Chris Adamson’s Blog: Bridge Tables and Many-to-many Relationships
Hide the many-to-many relationship. You can publish two versions of the schema : the full one for use by structured. Solved: Hi, I've got two tables that are related with a bridge table in the middle to create a many to many relationship. The data is coming from. Data Warehouse Design Techniques – Bridge Tables. 1. There will be times when you arrive at a many to many relationship between a fact and a dimension.
Dynamic query mode may return an error for some scenarios when bridge query subjects are present.
Data Modeling - Many-to-many Relationship
A string, mathematical, or logical expression using items or filters cannot reference items from query subjects on either side of a bridge query subject. The query is not supported. Query 'Query1' contains data item 'FullName' which has an expression that refers to opposing sides of a bridge query subject.
Query 'Query1' contains a detail filter which has an expression that refers to opposing sides of a bridge query subject The query is not supported. Query 'Query1' contains a summary filter which has an expression that refers to opposing sides of a bridge query subject Queries must have be set to perform automatic grouping and summarization of data.
Dynamic query mode does not allow a detail query which projects items from subjects on either side of a bridge query subject.
Chris Adamson’s Blog: Using A Hierarchy Bridge WIthout A Many-to-many Relationship
Query 'Query1' references a bridge query subject and therefore cannot have the property 'Auto Group and Summarize' set to No. Items which compute an aggregate value cannot include the pre-filter clause.
Query 'Query1' contains data item 'PrefilteredTotal' which has a pre-filter aggregate expression, this is not supported when the query references a bridge query subject.Database Design Tutorial on SQL Many-To-Many Relationships.
Average summaries within a bridge query are not supported. A bridge query that refers to a fact having its default aggregation set to average will fail with the following error. Query 'Query1' contains data item 'costs' which has an implicit average aggregate expression, this is not supported when the query references a bridge query subject.
Data Modeling - Many-to-many Relationship [Gerardnico]
Analytic expressions are not supported and will generate the following errors. Query 'Query1' contains data item 'theRank' which has an analytic aggregate expression, this is not supported when the query references a bridge query subject.
The bridge table allows us to link the repeating dimension to the facts, but as noted in the post, care must be taken not to double count. But we are not discussing entity-relationship modeling. In a dimensional model, this kind of relationship is allowed. Despite what you may think, it is also compatible with your relational database management system.
The figure above can be implemented as a physical design, and it will work. Unfortunately, some of our software tools do not understand this. Tool Problems Many of the software products we use in data warehousing were originally developed for transaction processing systems. Because of this lineage, the many-to-many relationship can be a problem. This is a problem for the model pictured above.
How to show the data in a many to many with bridge table
But it is not a primary key in either table. Luckily, this join will work even if it is not supported by any key constraints. Unfortunately, other tools may not be as forgiving. Some data modeling tools do not permit a many-to-many relationship in a physical model.
This presumably reflects their entity-relationship lineage. And if you use a SQL-generating BI tool to create reports, you may find it identifies bridge table as fact tables. Eliminating The Many-To-Many If your software products cannot live with a many-to-many relationship, there is a simple solution. You can resolve in the same way an entity-relationship modeler would: In the example above, a new table is added that contains one row for each group. An association or intersect table, will link the group to the salespeople.
- Tallan Blog
- Bridge tables
The result looks like this: For those of us who find humor in data modeling, this is a source of much amusement. And since the sole column is a surrogate key, the table's entire contents are likely to be a sequence of integers!