Discuss INNER JOIN ON vs WHERE clause (with multiple FROM tables)

When discussing the use of INNER JOIN ON versus the WHERE clause in SQL, particularly when dealing with multiple tables, it's important to understand both the functional and performance implications. Here’s a detailed explanation:

Functional Differences

INNER JOIN ON Clause

• Purpose: The ON clause is used to specify the condition for joining tables. It defines how rows from one table are matched with rows from another table.
• Syntax:
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  SELECT columns
  FROM table1
  INNER JOIN table2 ON table1.column = table2.column;

• Example:
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  SELECT employees.name, departments.name
  FROM employees
  INNER JOIN departments ON employees.department_id = departments.id;

• Behavior: The ON clause filters rows before the join operation is completed. This means that only rows that meet the join condition are included in the result set.

WHERE Clause

• Purpose: The WHERE clause is used to filter rows after the tables have been joined. It can be used to apply additional conditions to the result set.
• Syntax:
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  SELECT columns
  FROM table1, table2
  WHERE table1.column = table2.column AND additional_conditions;

• Example:
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  SELECT employees.name, departments.name
  FROM employees, departments
  WHERE employees.department_id = departments.id AND departments.location = 'New York';

• Behavior: The WHERE clause filters rows after the join operation. This means that all possible combinations of rows are first created (a Cartesian product), and then the WHERE clause filters out the rows that do not meet the specified conditions.

Performance Considerations

Execution Plan

• Modern RDBMS: For modern relational database management systems (RDBMS), there is generally no difference in performance between using INNER JOIN ON and the WHERE clause for inner joins. The query optimizer is capable of recognizing that both approaches are algebraically equivalent and will generate the same execution plan[2][4][5][10][13].
• Example:
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  -- Using INNER JOIN ON
  SELECT * 
  FROM A 
  INNER JOIN B ON A.ID = B.ID 
  WHERE B.name = 'Hazard';
  
  -- Using WHERE clause
  SELECT * 
  FROM A, B 
  WHERE A.ID = B.ID AND B.name = 'Hazard';

  Both queries will typically produce the same execution plan and have similar performance.

Readability and Maintainability

• INNER JOIN ON: Using the ON clause for join conditions and the WHERE clause for filtering conditions makes the query more readable and maintainable. It clearly separates the logic for joining tables from the logic for filtering rows[3][4][5][6].
• WHERE Clause: Using the WHERE clause for both joining and filtering can make the query harder to read and maintain, especially as the number of tables and conditions increases[3][4][5].

Special Cases

• Outer Joins: For outer joins (LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN), the placement of conditions in the ON clause versus the WHERE clause can affect the res...