mongo/jstests/core/null.js

t = db.null1;
t.drop();

t.save({x: 1});
t.save({x: null});

assert.eq(1, t.find({x: null}).count(), "A");
assert.eq(1, t.find({x: {$ne: null}}).count(), "B");

t.ensureIndex({x: 1});

assert.eq(1, t.find({x: null}).count(), "C");
assert.eq(1, t.find({x: {$ne: null}}).count(), "D");

// -----

assert.eq(2, t.find({y: null}).count(), "E");

t.ensureIndex({y: 1});
assert.eq(2, t.find({y: null}).count(), "E");

t.dropIndex({y: 1});

t.ensureIndex({y: 1}, {sparse: true});
assert.eq(2, t.find({y: null}).count(), "E");