SQL Joins: Why Use `LEFT JOIN` with Nested `INNER JOIN`s Instead of Multiple Tables in the `FROM` Clause?

SQL left join and multi-table join in FROM clause: Why the second syntax

is recommended

While the traditional SQL dialect allows joins both via WHERE clauses and LEFT JOIN, it is crucial to understand the advantages of using the LEFT JOIN syntax, especially when performing inner joins.

Avoid ambiguity

In modern databases, the traditional method of listing tables using WHERE clause conditions has been deprecated. This syntax can get confusing when combining inner and outer joins in the same query.

Example: Ambiguous Legacy Connection

Consider the following example:

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

This query is designed to list companies, their departments and employees. However, the query optimizer may choose to prioritize the inner join between the Department and Employee tables, causing companies without departments to be excluded.

Solution: LEFT JOIN syntax

The

LEFT JOIN syntax resolves this ambiguity by explicitly specifying the connection order. By placing the inner join in parentheses and performing a left join using the company table, the previous query becomes:

<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 ensures that all companies are included, even those without departments.

Extra control and flexibility

The

LEFT JOIN syntax also provides additional control over join conditions. For example, the following query filters for departments that contain the letter "X" in their name:

<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, along with inner joins, offers several advantages over the traditional method of joining tables on FROM rows. It removes ambiguity, provides greater control, and simplifies the query optimization process. Therefore, the use of LEFT JOIN and INNER JOIN is highly recommended in modern SQL applications.

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