Create composed unique key field with trigger after insert in SQLITE - sqlite

Question

I have a table with multiple columns and one (unique key) should be a value composed from the values of other two columns.
CREATE TABLE batches (
id TEXT PRIMARY KEY UNIQUE,
name TEXT NOT NULL,
project_id INTEGER);
On each insert, I want to generate the id based on the value of 'name' and 'project_id' (this one can be null):
INSERT INTO batches (name,project_id) VALUES
('21.01',NULL),
('21.01',1),
('21.02',2);
So, I have created a table TRIGGER but doesn't execute.
CREATE TRIGGER create_batches_id
AFTER INSERT ON batches FOR EACH ROW
BEGIN
UPDATE batches
SET id = SELECT quote(name ||"_"|| (CASE project_id
WHEN NULL THEN '' ELSE project_id END )
FROM batches WHERE rowid = (SELECT MAX(rowid) FROM batches))
WHERE rowid = (SELECT MAX(rowid) FROM batches);
END;
Error:
SQL Error [1]: [SQLITE_ERROR] SQL error or missing database (near "SELECT": syntax error)
I expect:
id = 21.01_
id = 21.01_1
id = 21.01_2
What am I doing wrong? If I run only the SELECT/CASE statment it returns ok: '21.01_2'
I have also tried without the quote() function, no success.
UPDATE I:
I have managed to execute the whole create trigger statement (parenthesis were missing):
CREATE TRIGGER create_batch_id
AFTER INSERT ON batches FOR EACH ROW
BEGIN
UPDATE batches
SET id = (SELECT name ||"_"|| (CASE project_id WHEN NULL THEN 0 ELSE project_id END ) FROM batches WHERE rowid = (SELECT MAX(rowid) FROM batches) )
WHERE rowid = (SELECT MAX(rowid) FROM batches);
END;
It seems my editor (DBeaver) has a glitch with the following new line character. If it is inside the selection it runs into this exception (or I am missing something):
SQL Error [1]: [SQLITE_ERROR] SQL error or missing database (incomplete input)
If I manually select only the above lines (from CREATE to ;), the trigger is created, however, not the expected result. If value in project_id is NULL, no id value is created.

Answer 1

Don't add the column id in the table.
Instead define the combination of name and project_id as the PRIMARY KEY of the table, so that it is also UNIQUE:
CREATE TABLE batches (
name TEXT NOT NULL,
project_id INTEGER,
PRIMARY KEY(name, project_id)
)
Then, whenever you need that id you can run a query:
SELECT name || '_' || COALESCE(project_id, '') AS id,
name,
project_id
FROM batches
Or create a view:
CREATE VIEW v_batches AS
SELECT name || '_' || COALESCE(project_id, '') AS id,
name,
project_id
FROM batches
and query the view:
SELECT * FROM v_batches
See the demo.
Or if your version of SQLite is 3.31.0+ you can have the column id as a generated column:
CREATE TABLE batches (
name TEXT NOT NULL,
project_id INTEGER,
id TEXT GENERATED ALWAYS AS (name || '_' || COALESCE(project_id, '')),
PRIMARY KEY(name, project_id)
);