What's the rowid of the sqlite data base? - sqlite

Question

I am new to database and have a sqlite database like this:
CREATE TABLE IF NOT EXISTS Tiles (id INTEGER NOT NULL PRIMARY KEY, X INTEGER NOT NULL, Y INTEGER NOT NULL, Zoom INTEGER NOT NULL, Type INTEGER NOT NULL,Date TEXT)
CREATE TABLE IF NOT EXISTS TilesData (id INTEGER NOT NULL PRIMARY KEY CONSTRAINT fk_Tiles_id REFERENCES Tiles(id) ON DELETE CASCADE, Tile BLOB NULL)
If I call the following insert call for the first time:
INSERT INTO Tiles(X, Y, Zoom, Type,Date) VALUES(?, ?, ?, ?,?)
INSERT INTO TilesData(id, Tile) VALUES((SELECT last_insert_rowid()), ?)
What would be the id of Tiles and TilesData?
What's the function of SELECT last_insert_rowid() here?

Answer 1

I built your queries on both SQLFiddle (to easily show you the results) and in a local SQLite instance (to confirm that SQLite (SQL.js) behaves the same as a local instance) using the following script:
CREATE TABLE IF NOT EXISTS Tiles (id INTEGER NOT NULL PRIMARY KEY, X INTEGER NOT NULL, Y INTEGER NOT NULL, Zoom INTEGER NOT NULL, Type INTEGER NOT NULL,Date TEXT);
CREATE TABLE IF NOT EXISTS TilesData (id INTEGER NOT NULL PRIMARY KEY CONSTRAINT fk_Tiles_id REFERENCES Tiles(id) ON DELETE CASCADE, Tile BLOB NULL);
INSERT INTO Tiles(X, Y, Zoom, Type,Date) VALUES(1, 2, 3, 4, 'Date');
INSERT INTO TilesData(id, Tile) VALUES((SELECT last_insert_rowid()), 'Stuff');
select * from Tiles;
select * from TilesData;
In both instances, the id in both tables is 1.
The purpose of select last_insert_rowid() is to determine what the rowid is of the last successful insert. In your example, it is returning the value from the INSERT INTO Tiles query (1) and then assigning that to the id of TilesData.
We can demonstrate this works by also specifying the id in your first insert and seeing that the id of TilesData also matches the forced id.
INSERT INTO Tiles(id, X, Y, Zoom, Type,Date) VALUES(123456, 10, 2, 3, 4, 'Date');
INSERT INTO TilesData(id, Tile) VALUES((SELECT last_insert_rowid()), 'Stuff');
In this example, I'm forcing the id in the Tiles table to be 123456. This value is utilized as the id for the TilesData.
select * from TilesData;
Results in:
id | Tile
--------------
123456 | Stuff