(See Part 1 and Part 2)
“A common term used in database design is a "relational database" -- but a database relation is not the same thing and does not imply, as its name suggests, a relationship between tables. Rather, a database relation simply refers to an individual table in a relational database. In a relational database, the table is a relation because it stores the relation between data in its column-row format. The columns are the table's attributes, while the rows represent the data records. A single row is known as a tuple to database designers.”
“A relation, or table, in a relational database has certain properties.”
“First off, its name must be unique in the database, i.e. a database cannot contain multiple tables of the same name.”
“Next ... as with the table names, no attributes can have the same name.”
“Next, no tuple (or row) can be a duplicate. In practice, a database might actually contain duplicate rows, but there should be practices in place to avoid this, such as the use of unique primary keys (next up). Given that a tuple cannot be a duplicate, it follows that a relation must contain at least one attribute (or column) that identifies each tuple (or row) uniquely. This is usually the primary key. This primary key cannot be duplicated. This means that no tuple can have the same unique, primary key. The key cannot have a NULL value, which simply means that the value must be known.”
“Further, each cell, or field, must contain a single value. For example, you cannot enter something like "Tom Smith" and expect the database to understand that you have a first and last name; rather, the database will understand that the value of that cell is exactly what has been entered.”
“Finally, all attributes—or columns—must be of the same domain, meaning that they must have the same data type. You cannot mix a string and a number in a single cell.”
“All these properties, or constraints, serve to ensure data integrity, important to maintain the accuracy of data.” --Definition of Database Relation
It is easy to discern when explanations of relational features are not grounded in the formal foundations of the RDM[1], but in industry practices. Here are some further clarifications and corrections.
Note: This is a re-write of an older post, to bring it into line with the McGoveran formalization and interpretation [1] of Codd's true RDM, which includes refinements, corrections, and extensions of his own.
“Codd has based his model on n-ary relations and that is the key mistake he has made; that leads to complex structure (absolutely not necessary) and situations where there are no values known and as a consequence the need of the concept we know too well -> the null pointers; binary relations (smallest possible) are sufficient to express any predicate/sententional formula and there is no possibility to have something like null; if a value is unknown then we do not know it thus it is not a fact for us thus it is not in our database.”
We suggest you test your foundation knowledge by figuring what's wrong with this picture before you proceed.
Note:
This is a re-write of two older posts (which now link here), to bring
them into line with the McGoveran formalization and interpretation of
Codd's real RDM, including his own refinements, corrections, and
extensions[1].
(Continued from Part 1)
I started Part 1
with comments exposing confusion (among the many misconceptions rampant
in the industry[2]) of database relations with tables (reinforced by
SQL), and of levels of representation. Debunking them requires
foundation knowledge lacking in the industry, so I provided some: I
explained the difference between mathematical relations, database
relations, and relational tables. We are now in a position to debunk the
comments.
Note: This is a re-write of two older posts (which now link here), to bring them into line with the McGoveran formalization and interpretation of Codd's real RDM, including his own refinements, corrections, and extensions[1]
“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.”
“Practically, a "Relation" in relational model can be considered as a "Table" in actual RDBMS products(Oracle, SQL Server, MySQL, etc), and "Tuples" in a relation can also be considered as "Rows" or "Records" in a table.”
“In common usage, however, when someone refers to a "relation" in a database course, they are referring to a tabular set of data either permanently stored in the database (a table) or derived from tables according to a mathematical description (a view or a query result).”
“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. Then from a table, a query can return a different relation.”
“Data is stored in two-dimensional tables consisting of columns (fields) and rows (records). Multi-dimensional data is represented by a system of relationships among two-dimensional tables.”
“I read [that] "Relations are multidimensional. They are not flat. They are not two dimensional. Don't let the term table mislead you." on the back cover of CJ Date's DATABASE IN DEPTH. Can anyone help how to visualize this multidimensional nature of relations?”
Because SQL DBMSs have been sold as relational databases (which they are not), and in SQL the data structure is the table, in the absence of foundation knowledge[2] most practitioners think that relational databases consist of tables, but do not ask themselves why and how is that significant for database practice. The subtitle of this post is a question I used to ask in presentations years ago that always got silence. I see no evidence of improvement -- in fact, it's gotten worse. To emulate Feynman, "Nobody understands the RDM".
That such a simple and commonly understood structure can visualize relations is an advantage of the RDM, but a table is not a relation and, SQL notwithstanding, confusing the two reflects a lack of understanding of the RDM, misses its significance for database practice, and prevents taking full advantage of its benefits.
