Q
Problem solve Get help with specific problems with your technologies, process and projects.

# Comparing current and previous row values

## My SupplyCosts table contains a list of supplies and the cost associated with that supply for every month. I want to select only the months where the cost has changed and the previous cost. SELECT DISTINCT doesn't work because I need the month field and it returns every record.

My SupplyCosts table contains a list of supplies and the cost associated with that supply for every month:

```SupplyCosts:
mth   item    cost
10-04 bandaid .20
11-04 bandaid .25
12-04 bandaid .30
01-05 bandaid .30```

Some months the cost remains the same and the only change is the month field. Other months, the cost changes. I want to select only the months where the cost has changed and the previous cost. From the example data above, I only want the records for the 11-04 and 12-04 months. SELECT DISTINCT doesn't work because I need the month field and it returns every record. Thanks so much.

The trick with this type of problem, where you need to compare a value on one row with a value on another row, is to use a self-join. This particular scenario is complicated by the fact that you need a pretty hairy join clause, to match the current month with the previous month.

To see why, let's ask ourselves how we should compare 12-04 and 01-05. We can not simply say:

`where t1.mth = t2.mth - 1`

The reason we can't is obvious: 12-04 and 01-05 aren't numbers. They aren't dates, either. They're strings.

However, all is not lost. We can convert strings to dates easily enough, although the actual mechanism is different in just about every database system. Here's the solution using MySQL syntax:

```select curr.item
, curr.mth
, curr.cost as current_cost
, prev.cost as previous_cost
from SupplyCosts as prev
inner
join SupplyCosts as curr
on prev.item = curr.item
cast(
concat('20',substr(prev.mth,4,2)
,'-',substr(prev.mth,1,2)
,'-01') as date)
, interval 1 month)
= cast(
concat('20',substr(curr.mth,4,2)
,'-',substr(curr.mth,1,2)
,'-01') as date)
where prev.cost
<> curr.cost
order
by curr.item
, curr.mth ```

The expressions involving DATE_ADD and CAST will change from one database system to another. Basically you convert the strings to dates (using the first of the month), and then use date arithmetic (in this case the DATE_ADD function) to add 1 month.

This was last published in August 2005

## Content

Find more PRO+ content and other member only offers, here.

#### Have a question for an expert?

Get answers from a TechTarget expert on whatever's puzzling you.

You will be able to add details on the next page.

#### Start the conversation

Send me notifications when other members comment.

## SearchDataManagement

• ### Connectedness is king, as Neo4j graph database ports to Spark

The Neo4j graph database emphasizes easy relationship mapping for diverse data points. Now, its related Cypher query language is ...

• ### NewSQL databases rise anew -- MemSQL, Spanner among contenders

The NewSQL database was almost hidden when Hadoop and NoSQL arose. Now, as more big data teams move toward production uses, ...

• ### Good data quality for analytics becomes an IT imperative

High-quality data is a must for analytics applications. That's driving more demand for data quality tools, but quality ...

• ### Automating machine learning puts analytical models on autopilot

With data scientists in short supply and the value of machine learning models growing ever more evident, software vendors are ...

• ### Hiring vs. training data scientists: The case for each approach

Hiring data scientists is easier said than done -- so should you try to train current employees in data science skills? That ...

• ### Tableau targets data preparation software with Project Maestro

As Tableau and other high-level applications look to automate more functionality, stand-alone data preparation tools have to ...

## SearchSAP

• ### Responsible sourcing can be good for business

A company's reputation and bottom line can be damaged if its suppliers engage in harmful practices. Responsible sourcing and risk...

• ### SAP TechEd 2017 focuses on next-gen development tools

At SAP TechEd 2017, SAP rolled out some new developer tools that are intended to extend the SAP platform and drive development of...

• ### SAP Analytics Cloud helps paper-maker tell a good business story

SAP Analytics Cloud software is helping paper manufacturer Pratt Industries tell the story of monthly forecasts more accurately, ...

## SearchSQLServer

• ### Microsoft Azure cloud database activity takes off at Connect();

Microsoft Connect(); 2017 saw the addition of MariaDB and Cassandra to the Azure cloud database lineup. Also discussed: A set of ...

• ### Microsoft technology refresh touches SQL Server, integration tooling

Microsoft is at work on a delicate technology refresh affecting database tuning and architecture, as well as data integration and...

• ### Microsoft boosts SQL Server machine learning services

Python and R are among the tools in the SQL Server machine learning toolkit. Native T-SQL scoring is also on the agenda, as ...

## TheServerSide.com

• ### Everything to know about overcoming digital transformation challenges

As organizations adopt new technologies, like blockchain, IoT devices and deep learning, strategies must be developed to address ...

• ### Can DevOps problems actually cause projects to fail?

DevOps isn't perfect. There are times when DevOps problems can overwhelm the potential benefits. So, why do some DevOps projects ...

• ### Owning the Java Platform is more of a burden or a blessing

Oracle became stewards of the Java platform as a by-product of their acquisition of Sun Microsystems. But looking back, it seems ...

## SearchDataCenter

• ### Security, vendor choices affect server purchases for IT buyers

Server selection is a quandary for IT, as security, the use of file servers and whether multiple servers of CPU systems will meet...

• ### How to choose a server based on your data center's needs

In an effort to optimize performance in the enterprise, IT should evaluate top priorities to determine how to choose a server and...

• ### Data center GPU use on the rise thanks to AI, big data

GPU vendors have added new devices and cards for data center servers, as data demanding workloads infiltrated the data center and...

## SearchContentManagement

• ### Q&A: New CEO bets on open source future for Acquia CMS

The Acquia CMS took the Red Hat model to content management by commercializing open source Drupal. What's next? We ask co-founder...

• ### CMS analytics arms businesses with a strategic planning edge

Content analytics for CMSes mines business value from free text in data lakes, so it's time to go prospecting for gold with this ...

• ### Enterprise content management systems boost intelligence

Content analytics moves beyond the tried-and-true web analytics style of insights, adding natural language processing and images ...

## SearchFinancialApplications

• ### Finance IT case study: Reporting secrets of Derek Rose

CEO Sacha Rose says specialist reporting tools have saved the company thousands by avoiding unnecessary mistakes.

• ### WestJet turns to gamification to help its Oracle ERP users soar

WestJet's initial gamification project focuses on expense reporting.

• ### The Transformation of HR is Underway

HR is being transformed while we watch.

Close