Is a JOIN Always Faster than a WHERE Clause for Related Table Queries?

Is a JOIN Fundamentally Faster than a WHERE?

When comparing two similar queries that retrieve data from linked tables, one using a WHERE clause and the other using a JOIN, the question arises: is there a significant performance difference?

Database Schema:

Consider the following database schema:

CREATE TABLE Document (
  Id INT PRIMARY KEY,
  Name VARCHAR 255
);

CREATE TABLE DocumentStats (
  Id INT PRIMARY KEY,
  DocumentId INT, -- foreign key to table Document
  NbViews INT
);

Query Comparison:

To retrieve documents with more than 500 views, we can use either a traditional WHERE clause or a JOIN operation:

-- WHERE clause
SELECT *
FROM Document, DocumentStats
WHERE DocumentStats.Id = Document.Id
  AND DocumentStats.NbViews > 500;

-- JOIN operation
SELECT *
FROM Document
INNER JOIN DocumentStats ON Document.Id = DocumentStats.Id
WHERE DocumentStats.NbViews > 500;

Theoretical Equivalence:

Theoretically, the two queries should produce equivalent results. The database query optimizer is designed to generate optimal execution plans that minimize execution time, regardless of the query syntax used.

However, in specific cases, some database engines may generate more efficient plans for one query over the other. For complex queries or database configurations, testing both approaches can reveal any potential performance advantages.

Therefore, while JOIN and WHERE clauses for similar queries are typically considered equivalent, it's always advisable to test them on the target database system to identify any database-specific optimizations or unexpected behaviors.

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