Exercise 7.7. Days between one customer's visits, with LAG().   [8 row(s)]
id    | day        | days_since_previous
------+------------+--------------------
B0013 | 2025-10-03 | NULL
B0029 | 2025-10-06 | 3.0
B0076 | 2025-10-15 | 9.0
B0097 | 2025-10-18 | 3.5
B0113 | 2025-10-21 | 2.5
B0140 | 2025-10-26 | 5.0
B0190 | 2025-11-03 | 8.4
B0208 | 2025-11-06 | 2.5

