Angularfire2 - Is possible to update the data of firebase list by query?

To update any of the quantity , you need the key of that particular item.

Say, the key of item1 in your example is '-KpZqM09AEDkhuIYAHSt' then in order to update the quantity of item1 you need the reference of

Project/items/-KpZqM09AEDkhuIYAHSt

So to update all those entry having quantity===0 to 30, you can do:

• Fetch all items using once() method of firebase with ref('items')

• iterate over the items using forEach and store the keys of all those items having quantity ===0 in an array. Use .key to get key of each item in items

  var keyArray = [];
  firebase.database().ref('items').once('value', data => {
      data.forEach(item => {
          if (item.val().qty === 0) {
              keyArray.push(item.key);
          }
      });
  });
  

• Once you get the keys, iterate over that key-array and for every key call firebase update() method with ref('items/'+keyOfItem)

  keyArray.forEach(keyOfItem => {
  firebase.database().ref('item/' + keyOfItem).update({
              quantity: 30
          });
  }
  //here keyOfItem is key of item having qty===0
  

  Don't forget that once() returns a promise which should be resolved first to get the second code working otherwise you will get keyArray as empty.
  Hope you find this helpful.