Left Join vs. Multiple Tables in FROM Clause: When Should I Use the Modern SQL Syntax?

Modern SQL syntax: LEFT JOIN and multiple table selection in the FROM clause

SQL provides two different table connection syntax: (1) List all tables in the FROM clause and use the WHERE clause to specify the connection conditions; (2) Use LEFT JOIN and INNER JOIN syntax. While most SQL dialects accept both syntaxes, the latter is preferable, especially when performing inner joins.

Traditional syntax: Ambiguity of outer joins

The traditional syntax (listing tables in the FROM clause) is prone to ambiguity when using both INNER JOIN and OUTER JOIN in the same query. 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 retrieve companies and their departments and employees, including companies without departments. However, ambiguity occurs when the WHERE clause specifies both inner and outer joins. Depending on the query optimizer logic and database engine settings, the query may exclude companies with missing departments, resulting in incorrect results.

Modern Grammar: Clarity and Control

The modern syntax using LEFT JOIN and INNER JOIN removes this ambiguity. It provides explicit control over how tables are matched by specifying the join condition in one line. In the above example, using modern syntax solves this problem:

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

In the query above:

• The INNER JOIN between Department and Employee ensures that only departments with employees are retrieved.
• A LEFT JOIN between Company and inner join results allows companies without departments to be included in the final output.

Other advantages

In addition to disambiguation, modern syntax has other advantages:

• Improved query plan generation: The query optimizer has a clearer understanding of join operations, allowing it to create more efficient query plans.
• Easier to read: Modern syntax is more intuitive and easier to understand, especially for complex queries.
• Compatibility: Modern syntax is supported in most modern databases, ensuring compatibility across multiple platforms.

Deprecation of traditional syntax

Due to the advantages of modern syntax, some vendors (such as Microsoft) have deprecated traditional outer join syntax (using the WHERE clause) in newer versions of their database software.

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