Question

What I want is if the result is empty, if both tables are empty I want to insert something into the database. The problem is with the current SQL-string

It should probably be something different than “SELECT * FROM incomplete, complete”
But what?

database.transaction(function(transaction){
    transaction.executeSql('SELECT * FROM incomplete, complete;', [], function(transaction, results) {
        var resultLength = results.rows.length;
        if(resultLength == 0) {
            transaction.executeSql('INSERT INTO complete (todo, description, date) VALUES (?, ?, ?);', ['Do Something', 'Description', date]);
        }
    });
});

Answer 1

Assuming standard SQL is allowed, you can execute the following statement:

SELECT * FROM complete UNION SELECT * FROM incomplete;

This only works if the structure of both tables is the same. And it is not a very efficient way to check whether data is in both tables.