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Finding NULL Values

You can use a WHERE clause to filter values by whether or not they're NULL.

IS NULL

SELECT name FROM users WHERE first_name IS NULL;

IS NOT NULL

SELECT name FROM users WHERE first_name IS NOT NULL;

Assignment

The way we store transactions at CashPal is interesting. The user identified by user_id is the "owner" of each transaction. Because user_id already identifies the owner, only the user on the other side needs another ID:

If the owner receives money, sender_id identifies the sender and recipient_id is NULL.
If the owner sends money, recipient_id identifies the recipient and sender_id is NULL.

From the transactions table, select all columns for transactions where the owner is receiving money.