Note: The table is the preferred way to picture relations, there are others (e.g., array).
First, the fundamentals.
Note: This is the second part of a multipart re-write of several older posts to bring them into line with the McGoveran formalization and re-interpretation of Codd's real RDM, including revisions, refinements, and extesions of his own[1].
(Continued from Part 1)
Part 1 started with an online exchange triggered by the question “Do I Have to Use Foreign Keys? If I am already manipulating data properly, are foreign keys required? Do they have another purpose that I’m just not aware of?” Both the question and the replies exhibit misconceptions about FKs (there are misconceptions about almost everything in the RDM[2]) rooted in lack of foundation knowledge, so we provided some FK fundamentals. We are now in a position to debunk the replies.
Note: This is the first part of a two-part re-write of several earlier posts, to bring them into line with McGoveran's formalization and re-interpretation of Codd's true RDM, which includes his own corrections, refinements and extensions[1]. For a more in-depth treatment see the series of papers available here.
“Do I Have to Use Foreign Keys? If I am already manipulating data properly, are foreign keys required? Do they have another purpose that I’m just not aware of? I appreciate the guidance!”
“... [we] wish to make a point. There is something which is bad design/good design/mandatory/optional. Please stop insisting that Primary and Foreign keys are mandatory. They are good design habits but by no means mandatory. However, life is much more complex than a Normalized DB structure. This includes tables serving as event logs; tables, serving as User maintained materialized query tables, tables, serving as supporting structures, reflecting state of complex transactional databases; persistent tables serving as Result Set or Session keepers. And I personally believe that if they were truly mandatory, Sybase, Oracle, SQL Server, Ingres, DB2, etc. would require them. Oh, sorry, forgot the SQL standard itself. This is not the relational model we're talking about. These are commercially available RDBMSs which, not surprisingly, DO tend to listen to their customers. If they didn't, they wouldn't be in business!! Since Sybrand is unlikely to get FKs required by the SQL standard or the major RDBMS vendors, it seems that mandatory means that his answer to the question "Do I have to use foreign keys?" is "You would if you worked in my shop!". I'm inclined to agree with that.”
“Databases can work with or without primary keys and foreign keys. The choice is yours... However ... enforcing referential integrity can be done by many methods ... TMTOWTDT = There is more than one way to do this ... It all depends on your approach... In the last ten years... every one is enforcing referential integrity with help of primary and foreign keys but before this ... a lot of applications were working without primary and foreign keys to enforce referential integrity and to avoid orphaned rows/avoid duplicate records.”
“We don't have every possible logical relationship enforced by the database. Sometimes you have to compromise for performance reasons, as too many foreign key validations can slow down high volume inserts. Other times you have to create breakpoints just to keep the web of relationships from becoming too tangled and connecting hundreds or thousands of tables.”
“I think it is preferable to have FK constraints as an additional security layer and they can be disable[d] during loading if required; however, you need to be 100% certain your ETL is enforcing the constraints. It is best to do both - have the ETL reject records which fail FK checks and report on these whilst also enforcing FK intergrity on insert/update, if appropriate. The only additional thing I can add is - when you delete from a FK enabled DB, make sure you do it in the correct order.”
“I think, you have to learn about data structures and logical data design (not only database, which is nowadays interpreted mainly as only RDBMS), to be clear about usage primary, alternate, and foreign keys, normal forms, data integrity-and database integrity, because your database will work suboptimally without these knowledge if it will work at all.”
Data practitioners have high levels of tool knowledge, but lack a good grasp of fundamentals, for which reason they cannot be considered data professionals. Now, do not get me wrong: I do not mean that good knowledge of tools is unimportant -- if you work with them you gotta know them real well -- but the ability to fully assess them, use them optimally, and compensate for any shortcomings is limited in the absence of foundation knowledge. So let's have some before tackling the exchange.
I have deleted my Facebook account and am focusing my presence on social media on two Twitter pages, DBDebunk, and The PostWest.
Follow: @DBDEBUNK @THEPOSTWEST
On DBDebunk I will tweet links to new posts on this site, To Laugh or Cry? and What's Wrong with This Picture? that I am bringing back there, and occasional links to and comments on items of interest. I have created a #RelModel hashtag for use with it.
On ThePostWest I will tweet links to evidence for, and my take on (1) Dystopian Western Decadence and Decline, including (2) the equivalence of The Only Acceptable Racism Left and The Weaponized Myth of a "Palestinian Nation". I will create a #ThePostWest later on for use with it.
Please make a note of it, disseminate, and follow.
