RDB (Relational Database)

关系型数据库源于relational model. Relational model? Relational model的内在逻辑是first-order (predicate) logic. (所有data表示为tuples, grouped into relations.) First-order logic? First-order Logic ：Constants, Variables, Predicates, Functions, Quantifiers... Example: Only one student failed in Mathematics. In this question, the predicate is "failed(x, y),"... ∃(x) [ student(x) → failed (x, Mathematics) ∧∀ (y) [¬(x==y) ∧ student(y) → ¬failed (x, Mathematics)].

SQL作为声明式语言(Declarative language)

Declarative language... WHAT. 你要query的系统已经实现了具体怎么取 (query optimizer)，声明式语言只须关注what results to get。 还有哪些常见的Declarative langages?

• XML, XPath, XQuery
• Regex
• Prolog, Datalog...

看起来声明式语言抽象层次更高（先有what再有how），所以学习这类语言，关键是准确理解具体query本身对应什么(what)功能、执行顺序如何。

SQL的运算

Logic and Databases Relational algebra基本操作：Union, Difference, Certesian Product, Projection, Selection, natural join. Query: The query evaluation problem, the query equivalence problem, the query containment problem. Relational algebra（数据库的传统运算) SQL_to_Relational_Algebra, SQL-Relational algebra计算器

SQL的执行顺序

Execution sequence of SQL FROM (JOIN)

• Joins: Inner Join? Left Join? Right Join? Natural Join? Self Join? ... etc.
• 笛卡尔积
• 超过两个表？

WHERE GROUP BY

• 常和SELECT Aggregation一起用
• 相同值的到同一行上

HAVING

• 限定group by的结果

SELECT ORDER BY

• Multi-columns?
• Using FIELD(col, value1, value2...)

LIMIT

SQL Query优化

• 用subqueries.
  • 　　https://www.sisense.com/blog/use-subqueries-to-count-distinct-50x-faster

• 多数情况下，join faster than subqueries.

SQL Tutorial

• Database Tutorial
• https://www.javatpoint.com/mysql-upsert
• https://www.mysqltutorial.org/mysql-having.aspx/