SqlDBM: Model revision history

In a previous post I presented sqlDBM, a really cool on-line tool for database modeling. You can use it to create the model of your SQL Server or MySQL RDBMS. Then use its forward engineering feature to generate an sql script that creates the whole of the database you have designed in the tool.

The tool has a very useful versioning feature: each time you click on the Save button and save the work on the model you are working with, the tool creates a new revision of the model. The picture below displays the revision history of my Payroll model so far:

You can easily open any revision you like by just clicking on the timestamp link. This is the latest revision of my database model:

What is really interesting on the screenshot above is that the tool, at the end of my last session, has automatically saved a draft of some changes I made after pressing the Save button. By pressing the Load Draft button I can load my work as it was just before I left the tool and continue my work exactly from the point I left it.

SqlDBM: A cool new tool for SQL Server Database design

I recently discovered a really cool new on-line tool for SQL Database design. The site hosting the tool is called SQL Database Modeler. After loging in with my Facebook account I started creating a new schema for SQL Server database. The GUI of the tool is intuitive and really easy to handle for someone accustomed to using desktop tools for database modelling like ErWin Data modeller.

The picture shows the project I've created: it's called Payroll and contains just one table, Employee.

Now I can start modeling my new SQL Server database! I can easiy login either from my desktop computer or my laptop at home, or my workstation at work and continue my modeling work from were I left it.

Calculate bus loads avoiding cursors

We have the following table structure, which is a representation of a bus route where passengers get on and off the bus with a door sensor:

Column	Data Type	Allow Nulls
ROUTE	char(4)	N
StopNumber	int	N
ONS	int	Y
OFFS	int	Y
SPOT_CHECK	int	Y

Field SPOT_CHECK represents a person who sits on that bus with a clipboard holding a spot count.

Sample data:

ROUTE	StopNumber	ONS	OFFS	SPOT_CHECK
Patras No. 2	1	5	0	NULL
Patras No. 2	2	0	0	NULL
Patras No. 2	3	2	1	NULL
Patras No. 2	4	6	3	8
Patras No. 2	5	1	0	NULL
Patras No. 2	6	0	1	7
Patras No. 2	7	0	3	NULL

Our aim:

Add a column LOAD to this table that calculates the load at each stop, according to the following logic:

LOAD = if (SPOT_CHECK is null)
       {
          Previous stops load + Current stop ONS - Current stop's OFFS
       }
       else SPOT_CHECK       

So, the expected results are:

ROUTE	StopNumber	ONS	OFFS	SPOT_CHECK	LOAD
Patras No. 2	1	5	0	NULL	5
Patras No. 2	2	0	0	NULL	5
Patras No. 2	3	2	1	NULL	6
Patras No. 2	4	6	3	8	8
Patras No. 2	5	1	0	NULL	9
Patras No. 2	6	0	1	7	7
Patras No. 2	7	0	3	NULL	4

We can implement the logic described using a cursor, but, the question is, can we solve this using a set based approach?

The tricky part:

Looks like a running total on ONS and OFF fields can solve the problem. Alas, this running total is being reset each time a not null SPOT_CHECK value is met.

Key idea:

Using a gaps and islands approach, identify sub-groups of consecutive records where a running total can be applied in order to calculate LOAD.

Query based on the key idea:

SELECT ROUTE, StopNumber, ONS, OFFS, SPOT_CHECK,
       COALESCE(SPOT_CHECK, ONS - OFFS) AS ld,
       SUM(CASE 
              WHEN SPOT_CHECK IS NULL THEN 0 
              ELSE 1 
           END)
       OVER (PARTITION BY ROUTE ORDER BY StopNumber) AS grp
FROM BusLoad

Output:

ROUTE	StopNumber	ONS	OFFS	SPOT_CHECK	LOAD	ld	grp
Patras No. 2	1	5	0	NULL	5	5	0
Patras No. 2	2	0	0	NULL	5	0	0
Patras No. 2	3	2	1	NULL	6	1	0
Patras No. 2	4	6	3	8	8	8	1
Patras No. 2	5	1	0	NULL	9	1	1
Patras No. 2	6	0	1	7	7	7	2
Patras No. 2	7	0	3	NULL	4	-3	2

All we want now is the running total of ld over [ROUTE, grp] partitions of data:

;WITH CTE AS (
    ...
    previous query here
    ...
    )
    SELECT ROUTE, StopNumber, ONS, OFFS, SPOT_CHECK, 
           SUM(ld) OVER (PARTITION BY ROUTE, grp ORDER BY StopNumber) AS LOAD
    FROM cte

Click here for a live demo.

Final note: The above query works for SQL Server versions starting from 2012. If you want a query for 2008 you have to somehow simulate sum() over (order by ...).