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Don Burleson Blog 







SQL to compare two date ranges

Oracle Database Tips by Donald BurlesonJune 14, 2015

In Oracle reporting it's not uncommon for the end-users to requests comparison of two point-in-time measures (i.e. compare June 1, 2015 with June 1 2015), but it gets more complex when you need to compare values for two ranges of dates (e.g. compare June 2015 sales with June 2015 sales).

How do we formulate SQL to compare two ranges of date values?

Traditional SQL might formulate this query: "Show me the difference in March sales between 2004 and 2005, by city, by region".

Comparing date ranges with Oracle SQL

Comparing date ranges can be done in a variety of ways, each with different performance.

There is a significant run-time overhead of converting and testing each date in a multi-million row table while maintaining sub-second response time!

This range comparison query would require complex date transformation at the SQL level, where the transaction requires one date group to be compared with another date range.

This range-based self-join has notoriously poor performance.  It's easy to write SQL to compare values within two date ranges, but it's more difficult to write SQL that compares two date ranges with fast performance.

The query to compare two date ranges might look something like this, using a pair of greater-than and less-than operators:

   transaction_table mar2004,
    trans_date >= to_date('1-mar-2004')
    trans_date < to_date('31-mar-2004')
group by trans_date
   transaction_table mar2005,
    trans_date >= to_date('1-mar-2005')
    trans_date < to_date('31-mar-2005')
group by trans_date;

Comparing ranges of values within he same table (even with partitioning) can generate very long-running queries, as in the date range comparison SQL above. There are several approaches to this range date time constraint issue when comparing two date ranges.

  1. Add a redundant day-month-year column
  2. Add a date lookup table
  3. Use the new SQL "overlaps" operator


Approach one for comparing date ranges:  Add redundant day-month-year

One approach is to slice-out the day-month-year into separate columns in the transaction table:

trans_year   trans_month trans_day  trans_date . . . .
2005         04          25         2005-04-25 17:25:43
2005         04          25         2005-04-25 17:25:43
2005         04          25         2005-04-25 17:25:43

This redundancy greatly simplifies range comparisons and performance.  Because the redundant date slice-off columns (trans_year, trans_month, trans_day) become separate dimensions, it simplifies the OLAP model.

It also improves query performance (psuedocode below- may not be syntactically accurate):

  sum(mar2004.sales) - sum(mar2005.sales)
   transaction_table mar2004,
   transaction_table mar2005
   mar2004.trans_year = 2004
   mar2004.trans_month = 3
   mar2004.trans_year = 2005
   mar2004.trans_month = 3;  

Approach two for comparing date ranges:  Add a date lookup table

Michael Armstrong-Smith, author of the bestselling Oracle Press book Oracle Discoverer Handbook notes his approach to the issue:

What works best is to create a routine that preloads calendar dates, along with their corresponding fiscal quarters, months and years into a single master date table. Then inside Discoverer we would join the transactional dates to this table.

The secret is to fully de-normalize the master table such that every date would have corresponding month, quarter and year data.  Thus for any given date, in any table, we could look up the corresponding month, quarter and year. We should also add start and end dates.

Approach 3 for comparing date ranges - Use the Oracle "overlaps" and "interval" SQL predicates

David Aldridge shows a demonstration of the Oracle SQL "Overlaps" predicate and how it can be used to easily compare two ranges of dates:

SQL> select * from dual
  2  where          (date ?2007-01-01′, date ?2008-01-01′)
  3        overlaps (date ?2005-01-01′, date ?2006-01-01′);

David also introduces the "interval" predicate and demonstrates how it performs with the "overlaps" predicate to compare two date ranges:

SQL> select * from dual
  2  where          (date ?2007-01-01′, interval ?5′ year)
  3        overlaps (date ?2005-01-01′, interval ?10′ year;





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