SQL LEFT JOIN vs. Multiple Tables in FROM Clause: Which is Better?

SQL JOIN syntax: LEFT JOIN and multi-table joins in the FROM clause

When performing an inner SQL join, you have two options: the old syntax, which lists the tables in the FROM clause and specifies the join conditions in the WHERE clause, or the newer syntax, which uses it directly in the FROM clause LEFT JOIN. Although both syntaxes can produce the same results, there are many reasons why using the newer syntax is preferable.

Ambiguity of old syntax

The old syntax was ambiguous in queries that combined INNER JOIN and OUTER JOIN. Depending on the query optimizer's strategy, the order in which joins are performed affects the results. For example, a LEFT JOIN followed by an INNER JOIN may exclude matching rows that do not meet the inner join criteria, while the reverse order may include unmatched rows.

Clarity and Explicitness

New syntax using LEFT JOIN provides greater clarity and unambiguity. It specifies logical relationships between tables, allowing the query optimizer to handle joins efficiently. By separating the join condition from the row filter condition, the LEFT JOIN clause ensures that all rows from the left table are included regardless of whether matching rows exist in other tables.

Performance improvements

By using LEFT JOIN in the FROM clause, the query optimizer can optimize the join operation independently of row filters. This can improve performance by reducing the number of intermediate join results and minimizing the impact of WHERE clause filtering on join execution.

Consistency and standardization

The new syntax is more consistent across different database vendors, especially Microsoft SQL Server, which deprecated the old outer join syntax but not the inner join syntax. This helps ensure query compatibility and portability between databases.

Example

Suppose you have three tables: Company, Department, and Employee, with a hierarchical relationship between them. To retrieve all companies, departments and employees, and filter departments with at least one employee, you can use the following query:

<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 query specifies a LEFT JOIN between Company and the inner joined Department and Employee tables. It ensures that all companies are included regardless of whether they have departments or employees. Rows for departments or employees that do not match will have NULL values ​​in the corresponding columns.

Conclusion

While both the old and new syntax for inner joins in SQL can produce the same results, using a LEFT JOIN in the FROM clause is the preferred approach. It provides clarity, unambiguity, improved performance, consistency across database vendors, and greater flexibility in specifying join conditions and row filtering conditions.

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