Why Do Explicit and Implicit JOINs Cause SQL Errors?

SQL Errors from Combining Explicit and Implicit Joins

Combining explicit and implicit JOINs in a single SQL query can lead to errors. 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 produces errors in both MSSQL 2000/2008 and MySQL:

• MSSQL 2000: "The column prefix 'e1' does not match with a table name or alias name used in the query."
• MySQL: "Unknown column 'e1.managerEmployeeID' in 'on clause'."

Understanding the Syntax Issue

The error stems from a precedence conflict. In SQL, the JOIN keyword takes precedence over the comma-separated table listing in the FROM clause. The alias e1 isn't recognized within the ON clause of the JOIN because the parser hasn't yet processed the FROM clause completely. Essentially, e1 is undefined at the point it's referenced in the JOIN condition.

Hibernate and Join Syntax

There's a known issue in Hibernate concerning the generation of incorrect SQL when mixing explicit and implicit JOINs. While forcing Hibernate to exclusively use explicit JOINs is a potential workaround, specific details on achieving this are currently unavailable due to limitations in accessible HQL documentation.

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