SELECT * FROM employees WHERE salary = NULL;

WITH RECURSIVE NumberSeries AS 
( SELECT 1 AS number 
UNION ALL 
SELECT number + 1 FROM NumberSeries 
WHERE number < 5 ) 
SELECT * FROM NumberSeries;

SELECT * FROM employees; 

SELECT id, name, salary FROM employees;  

SELECT * FROM employees 
      WHERE salary IN (NULL,       
        50000, 60000);

SELECT t1.id, 
       (SELECT SUM(value) FROM transactions t2 WHERE t2.customer_id = t1.id AND t2.transaction_date <= t1.transaction_date) AS daily_balance
FROM transactions t1;

SELECT id, salary, 
       (SELECT COUNT(*) FROM employees e2 WHERE e2.salary >= e1.salary) AS rank
FROM employees e1;

 
SELECT id, salary, 
       (SELECT COUNT(*) FROM employees WHERE salary < e.salary) AS rank
FROM employees e;

SELECT id, salary, 
       (SELECT COUNT(*) FROM employees WHERE salary >= salary) AS rank
FROM employees;

SELECT id, salary, 
       (SELECT COUNT(*) FROM employees e2 WHERE e2.salary < e.salary) AS rank
FROM employees e;

SELECT e.id, e.name, 
       (SELECT AVG(salary) FROM employees e2 WHERE e2.department_id = e.department_id) AS avg_department_salary
FROM employees e;

Таблица t1

| a1      | 
|-------- | 
| 2       | 
| 4       | 
| 4       | 
| 5       |
| null    | 

Таблица t2
-----------
| a2      |
|---------|
| 2       |
| 2       | 
| 4       |
| 4       |
| 7       |
| null    |
| null    |