在PostgreSQL中使用CASE选择不同的FROMs

SELECT column1, column2, ...
FROM
    (SELECT column1, column2, ...
    FROM table1
    WHERE condition1
    ) AS from1
JOIN
    (SELECT column1, column2, ...
    FROM table2
    WHERE condition2
    ) AS from2
ON condition
WHERE CASE
    WHEN condition3 THEN from1
    WHEN condition4 THEN from2
    ELSE from3
END;

SELECT *
FROM
    (SELECT employee_id, name, position
    FROM employees
    WHERE department = 'IT'
    ) AS employees
LEFT JOIN
    (SELECT employee_id, manager_id, department
    FROM managers
    WHERE department = 'IT'
    ) AS managers
ON employees.employee_id = managers.employee_id
WHERE CASE
    WHEN position = 'Manager' THEN managers.department = 'IT'
    ELSE TRUE
END;