#
msgid ""
msgstr ""
msgid ""
"title: A Relational Model of Data for Large Shared Data Banks - article-"
"review"
msgstr ""
msgid "date: 2021-04-29"
msgstr ""
msgid "layout: post"
msgstr ""
msgid "lang: en"
msgstr ""
msgid ""
"ref: a-relational-model-of-data-for-large-shared-data-banks-article-review"
msgstr ""
msgid ""
"This is a review of the article \"[A Relational Model of Data for Large "
"Shared Data Banks](https://www.seas.upenn.edu/~zives/03f/cis550/codd.pdf)\","
" by E. F. Codd."
msgstr ""
msgid "Data Independence"
msgstr ""
msgid ""
"Codd brings the idea of *data independence* as a better approach to use on "
"databases. This is contrast with the existing approaches, namely "
"hierarquical (tree-based) and network-based."
msgstr ""
msgid ""
"His main argument is that queries in applications shouldn't depende and be "
"coupled with how the data is represented internally by the database system. "
"This key idea is very powerful, and something that we strive for in many "
"other places: decoupling the interface from the implementation."
msgstr ""
msgid ""
"If the database system has this separation, it can kep the querying "
"interface stable, while having the freedom to change its internal "
"representation at will, for better performance, less storage, etc."
msgstr ""
msgid ""
"This is true for most modern database systems. They can change from B-Trees "
"with leafs containing pointers to data, to B-Trees with leafs containing the"
" raw data , to hash tables. All that without changing the query interface, "
"only its performance."
msgstr ""
msgid ""
"Codd mentions that, from an information representation standpoint, any index"
" is a duplication, but useful for perfomance."
msgstr ""
msgid ""
"This data independence also impacts ordering (a *relation* doesn't rely on "
"the insertion order)."
msgstr ""
msgid "Duplicates"
msgstr ""
msgid ""
"His definition of relational data is a bit differente from most modern "
"database systems, namely **no duplicate rows**."
msgstr ""
msgid ""
"I couldn't find a reason behind this restriction, though. For practical "
"purposes, I find it useful to have it."
msgstr ""
msgid "Relational Data"
msgstr ""
msgid ""
"In the article, Codd doesn't try to define a language, and today's most "
"popular one is SQL."
msgstr ""
msgid ""
"However, there is no restriction that says that \"SQL database\" and "
"\"relational database\" are synonyms. One could have a relational database "
"without using SQL at all, and it would still be a relational one."
msgstr ""
msgid ""
"The main one that I have in mind, and the reason that led me to reading this"
" paper in the first place, is Datomic."
msgstr ""
msgid ""
"Is uses an [edn]-based representation for datalog queries[^edn-queries], and"
" a particular schema used to represent data."
msgstr ""
msgid ""
"Even though it looks very weird when coming from SQL, I'd argue that it "
"ticks all the boxes (except for \"no duplicates\") that defines a relational"
" database, since building relations and applying operations on them is "
"possible."
msgstr ""
msgid ""
"Compare and contrast a contrived example of possible representations of SQL "
"and datalog of the same data:"
msgstr ""
msgid ""
"-- create schema\n"
"CREATE TABLE people (\n"
" id UUID PRIMARY KEY,\n"
" name TEXT NOT NULL,\n"
" manager_id UUID,\n"
" FOREIGN KEY (manager_id) REFERENCES people (id)\n"
");\n"
"\n"
"-- insert data\n"
"INSERT INTO people (id, name, manager_id) VALUES\n"
" (\"d3f29960-ccf0-44e4-be66-1a1544677441\", \"Foo\", \"076356f4-1a0e-451c-b9c6-a6f56feec941\"),\n"
" (\"076356f4-1a0e-451c-b9c6-a6f56feec941\", \"Bar\");\n"
"\n"
"-- query data, make a relation\n"
"\n"
"SELECT employees.name AS 'employee-name',\n"
" managers.name AS 'manager-name'\n"
"FROM people employees\n"
"INNER JOIN people managers ON employees.manager_id = managers.id;\n"
msgstr ""
msgid "{% raw %}"
msgstr ""
msgid ""
";; create schema\n"
"#{ {:db/ident :person/id\n"
" :db/valueType :db.type/uuid\n"
" :db/cardinality :db.cardinality/one\n"
" :db/unique :db.unique/value}\n"
" {:db/ident :person/name\n"
" :db/valueType :db.type/string\n"
" :db/cardinality :db.cardinality/one}\n"
" {:db/ident :person/manager\n"
" :db/valueType :db.type/ref\n"
" :db/cardinality :db.cardinality/one}}\n"
"\n"
";; insert data\n"
"#{ {:person/id #uuid \"d3f29960-ccf0-44e4-be66-1a1544677441\"\n"
" :person/name \"Foo\"\n"
" :person/manager [:person/id #uuid \"076356f4-1a0e-451c-b9c6-a6f56feec941\"]}\n"
" {:person/id #uuid \"076356f4-1a0e-451c-b9c6-a6f56feec941\"\n"
" :person/name \"Bar\"}}\n"
"\n"
";; query data, make a relation\n"
"{:find [?employee-name ?manager-name]\n"
" :where [[?person :person/name ?employee-name]\n"
" [?person :person/manager ?manager]\n"
" [?manager :person/name ?manager-name]]}\n"
msgstr ""
msgid "{% endraw %}"
msgstr ""
msgid ""
"(forgive any errors on the above SQL and datalog code, I didn't run them to "
"check. Patches welcome!)"
msgstr ""
msgid ""
"This employee example comes from the paper, and both SQL and datalog "
"representations match the paper definition of \"relational\"."
msgstr ""
msgid ""
"Both \"Foo\" and \"Bar\" are employees, and the data is normalized. SQL "
"represents data as tables, and Datomic as datoms, but relations could be "
"derived from both, which we could view as:"
msgstr ""
msgid ""
"employee_name | manager_name\n"
"----------------------------\n"
"\"Foo\" | \"Bar\"\n"
msgstr ""
msgid ""
"[^edn-queries]: You can think of it as JSON, but with a Clojure taste. "
"[edn]: https://github.com/edn-format/edn"
msgstr ""
msgid "Conclusion"
msgstr ""
msgid ""
"The article also talks about operators, consistency and normalization, which"
" are now so widespread and well-known that it feels a bit weird seeing "
"someone advocating for it."
msgstr ""
msgid ""
"I also stablish that `relational != SQL`, and other databases such as "
"Datomic are also relational, following Codd's original definition."
msgstr ""
