SQL compilation error: unsupported subquery type cannot be evaluated

This error message generally indicates that you are using a subquery that is not supported by your database engine or you are using it in an unsupported context. It could be due to the database version or the SQL dialect you’re using. Here are some possible reasons and their solutions.

Reason 1: Correlated subqueries not supported

Some databases do not support correlated subqueries, or they might have limitations. Here’s an example of a correlated subquery:

SELECT e1.*
FROM employees e1
WHERE salary > (
    SELECT AVG(e2.salary)
    FROM employees e2
    WHERE e2.department_id = e1.department_id
);

Solution: Use a derived table or join instead

SELECT e.*
FROM employees e
JOIN (
    SELECT department_id, AVG(salary) AS avg_salary
    FROM employees
    GROUP BY department_id
) AS dept_avg ON e.department_id = dept_avg.department_id
WHERE e.salary > dept_avg.avg_salary;

Reason 2: Subquery in the SELECT clause

Some databases do not support subqueries in the SELECT clause. Here’s an example:

SELECT e.employee_id, e.first_name,
    (SELECT d.department_name
     FROM departments d
     WHERE d.department_id = e.department_id) AS department_name
FROM employees e;

Solution: Use a JOIN instead

SELECT e.employee_id, e.first_name, d.department_name
FROM employees e
JOIN departments d ON e.department_id = d.department_id;

Reason 3: Nested subqueries too deep

Some databases have limitations on the depth of nested subqueries. Here’s an example with 3 levels of nesting:

SELECT *
FROM employees
WHERE department_id IN (
    SELECT department_id
    FROM departments
    WHERE location_id IN (
        SELECT location_id
        FROM locations
        WHERE country_id = 'US'
    )
);

Solution: Break down the query into separate queries or use JOINs

SELECT e.*
FROM employees e
JOIN departments d ON e.department_id = d.department_id
JOIN locations l ON d.location_id = l.location_id
WHERE l.country_id = 'US';

In order to find the most suitable solution, it is important to understand the SQL dialect and version you are using, as well as any limitations it might have. Check the documentation for your specific database engine to find out more about supported subquery types and how to use them.