是否可将所有相关SQL子查询去关联化？（通用理论视角，非特定查询）

Hey folks, during my university database courses, I’ve dug into how SQL databases actually execute the queries we write, and it’s way more structured than you might think! Here’s a breakdown of the key bits I’ve picked up:

The Core Translation Step: SQL → Relational Algebra #

When you fire off a SQL query, the database doesn’t just run it directly. First, it parses and translates the query into relational algebra—this is the foundation for building an efficient execution plan. Relational algebra gives the database a formal, mathematical way to represent the operations needed to fetch your result set.

Handling Non-Correlated Subqueries with Ease #

One of the biggest strengths of relational algebra here is how cleanly it handles non-correlated subqueries (those that don’t depend on values from the outer query). Unlike trying to reason through nested SQL syntax, relational algebra lets you represent these subqueries as standalone relational operations first.

For example, say you have this SQL query to find customers who’ve placed orders over $100:

SELECT customer_id, name
FROM customers
WHERE customer_id IN (
    SELECT customer_id
    FROM orders
    WHERE total_amount > 100
);

In relational algebra, this translates to a semi-join (a more efficient alternative to set intersection) between the customers relation and the subset of orders where total_amount > 100. The database can then choose to optimize this into a direct join operation instead of executing the subquery separately if that’s faster.

Combining Operations: Joins & Set Operations as Needed #

Relational algebra’s modular nature means the database can mix and match operations to create the most efficient plan. If a subquery can be rewritten as a join (like the example above), it will often do that because joins are highly optimized in most databases. Similarly, set operations like UNION, INTERSECT, or EXCEPT map directly to relational algebra’s union, intersection, and difference operations, making it straightforward to translate and optimize those too.

It’s fascinating to see how this underlying mathematical model lets databases make smart decisions about how to run our queries efficiently—way beyond just reading the SQL as-is!

内容的提问来源于stack exchange，提问作者TheHans255