Querying D1

First, create a D1 client using the builder:

D1 d1 = new D1.builder()
//    .withLogLevel(LogLevel.ALL) // Useful for debugging
    .withAccountId("...")
    .withDatabaseId("...")
    .withApiToken("...")
    .withApiUrl("https://...")    // If for some reason you have access to some private API, omit to use the public Cloudflare API.
    .build();

Then, using that d1 instance, use query(). Here, we'll retrieve the first result as a JsonObject:

System.out.println(
    d1
        .query("SELECT * FROM myTable")
        .first()
);

That prints: {"field":123123}.

You can also use WHERE and pass in parameters:

System.out.println(
    d1
        .query("SELECT * FROM myOtherTable WHERE id = ?", "someUniqueId")
        .first()
);

We can also just get the value of field directly:

System.out.println(
    d1
        .query("SELECT * FROM myTable")
        .first("field")
);

You can also convert the Json types into your own POJO:

d1
    .query("SELECT * FROM myTable")
    .first()
    .as(MyClass.class);

// Or, if you want an array...
d1
    .query("SELECT * FROM myTable")
    .all()
    .as(MyClass[].class);

// Or, if you want a collection...
d1
    .query("SELECT * FROM myTable")
    .all()
    .as(new TypeToken<List<MyClass>>() {});