Add programming/about_relational_databases

diff --git a/programming/about_relational_databases.md b/programming/about_relational_databases.md
new file mode 100644 (file)
index 0000000..464de33
--- /dev/null
+++ b/programming/about_relational_databases.md
@@ -0,0 +1,30 @@
+# About relational databases
+
+## What is a relation?
+
+A common misconception is that the "relations" in a relational database are about relations between database tables.
+
+Actually, the relations in a relational database are the tables.
+
+A relation "relates" a set of values with another set of values.
+
+For example, a relation can relate the name of a person with their birth date and birth place.
+For example:
+
+```
+(person name) => (birth date, birth place)
+(Alice) => (1979-12-03, Barcelona)
+(Bob) => (1995-03-04, Paris)
+...
+```
+
+Many computer languages have similar concepts:
+
+* [Python mapping types such as `dict`](https://docs.python.org/3/library/stdtypes.html#mapping-types-dict)
+* C++ `std::map`
+* Java `java.util.Map`
+* [C# `System.Collections.Generic.Dictionary`](https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.dictionary-2?view=net-9.0)
+* [Javascript `Object`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object)
+* [PHP arrays](https://www.php.net/manual/en/language.types.array.php)
+
+Relations are a natural concept, so although non-relational data systems exist, most data can be stored as relations.