Why Does Mixing Implicit and Explicit Joins in SQL Cause Syntax Errors?

The reason why mixing implicit and explicit joins in SQL leads to syntax errors

In database programming, using both implicit and explicit joins can cause problems. This article provides an in-depth look at SQL syntax exceptions caused by incorrectly mixing these connection types.

Example schema and data

Consider the following simplified pattern:

<code class="language-sql">CREATE TABLE Employee (
    employeeID INT,
    name VARCHAR(255),
    managerEmployeeID INT   
);

INSERT INTO Employee (employeeID, name) VALUES (1, 'Gary');
INSERT INTO Employee (employeeID, name, managerEmployeeID) VALUES (2, 'Bob', 1);</code>

Valid query

Using explicit or implicit joins respectively will generate valid SQL statements:

<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>

Invalid query

However, mixing explicit and implicit joins generates errors in SQL Server and MySQL:

<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>

Root cause of invalid syntax

This syntax error is caused by the JOIN keyword taking precedence over commas in SQL. Therefore, the JOIN expression is processed before the table reference (alias) is resolved. As a result, the reference to e1 in the JOIN condition (e1.managerEmployeeID) is invalid because e1 has not been defined in the preceding comma-separated table reference.

Is there a solution?

Regarding the additional question, it is not clear from the information provided whether it is possible to force Hibernate to only use explicit connections. Further research is needed to explore this possibility.

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