Why Does Mixing Implicit and Explicit JOINs in SQL Result in Invalid Syntax?

SQL Syntax Error: Combining Implicit and Explicit JOINs

Using both implicit and explicit JOINs within a single SQL query often results in a syntax error. The following example illustrates this problem:

<code class="language-sql">SELECT e1.name, e2.name, e1Manager.name
FROM Employee e1, Employee e2
INNER JOIN Employee e1Manager
ON e1.managerEmployeeID = e1Manager.employeeID</code>

This query fails due to operator precedence. The JOIN keyword takes precedence over the comma, meaning the database attempts to process the JOIN before fully understanding the FROM clause. Consequently, e1 is referenced in the ON condition before it's defined, leading to an error.

Understanding the Precedence Issue

SQL standards dictate that JOIN operations have higher precedence than comma-separated table listings in the FROM clause. The query parser interprets the JOIN first, encountering e1 in the ON clause before its definition, causing the syntax error.

Controlling JOINs in Hibernate (Explicit JOINs)

Forcing Hibernate to exclusively use explicit JOINs depends on the Hibernate version. Older versions might not have fully supported explicit JOINs. However, newer versions likely provide this capability.

Currently, comprehensive Hibernate Query Language (HQL) documentation is unavailable due to the redirection of Hibernate.org to jboss.org. Once this issue is resolved, refer to the updated HQL documentation to determine explicit JOIN support in your specific Hibernate version.

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