MySQL查询父节点含指定值的子节点：哪种查询性能更优？

Great question! Let's break down these two SQL approaches and their performance characteristics clearly.

First, let's clarify what each query is doing:

• Your original query uses a subquery with IN: it first fetches the id of the parent folder where uniq='w2d4f6', then finds all child folders whose parent_id matches that value.
• The second query uses an implicit inner join (you could rewrite it with explicit INNER JOIN syntax for better clarity) to directly link parent and child records by matching a.id to b.parent_id, then filters for the parent's uniq value.

Now, onto performance:

• Most modern database optimizers (like MySQL, PostgreSQL, SQL Server) are smart enough to rewrite these two queries into the same underlying execution plan. Even though your EXPLAIN shows 2 subqueries, the optimizer is likely treating them as equivalent join operations under the hood. So in most production scenarios, you won't see a meaningful performance difference between the two.
• That said, in much older database versions (think pre-2010), subqueries using IN could sometimes be less efficient because the database might execute the subquery first, store the result set, then scan the outer table to match against it. Joins, by contrast, could be optimized to use indexes more directly. But this is a rare edge case these days.
• The bigger factor here is readability and maintainability. Rewriting the second query with explicit INNER JOIN syntax makes the parent-child relationship much clearer for other developers (or future you):

  SELECT b.label, b.parent_id 
  FROM folders a 
  INNER JOIN folders b ON b.parent_id = a.id 
  WHERE a.uniq = 'w2d4f6'
  

  This syntax is easier to parse, especially if you ever need to add more conditions or join additional tables later.
• Don't forget about indexes! Regardless of which query you use, performance will depend heavily on having proper indexes. Make sure uniq has an index (to quickly locate the parent folder), and parent_id has an index (to quickly find all child folders for that parent). Since id is almost always the primary key, it's already indexed by default.

In short: For modern databases, both queries will perform nearly identically. The explicit join approach is preferable for readability, and it aligns with standard SQL best practices.

内容的提问来源于stack exchange，提问作者Thijn