No Such Column in Correlated Subquery - sql

Question

So I wrote a query pretty much the same as the one described in this answer. However I'm getting an error stating that: No such column b.when.
My select statement:
SELECT
mileage,
(SELECT b.mileage FROM MileageEvents as b WHERE `b.when` < `a.when` ORDER BY `b.when` DESC LIMIT 1) as last_mileage,
gallons,
cost_per_gallon,
`when`
FROM MileageEvents as a
I know I've written such queries previously, but I can't seem to figure out what is going on with this query. What am I doing wrong?
A dump of my database:
PRAGMA foreign_keys=OFF;
BEGIN TRANSACTION;
CREATE TABLE IF NOT EXISTS "MileageEvents" (
"mileage" INTEGER,
"when" TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
"cost_per_gallon" INTEGER,
"gallons" INTEGER,
"id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
"car_id" INTEGER NOT NULL
);
INSERT INTO MileageEvents VALUES(150000,'2019-03-12',3.3500000000000000888,12,1,1);
INSERT INTO MileageEvents VALUES(150300,'2019-03-19',3.25,12,2,1);
INSERT INTO MileageEvents VALUES(150693,'2019-03-22',3.4500000000000001776,12,3,1);
INSERT INTO MileageEvents VALUES(151000,'2019-03-25',3.3900000000000001243,12,4,1);
INSERT INTO MileageEvents VALUES(151600,'2019-04-01',2.25,12,5,1);
INSERT INTO MileageEvents VALUES(151883,'2019-06-10 23:01:43',2.4500000000000001776,11.695999999999999729,6,1);
DELETE FROM sqlite_sequence;
INSERT INTO sqlite_sequence VALUES('MileageEvents',6);
COMMIT;

Answer 1

(First, I'd use normal double quote escaping instead of MySQL style backticks, or better yet, an identifier that's not a reserved keyword)
You need to use b."when". What you're doing treats the entire string b.when as an identifier, with no split between table name and column name.
Rewritten:
SELECT
mileage,
(SELECT b.mileage FROM MileageEvents as b WHERE b."when" < a."when" ORDER BY b."when" DESC LIMIT 1) as last_mileage,
gallons,
cost_per_gallon,
"when"
FROM MileageEvents as a;
Note: if using sqlite 3.25 or newer, you can avoid the subquery completely:
SELECT
mileage,
lag(mileage) OVER (ORDER BY "when") AS last_mileage,
gallons,
cost_per_gallon,
"when"
FROM MileageEvents;