android - IN clause and placeholders

Question

I'm attempting to do the following SQL query within Android:

    String names = "'name1', 'name2";   // in the code this is dynamically generated

    String query = "SELECT * FROM table WHERE name IN (?)";
    Cursor cursor = mDb.rawQuery(query, new String[]{names});

However, Android does not replace the question mark with the correct values. I could do the following, however, this does not protect against SQL injection:

    String query = "SELECT * FROM table WHERE name IN (" + names + ")";
    Cursor cursor = mDb.rawQuery(query, null);

How can I get around this issue and be able to use the IN clause?

Answer 1

A string of the form "?, ?, ..., ?" can be a dynamically created string and safely put into the original SQL query (because it is a restricted form that does not contain external data) and then the placeholders can be used as normal.

Consider a function String makePlaceholders(int len) which returns len question-marks separated with commas, then:

String[] names = { "name1", "name2" }; // do whatever is needed first
String query = "SELECT * FROM table"
    + " WHERE name IN (" + makePlaceholders(names.length) + ")";
Cursor cursor = mDb.rawQuery(query, names);

Just make sure to pass exactly as many values as places. The default maximum limit of host parameters in SQLite is 999 - at least in a normal build, not sure about Android :)

Happy coding.

---

Here is one implementation:

String makePlaceholders(int len) {
    if (len < 1) {
        // It will lead to an invalid query anyway ..
        throw new RuntimeException("No placeholders");
    } else {
        StringBuilder sb = new StringBuilder(len * 2 - 1);
        sb.append("?");
        for (int i = 1; i < len; i++) {
            sb.append(",?");
        }
        return sb.toString();
    }
}