Move about relational databases to blog website

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 => tech/migadu Migadu
 => tech/ripping Ripping
 => tech/about-apis About APIs
+=> tech/about-relational-databases About relational databases
 
 ### Gadgets
 
diff --git a/blog/content/notes/tech/about-relational-databases.gmi b/blog/content/notes/tech/about-relational-databases.gmi
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+# 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
+* C++ std::map
+* Java java.util.Map
+* C# System.Collections.Generic.Dictionary
+* Javascript Object
+* PHP arrays
+
+Relations are a natural concept, so although non-relational data systems exist, most data can be stored as relations.