Wrong number of statement arguments with "like '%?%'"

[Timac]

What did you do?

I am trying to perform some SQLite queries using the like operator.
For example I would like to delete all rows from mytable where the title column contains the string "something":

do {
	try databasePool.write { db in
		try db.execute(sql: "DELETE FROM mytable where title like '%?%'", arguments: ["something"])
	}
} catch let error as DatabaseError {
	debugPrint("Failed with error \(error)")
} catch {
	debugPrint("Failed with unknown error")
}

What did you expect to happen?

The SQLite query looks valid. I would expect the query to delete all rows from mytable where the substring is found.

What happened instead?

I see instead an error SQLite error 21: wrong number of statement arguments: 1
Note that if I use instead the following, I don't see the error and the expected rows are deleted:

try db.execute(sql: "DELETE FROM mytable where title like '%\(something)%'")

A similar issue occurs when using SELECT:

try db.execute(sql: "SELECT * FROM mytable where title like '%?%'", arguments: ["something"])

Environment

GRDB flavor(s): GRDB
GRDB version: 4.0 (revision 0e2daff)
Installation method: manual
Xcode version: 10.2.1
Swift version: 5
Platform(s) running GRDB: iOS
macOS version running Xcode: 10.14.4

[groue]

Please see Nike-Inc/SQift#9 for an identical issue. It is a misunderstanding of what SQLite parameters are.

[Timac]

@groue Thanks a lot for the clarification!
Since I am not the first one asking this question and probably not the last one, I added a paragraph to the README to clarify this point.

See #531

[groue]

That's lovely, @Timac! You're right, this is a FAQ :-) I'll soon look into your PR.

[sobri909]

It also works if you put the percent marks in the argument.

let notes = Stalker.store.notes(
    where: "body LIKE ? AND deleted = 0 ORDER BY date DESC",
    arguments: ["%\(query)%"])