The Typical Transaction
The fraud model at Northline Bank needs a baseline for how big a typical movement is, so it can notice when one is not. Averaging the ledger as stored would let deposits and withdrawals cancel each other out and produce a "typical" figure far smaller than anything that actually happens.
transactions — one row per movement of money. customer_id points at a row in customers; type is one of deposit, withdrawal or transfer; txn_date is the day it settled. The ledger is signed: money arriving in an account is stored as a positive amount, and money leaving it as a negative one, so a 600-dollar withdrawal is recorded as -600.
| id | customer_id | amount | type | txn_date |
|---|---|---|---|---|
| 1 | 1 | 500 | deposit | 2023-07-01 |
| 2 | 1 | -120 | withdrawal | 2023-07-03 |
| 3 | 2 | 1000 | deposit | 2023-07-02 |
| 4 | 2 | -300 | withdrawal | 2023-07-05 |
| 5 | 3 | 250 | deposit | 2023-07-04 |
| 6 | 3 | -600 | withdrawal | 2023-07-06 |
| 7 | 4 | 800 | deposit | 2023-07-07 |
| 8 | 4 | -450 | withdrawal | 2023-07-08 |
| 9 | 5 | 2000 | deposit | 2023-07-09 |
| 10 | 5 | -1500 | withdrawal | 2023-07-10 |
| 11 | 1 | -200 | transfer | 2023-07-11 |
| 12 | 3 | 400 | deposit | 2023-07-12 |
The table already exists in the database — there is nothing to create or load.
Task: Write a query that returns exactly one row with one column, avg_size, holding the mean amount of money moved per transaction ignoring direction, rounded to 2 decimal places.
Example output — the shape, on an invented figure. A ledger averaging nine hundred and thirteen dollars forty-five cents a movement reads:
| avg_size |
|---|
| 913.45 |
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