Question

I’m writing some SQL queries with several subqueries and lots of joins everywhere, both inside the subquery and the resulting table from the subquery.

We’re not using views so that’s out of the question.

After writing it I’m looking at it and scratching my head wondering what it’s even doing cause I can’t follow it.

What kind of formatting do you use to make an attempt to clean up such a mess? Indents perhaps?

Answer 1

With large queries I tend to rely a lot on named result sets using WITH. This allows to define the result set beforehand and it makes the main query simpler. Named results sets may help to make the query plan more efficient as well e.g. postgres stores the result set in a temporary table.

Example:

WITH 
  cubed_data AS (
     SELECT 
        dimension1_id,
        dimension2_id,
        dimension3_id,
        measure_id,
        SUM(value) value
     FROM
        source_data
     GROUP BY
        CUBE(dimension1, dimension2, dimension3),
        measure
  ), 
  dimension1_label AS(
     SELECT 
        dimension1_id,
        dimension1_label
     FROM 
        labels 
     WHERE 
        object = 'dimension1'
  ), ...
SELECT 
  *
FROM  
  cubed_data
  JOIN dimension1_label USING (dimension1_id)
  JOIN dimension2_label USING (dimension2_id)
  JOIN dimension3_label USING (dimension3_id)
  JOIN measure_label USING (measure_id)

The example is a bit contrived but I hope it shows the increase in clarity compared to inline subqueries. Named result sets have been a great help for me when I’ve been preparing data for OLAP use. Named results sets are also must if you have/want to create recursive queries.

WITH works at least on current versions of Postgres, Oracle and SQL Server