Sunday, November 26, 2017

What Relations Really Are and Why They Are Important

Note: Some of the References have been re-written to bring them into line with the McGoveran formalization and interpretation [1] of Codd's real RDM -- re-reading is recommended.

Here's what's wrong with the picture of two weeks ago, namely:

"In SQL RDBMSes (such as MS SQL Server and Oracle] tables are permently stored relations, where the column names defined in the data dictionary form the "heading" and the rows are the "tuples" of the relation."

"A relation can be represented by a table in database. A relation in the context of modeling a problem will include the fields and possibly the identification of fields which have relationships with other relations..."

"Put simply, a "relation" is a table, the heading being the definition of the structure and the rows being the data."

"In simple English: relation is data in tabular format with fixed number of columns and data type of each column. This can be a table, a view, a result of a subquery or a function etc."

"A relation is a table, which is a set of data. A table is the result of a query."

--What is a relation in database terminology?, StackOverflow.com

Sunday, November 19, 2017

This Week

1. Database Truth of the Week

"Codd's original 'normal form' in 1969-70 is not equivalent to the current 1NF. The former and 'normalization' were tied to Codd's first definition of join in 1969. Multiple normal forms (1NF-5NF) and 'further normalization' are a consequence of Codd's re-definition of join in 1970 as the one of today. Once 1NF and further normalization to at least 2NF have been introduced, 'the one normal form' makes no longer sense. Thus, there is no way to answer "what is the difference between the normal form and 1NF" without taking into account the definition of join, and -- if defined as we now do -- no way to understand the former, except to say that it was to the 1969-70 join what the 5NF is to the current join.That is one reason I personally strongly believe that while relations are at least in 1NF -- a relation that is not, cannot be represented formally as a relation, nor do the formal operators of the relational algebra work correctly otherwise -- even this is insufficient and formal relations must be in 5NF. Otherwise put, there is no such thing as a non-5NF database relation." --David McGoveran

2. What's Wrong With This Database Picture?

"A relation is a data structure which consists of a heading and an unordered set of tuples which share the same type", according to Wikipedia on 'Relation (database)'."
"In SQL RDBMSes (such as MS SQL Server and Oracle] tables are permanently stored relations, where the column names defined in the data dictionary form the "heading" and the rows are the "tuples" of the relation."
"A relation can be represented by a table in database. A relation in the context of modeling a problem will include the fields and possibly the identification of fields which have relationships with other relations..."
"Put simply, a "relation" is a table, the heading being the definition of the structure and the rows being the data."
"In simple English: relation is data in tabular format with fixed number of columns and data type of each column. This can be a table, a view, a result of a subquery or a function etc."
"A relation is a table, which is a set of data. A table is the result of a query." --What is a relation in database terminology?, StackOverflow.com

Wednesday, November 8, 2017

Data Model: The RDM Is, the E/RM Isn't

Minor touches, 11/17/17.


Note: This is a 11/08/17 rewrite of two older posts to bring them in line with the McGoveran's formalization and interpretation [1] of Codd's true RDM.

Here's what is wrong with the picture of two weeks ago, namely:

"ERM is a data model -- So says Date, Chen, etc. So says the majority of current industry experts. Refer to Date 6th edition p347. With very strong references to Codd (who he worked with), Date elegantly explains the differences between RM and ERM -- but clearly believes both are data models (even allowing for the charitable comment). If we take a RDB as the ultimate target implementation of data, and an ERM (or extended) can correctly design all the artifacts that are implemented, this means it is modelling [sic] the data. Granted, an ERM does not explicitly model some of the non-structural aspects of the original Codd definition.

Out of interest, is there a common Relational Modelling tool, that is not also an ERM tool and models the full Codd definition? There are also several other methods of modeling data -- ERM is more a mechanism to represent the data. If ERMs are used by IT professionals across the world to direct the design and build of the majority of applications guided by standard methodologies, is the view of this argument that these were all build wrongly? Regardless of success? Is the inferred conclusion that only the RM models data, and ERM, [or] any other techniques do not? [If so] that is a little limiting."
  
Chen's E/RM [2] (1976) preceded Codd's definition of a formal data model [3] (1980) and it would be unfair to hold it to that definition. But even if Chen used "unified view of data" in his paper's title, there is no excuse for "the majority of industry experts" claiming, post-1980, that the E/RM is a data model.

Sunday, November 5, 2017

This Week

1. Database Truth of the Week

"Logical (aka. syntactic) validity: Every logical inference (application of any number of rules of inference) from syntactically valid and well-formed premises yields well-formed and syntactically valid consequents (i.e., the rules of inference preserve syntactic validity and well-formedness.) This property is independent of any interpretations of the symbols. A query result is said to be logically valid iff it is derived by any sequence of RA operations on one or more relations.

"Semantic correctness: Every interpretation of the symbols (meaning and truth value assignment) that makes the axioms true, makes the theorems true. When we extend a logical model with semantics (specific to the subject matter and its "business" rules) via constraints, those constraints become axioms that must be true. A query result is said to be semantically correct iff for every assignment of meaning to relations that are the RA operands under which their tuples represent true facts, the tuples of the result relations also represent true facts.
If relations are not in 5NF, query results aren’t guaranteed to be semantically correct. Any update anomaly can appear in a result -- a join might deliver extra tuples that are anomalous. A database design that permits update anomalies does not preserve semantic correctness!" --David McGoveran


2. What's Wrong With This Database Picture?

"If we take a RDB as the ultimate target implementation of data, and an ERM (or extended) can correctly design all the artifacts that are implemented, this means it is modelling the data. Granted, an ERM does not explicitly model some of the non-structural aspects of the original Codd definition.

"ERM is a data model -- So says Date, Chen, etc. So says the majority of current industry experts. Refer to Date 6th edition p347. With very strong references to Codd (who he worked with), Date elegantly explains the differences between RM and ERM – but clearly believes both are data models (even allowing for the charitable comment).

Out of interest, is there a common Relational Modelling tool, that is not also an ERM tool and models the full Codd definition? There are also several other methods of modeling data -- ERM is more a mechanism to represent the data. If ERMs are used by IT professionals across the world to direct the design and build of the majority of applications guided by standard methodologies, is the view of this argument that these were all build wrongly? Regardless of success? Is the inferred conclusion that only the RM models data, and ERM, [or] any other techniques do not? [If so] that is a little limiting." --LinkedIn.com