Which is Better: Multiple JOIN Clauses or an OR Condition in the ON Clause for Duplicate Table Lookups?

Optimizing Table Joins for Duplicate Lookups

When joining the same table twice, there are generally two approaches: using multiple JOIN clauses or incorporating an OR condition in the ON clause.

Multiple JOIN Clauses (Method 1):

As demonstrated in your example, this method involves creating a separate JOIN for each matching condition:

SELECT t1.PhoneNumber1, t1.PhoneNumber2, 
   t2.SomeOtherFieldForPhone1, t3.someOtherFieldForPhone2
FROM Table1 t1
INNER JOIN Table2 t2
   ON t2.PhoneNumber = t1.PhoneNumber1
INNER JOIN Table2 t3
   ON t3.PhoneNumber = t1.PhoneNumber2

This approach is straightforward and ensures that the resulting data contains all the required columns.

OR Condition in ON Clause (Method 2):

This method attempts to simplify the query by combining the JOIN conditions into one:

SELECT ...
FROM Table1
INNER JOIN Table2 
   ON Table1.PhoneNumber1 = Table2.PhoneNumber OR
      Table1.PhoneNumber2 = Table2.PhoneNumber

However, this approach can lead to duplicate rows in the result set if both matching conditions are met for a given row.

Best Practice:

The preferred method is Method 1 with multiple JOIN clauses. It provides a more explicit and reliable way to join the table based on specific criteria while avoiding data duplication.

Avoiding Redundant Joins:

It's important to avoid redundant joins by using appropriate aliases and naming conventions. For instance, the following query can be simplified:

SELECT t.PhoneNumber1, t.PhoneNumber2, 
   t1.SomeOtherFieldForPhone1, t1.someOtherFieldForPhone1
FROM Table1 t
JOIN Table2 t1 ON t1.PhoneNumber = t.PhoneNumber1
JOIN Table2 t2 ON t2.PhoneNumber = t.PhoneNumber2

By aliasing the joined tables (e.g., t1 and t2), you can reduce redundancy and make the query more readable.

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