SQL Joins: Left Join vs. Multiple Tables on the FROM Clause – Which is Better?

SQL left join and multiple tables in FROM clause: how to choose?

In SQL, inner joins can be specified using multiple tables listed in the FROM line or left join syntax. While both produce the same results for simple joins, there are significant differences in their behavior when handling ambiguous queries.

Priority and execution order

The main difference is the priority of the connection. In multi-table syntax, all tables are listed in the FROM row without specifying join conditions. This means that the order of execution is determined by the database engine, which can lead to unexpected results.

In contrast, left join syntax clearly defines the join conditions between tables. By using parentheses or nested connections, you can control connection priority and ensure predictable execution.

Ambiguity and data loss

Multi-table syntax is prone to ambiguity when combining inner joins and outer joins. Consider an example where you want to list companies, departments, and employees, including companies without departments. Using multi-table syntax and an inner join with Department.ID = Employee.DepartmentID may result in missing company data without departments.

The left join syntax allows you to specify the priority of the join, ensuring that inner joins are performed first and only departments with employees are returned. By left joining the result with the companies table you can keep all company data even if they don't have departments.

Performance and Optimization

Left join syntax provides better performance optimization opportunities. When join conditions are explicitly specified, the query optimizer can more easily determine the optimal execution plan. This optimization usually results in faster query execution times.

Modernization and Deprecation

Many modern database vendors have deprecated the multi-table syntax for joins, especially for external joins. Left join syntax is preferred due to its clarity, consistency, and improved performance. By adopting left join syntax, you ensure compatibility with modern database systems and avoid potential data ambiguity or loss issues.

So while the multi-table syntax may still be supported in legacy environments, the left join syntax is the recommended and future-proof way to handle joins in SQL. It provides better query performance, reduces ambiguity, and ensures consistent data retrieval across different database engines.

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