DBGrid - how to show the differnce between a column value in two adjacent rows as another column?

Question

Let's say I record my car's latitude & longitude every minute or so, adding a row to a table each time.

I want to have a DB grid with 4 columns

1. latitude
2. longitude
3. distance since last measurment
4. curent street address, if known

Number 4, I can try to retrieve from Google Maps and I either get a text or blank, so let's ignore that.

How do I get #3? Should I calculate it in my application or in MySql (using a stored procedure or just a complicated SELECT)?

Whichever answer, can someone provide some sample code, or a link to an example? Google is not my friend today :-(

Answer 1

You can do it either way:

• Calculate it in your query, and display it in a calculated column (such as when showing line totals where you're displaying ITEM_PRICE, QUANTITY, ITEM_PRICE * QUANTITY AS LINE_COST.

• Calculate it in your application, using a calculated field (double-click the TTable component to open the Fields Editor, add field, set the type to ftCalculated, and write code in the OnCalcEvent for that calculated field.

The first choice is usually preferable, because the DB is usually more efficient at doing those sorts of thing on sets of data than when your code does it by row instead. (Set operations are obviously more efficient than row operations.)

Answer 2

Ken’s answer is goes far as it goes, because the most difficult task of solving your problem is getting data from two rows at the same time in SQL. If you were using Oracle or SQL Server, you could use the analytical/windowing LAG function. With MySQL you have to do it yourself. Here are 2 articles on how to simulate LAG in MySQL

http://www.onlamp.com/pub/a/mysql/2007/03/29/emulating-analytic-aka-ranking-functions-with-mysql.html?page=1

http://explainextended.com/2009/03/12/analytic-functions-optimizing-lag-lead-first_value-last_value/

Plus there are StackOverflow questions on how to get data from the previous row.

How do I lag columns in MySQL?

Calculate delta(difference of current and previous row) in sql

I cannot offer any opinion on any of them because I seldom work with MySQL.