SQL JOIN - WHERE clause vs. ON clause

Question

After reading it, this is not a duplicate of Explicit vs Implicit SQL Joins. The answer may be related (or even the same) but the question is diffe

Answer 1

They are equivalent, literally.

In most open-source databases (most notable examples, in MySql and postgresql) the query planning is a variant of the classic algorithm appearing in Access Path Selection in a Relational Database Management System (Selinger et al, 1979). In this approach, the conditions are of two types

• conditions referring to a single table (used for filtering)
• conditions referring to two tables (treated as join conditions, regardless of where they appear)

Especially in MySql, you can see yourself, by tracing the optimizer, that the join .. on conditions are replaced during parsing by the equivalent where conditions. A similar thing happens in postgresql (though there's no way to see it through a log, you have to read the source description).

Anyway, the main point is, the difference between the two syntax variants is lost during the parsing/query-rewriting phase, it does not even reach the query planning and execution phase. So, there's no question about whether they are equivalent in terms of performance, they become identical long before they reach the execution phase.

You can use explain, to verify that they produce identical plans. Eg, in postgres, the plan will contain a join clause, even if you didn't use the join..on syntax anywhere.

Oracle and SQL server are not open source, but, as far as I know, they are based equivalence rules (similar to those in relational algebra), and they also produce identical execution plans in both cases.

Obviously, the two syntax styles are not equivalent for outer joins, for those you have to use the join ... on syntax