Why Does Mixing Implicit and Explicit JOINs in Hibernate Lead to SQL Errors?

Mixing implicit and explicit JOIN causes Hibernate SQL error

In database query languages, the JOIN statement is used to combine data from multiple tables based on one or more common columns. In Hibernate, a popular Java ORM framework, there are two types of JOINs: implicit and explicit.

Question

Mixing implicit and explicit JOINs in Hibernate can result in invalid SQL, throwing errors such as "Column prefix 'e1' does not match the table name or alias used in the query" in MS2000 and in MySQL "Unknown column 'e1.managerEmployeeID' in 'on clause'".

Reason

According to SQL standards, the JOIN keyword has a higher priority than comma. Therefore, in the following statement:

<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>

JOIN clauses are evaluated before table aliases (e1, e2). As a result, the reference to e1 in the ON expression is invalid because e1 has not been defined in the FROM clause.

Solution

To resolve this issue, make sure all JOINs use the same join syntax, either implicit or explicit.

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

-- 隐式JOIN:
SELECT e1.name,
       e2.name,
       e1Manager.name
  FROM Employee e1,
       Employee e2,
       Employee e1Manager
 WHERE e1.managerEmployeeID = e1Manager.employeeID</code>

Bonus Tip: Force Hibernate to use explicit JOIN

Unfortunately, there is no documented way to force Hibernate to only use explicit JOINs. However, you can try the following workarounds:

1. Create a CustomQueryInterceptor implementation.
2. In the onPrepareStatement() method, replace all implicit JOINs with explicit JOINs.

Please note that this workaround is not officially supported by Hibernate and may have unintended consequences.

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