Simplifying complex SQL queries

There are two types of complexity that you can encounter in SQL queries.

First, the complexity can be directly visible in the query, ifit hashundredsoreventhousandsof rows of SQL code in a single query. This can cause both maintenance headaches and slowexecution.

The complexity can also be hidden in subviews, so the SQL code of the query may seem simple, but it uses other views and/or functions to do part of the work, which can, in turn, use others. This is much better for maintenance, but it can still cause performance problems.

Both types of queries can either be written manually by programmers or data analysts, or emerge as a result of a query generator.

Getting ready

First, verify that you really have a complex query.

A query that simply returns lots of database fields is not complex by itself. In order to be complex, the query has to join lots of tables in complex ways.

The easiest way to find out whether the query is complex is to look at the output of EXPLAIN...