Question

It seems like to combine two or more tables, we can either use join or where. What are the advantages of one over the other?

Answer 1

Any query involving more than one table requires some form of association to link the results from table “A” to table “B”. The traditional (ANSI-89) means of doing this is to:

1. List the tables involved in a comma separated list in the FROM clause

2. Write the association between the tables in the WHERE clause

   SELECT *
     FROM TABLE_A a,
          TABLE_B b
    WHERE a.id = b.id
   

Here’s the query re-written using ANSI-92 JOIN syntax:

SELECT *
  FROM TABLE_A a
  JOIN TABLE_B b ON b.id = a.id

From a Performance Perspective:

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Where supported (Oracle 9i+, PostgreSQL 7.2+, MySQL 3.23+, SQL Server 2000+), there is no performance benefit to using either syntax over the other. The optimizer sees them as the same query. But more complex queries can benefit from using ANSI-92 syntax:

• Ability to control JOIN order – the order which tables are scanned
• Ability to apply filter criteria on a table prior to joining

From a Maintenance Perspective:

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There are numerous reasons to use ANSI-92 JOIN syntax over ANSI-89:

• More readable, as the JOIN criteria is separate from the WHERE clause
• Less likely to miss JOIN criteria
• Consistent syntax support for JOIN types other than INNER, making queries easy to use on other databases
• WHERE clause only serves as filtration of the cartesian product of the tables joined

From a Design Perspective:

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ANSI-92 JOIN syntax is pattern, not anti-pattern:

• The purpose of the query is more obvious; the columns used by the application is clear
• It follows the modularity rule about using strict typing whenever possible. Explicit is almost universally better.

Conclusion

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Short of familiarity and/or comfort, I don’t see any benefit to continuing to use the ANSI-89 WHERE clause instead of the ANSI-92 JOIN syntax. Some might complain that ANSI-92 syntax is more verbose, but that’s what makes it explicit. The more explicit, the easier it is to understand and maintain.