Information Design Tool (**Universe

Design Tool)

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Creating a Universe using IDT

**Here the data is stored in a MS Access DB and connection to the DB is made

using ODBC**

Our intension is to design/develop a Universe, which can be used by the webI

users to create reports as per the requirements. The process is as follows;

1: Creating a project

2: Creating Database Connections

3: Creating Data Foundation layer

4: Insert Tables and Joins to DFX layer

5: Creating Business Layer

6: Creating Folders/Classes and Objects in BLX layer

7: Creating Queries to Test

8: Publish to repository

Tutorial 5

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SQL Traps

A trap is a situation during the Universe design where the query returns higher

number of rows, than expected results. There are two types of SQL traps;

� CHASM trap

This is a problem with join path between three tables, where a single table

has 2 one-to-many join paths between the other two tables and there is no

context to separate the converging path. The relationship between three

tables will be “many-to-one-to-many”.

In above case when a query includes object from table B and Table C and

objects from table A, the CHASM trap causes a query to return every

possible combination of one measure with other. This result gets multiplied

by number of rows in result set and output is similar like a Cartesian

product. Chasm trap is not dependent on object types.

� FAN trap

This occurs when there is a one-to-many join to a table that fans out into

another one-to-many join to another table. It does not result into fan trap

unless a query includes a measure object on the middle table ('B') of the

table path and an object (of any kind) from the subsequent table ('C'). The

trap only occurs where a column in table B holds data values which are

already a sum of those values held at table C.

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If we run a query with objects Y and Z, the SQL will relate table tables B and

C which have 1: N relationship. The result will be inflated as the value for Y

object being multiplied by the number of values of the Z object related to

the Y object value. We cannot automatically detect FAN traps but needs to

visually examine the direction of the cardinalities.

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How to resolve Chasm Trap?

Analyze one –to-many join paths in the schema, Chasm trap is resolved by

executing separate SELECT statements for object Y and object Z.

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This is why the Sale and Rental revenues got doubled

Either modify the SQL parameters for the universe and generate separate SQL

queries for each measure or simply use context for each fact table.

How to resolve FAN Trap?

For resolving the FAN trap the best method will be to use a combination of Aliases

and Contexts. Create an Alias for the table which contain the aggregated value (B

table in the example) and then detect and implement contexts to separate the

query.

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Create an alias for a middle table (B). Create a join between new alias table (Ba)

and first table (A) which is at one end. Set the cardinality and set context for the

join. Change the parameter in Business layer of measure “Y” to refer to new alias

table.