Left JOIN vs. Multiple Tables in FROM Clause: Which SQL Join Method is Better?

Comparison of SQL JOIN methods: LEFT JOIN and multiple tables listed in the FROM clause

Most SQL dialects support both methods, but it's important to understand the pros and cons of each method when using database connections. This article explores the potential benefits and drawbacks of using LEFT JOIN syntax in inner joins versus listing multiple tables in the FROM clause.

Gradual elimination of multi-table methods

The traditional syntax (simply listing the tables in the FROM clause and using the WHERE clause to establish the join conditions) is being phased out in favor of the LEFT JOIN syntax. This is not just an aesthetic choice, it also helps alleviate potential ambiguity when combining inner and outer joins in a single query.

Ambiguity and join order dependence

Consider the following scenario: When querying three interrelated tables (company, department, and employee), ambiguity occurs using the old syntax. Let's say the goal is to list all companies with their departments and employees, where departments must have employees to be included, but companies without departments must still show up.

Use old syntax:

<code class="language-sql">SELECT * -- 为简单起见
FROM Company, Department, Employee
WHERE Company.ID *= Department.CompanyID
  AND Department.ID = Employee.DepartmentID</code>

The LEFT JOIN syntax, on the other hand, offers greater flexibility:

<code class="language-sql">SELECT *
FROM Company
     LEFT JOIN (
         Department INNER JOIN Employee ON Department.ID = Employee.DepartmentID
     ) ON Company.ID = Department.CompanyID</code>

This syntax explicitly specifies that the department-employee join should be performed first, and then the results should be left-joined with company.

Filter the join column

Another advantage of the LEFT JOIN syntax is its ability to filter on the join column without affecting the parent row. For example, if you need departments whose names contain the letter "X", the LEFT JOIN syntax can achieve this:

<code class="language-sql">SELECT *
FROM Company
     LEFT JOIN (
         Department INNER JOIN Employee ON Department.ID = Employee.DepartmentID
     ) ON Company.ID = Department.CompanyID AND Department.Name LIKE '%X%'</code>

Conclusion

The LEFT JOIN syntax provides a greater degree of clarity and control over the join operation than listing multiple tables in the FROM clause for an inner join. It removes ambiguity, allows more flexible filtering, and is less susceptible to order dependency issues. Therefore, the LEFT JOIN syntax is becoming the preferred method for performing efficient and unambiguous database joins.

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