SQL Server - conditional aggregation with correlation

Question

Background:

The original case was very simple. Calculate running total per user from highest revenue to lowest:

CREATE TABLE t(Customer INTEGER  NOT NULL         


        

Answer 1

There is an easier solution:

SELECT c.Customer, c."User", c."Revenue",
       1.0 * Revenue/ NULLIF(c2.sum_total, 0) AS percentage,
       1.0 * c2.sum_running / NULLIF(c2.sum_total, 0) AS running_percentage
FROM t c CROSS APPLY
     (SELECT SUM(c2.Revenue) AS sum_total,
             SUM(CASE WHEN c2.Revenue >= x.Revenue THEN c2.Revenue ELSE 0 END) 
                 as sum_running
      FROM t c2 CROSS JOIN
           (SELECT c.REVENUE) x
      WHERE c."User" = c2."User"
     ) c2
ORDER BY "User", Revenue DESC;

I am not sure why or if this limitation is in the SQL '92 standard. I did have it pretty well memorized 20 or so years ago, but I don't recall that particular limitation.

I should note:

• At the time of the SQL 92 standard, lateral joins were not really on the radar. Sybase definitely had no such concept.
• Other databases do have problems with outer references. In particular, they often limit the scoping to one level deep.
• The SQL Standard itself tends to highly political (that is, vendor-driven) rather than driven by actual database user requirements. Well, over time, it does move in the right direction.

Answer 2

There is no such limitation in the SQL standard for LATERAL. CROSS APPLY is a vendor-specific extension from Microsoft (Oracle adopted it later for compatibility) and its limitations are obviously not owed to the ISO/IEC SQL standard, since the MS feature pre-dates the standard.

LATERAL in standard SQL is basically just a modifier for joins to allow lateral references in the join tree. There is no limit to the number of columns that can be referenced.

I wouldn't see a reason for the odd restriction to begin with. Maybe it's because CROSS APPLY was originally intended to allow table-valued functions, which was later extended to allow sub-SELECTs.

The Postgres manual explains LATERAL like this:

The LATERAL key word can precede a sub-SELECT FROM item. This allows the sub-SELECT to refer to columns of FROM items that appear before it in the FROM list. (Without LATERAL, each sub-SELECT is evaluated independently and so cannot cross-reference any other FROM item.)

The Postgres version of your query (without the more elegant window functions) can be simpler:

SELECT c.*
     , round(revenue        / c2.sum_total, 2) END AS percentage 
     , round(c2.running_sum / c2.sum_total, 2) END AS running_percentage 
FROM   t c, LATERAL (
   SELECT NULLIF(SUM(revenue), 0)::numeric AS sum_total  -- NULLIF, cast once
        , SUM(revenue) FILTER (WHERE revenue >= c.revenue) AS running_sum
   FROM   t
   WHERE  "User" = c."User"
   ) c2
ORDER  BY c."User", c.revenue DESC;

• Postgres 9.4+ has the more elegant aggregate FILTER for conditional aggregates.

• NULLIF makes sense, I only suggest a minor simplification.

• Cast sum_total to numeric once.

• Round result to match your desired result.