SQL Query Output

Argo Data processes check and payment items for banks. Here is a small slice of its account and transaction tables, already loaded with sample rows.

CREATE TABLE accounts (
    account_id     INT PRIMARY KEY,
    customer_name  TEXT,
    balance        NUMERIC
);

INSERT INTO accounts (account_id, customer_name, balance) VALUES
    (1, 'Alice', 500),
    (2, 'Bob', 1200),
    (3, 'Chris', 0);

CREATE TABLE transactions (
    txn_id      INT PRIMARY KEY,
    account_id  INT REFERENCES accounts(account_id),
    txn_type    TEXT,
    amount      NUMERIC
);

INSERT INTO transactions (txn_id, account_id, txn_type, amount) VALUES
    (101, 1, 'deposit', 150),
    (102, 1, 'withdrawal', -50),
    (103, 2, 'deposit', 300),
    (104, 3, 'deposit', NULL);

Now consider this query:

SELECT
    a.customer_name,
    COUNT(t.txn_id) AS txn_count,
    SUM(t.amount) AS total_amount
FROM accounts a
LEFT JOIN transactions t ON a.account_id = t.account_id
GROUP BY a.customer_name
ORDER BY total_amount DESC NULLS LAST;

Write out exactly what this query returns: every row, every column value, and the order the rows come back in. Pay close attention to what NULL does inside SUM.