```sql
SELECT name FROM users WHERE first_name IS NULL;
SELECT name FROM users WHERE first_name IS NOT NULL;
CREATE TABLE transactions (
id INTEGER PRIMARY KEY,
user_id INTEGER,
recipient_id INTEGER,
sender_id INTEGER,
amount INTEGER
);
INSERT INTO transactions(user_id, recipient_id, sender_id, amount)
VALUES (1, 2, NULL, -10);
INSERT INTO transactions(user_id, recipient_id, sender_id, amount)
VALUES (1, NULL, 2, 25);
INSERT INTO transactions(user_id, recipient_id, sender_id, amount)
VALUES (1, 5, NULL, -20);
INSERT INTO transactions(user_id, recipient_id, sender_id, amount)
VALUES (1, NULL, 3, 10);
SELECT * FROM transactions WHERE sender_id IS NOT NULL AND recipient_id IS NULL;
UPDATE employees
SET job_title = 'Backend Engineer', salary = 150000
WHERE id = 251;
UPDATE users
SET is_admin = TRUE
WHERE id = 9;
SELECT COUNT(*) FROM users WHERE country_code IS 'US';
ALTER TABLE posts
RENAME author_id TO poster_id;
SELECT *,
IIF(was_successful = TRUE, 'No action required', 'Perform an audit') AS audit
FROM transactions;
```