = '#somedate#'

So as you see, there is a subquery in the SELECT clause, which I want to return a single value: b.date2. This does work... some of the time. However, if b.ID returns more than one record where date2 is within the last 60 days of date1, then the query fails.

What I want to do is to limit the subquery's output to the single most recent value of date2. I have tried all sorts of things: MAX, LIMIT 1, DISTINCT... however the SQL will not compile, telling me that I have errors. For example the following TOP 1 produces an error saying "FROM keyword not found where expected":

    (
    SELECT
      TOP 1 b.date2
    FROM
      tbl b
    WHERE
      b.ID = a.ID
      AND
      b.date2 >= a.date1-60
    ) AS SUBDATE

The following ORDER BY/LIMIT produces an error saying "Missing right parenthesis":

    (
    SELECT
      b.date2
    FROM
      tbl b
    WHERE
      b.ID = a.ID
      AND
      b.date2 >= a.date1-60
      ORDER BY b.date2 LIMIT 1
    ) AS SUBDATE

(Reproduced subqueries only above).

From researching these errors, I see various advice about subqueries after the main WHERE clause; however nothing about subqueries in the way I want to use them.

Can anyone help me understand why I am getting errors, and what the correct way should be do do what I want?

Thanks.

Answer 1

Oracle can be a pain in this case (unless you are using Oracle 12). One method uses keep . However, in your case max() is sufficient:

SELECT . . . 
     (SELECT MAX(b.date2)
      FROM tbl b
      WHERE b.ID = a.ID AND
            b.date2 >= a.date1 - 60
     ) AS SUBDATE
. . .

Note: You don't need the condition on date2 in the subquery, unless you really want to limit results to the last 60 days.