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SUM
The SUM aggregation function returns the sum of a set of values.
For example, the query below returns a single record containing a single field. The returned value is equal to the total salary being collected by all of the employees in the employees table.
SELECT SUM(salary)
FROM employees;
Which returns:
| SUM(SALARY) |
|---|
| 2483 |
Assignment
We need to be able to calculate the current balance for a given user because we don't (yet) store the running balance on each individual transaction record.
Write a query that returns the SUM aggregation of the amounts for all of Bob's successful transactions (user_id is 9). Check the file 001_up.sql for more details about the table.