chore(postgres): dump learned queries and concepts

knowledge base/sql.md

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+# Structured Query Language
+
+Standard language for relational database management systems.
+
+## TL;DR
+
+`NULL` represents **unknown** or **missing** information.<br/>
+It is **not** the same as an empty string or the number 0.
+
+| Constraint | Description                                    |
+| ---------- | ---------------------------------------------- |
+| `NOT NULL` | The value cannot be `NULL`                     |
+| `UNIQUE`   | The value must be unique in the table's column |
+
+_Primary keys_ are one or more columns in a table used to uniquely identify a single row.<br/>
+Tables can have **zero or one** primary keys. They **cannot** have more than one primary key.
+
+Technically, primary key constraints are the combination of a `NOT NULL` constraint and a `UNIQUE` constraint.
+
+_Foreign keys_ are one or more columns in a table that uniquely identify a row in **another** table.<br/>
+They reference either the primary key or another `UNIQUE` constraint of the referenced table.
+
+## Further readings
+
+- [The SQL Standard - ISO/IEC 9075:2023 (ANSI X3.135)]
+- [PostgreSQL]
+- [MySQL]
+
+<!--
+  Reference
+  ═╬═Time══
+  -->
+
+<!-- In-article sections -->
+<!-- Knowledge base -->
+[mysql]: mysql.md
+[postgresql]: postgresql.md
+
+<!-- Files -->
+<!-- Upstream -->
+[the sql standard - iso/iec 9075:2023 (ansi x3.135)]: https://blog.ansi.org/sql-standard-iso-iec-9075-2023-ansi-x3-135/
+
+<!-- Others -->