Retrieve count data from Firebase like MySQL

问题

When trying to pull data from a MySQL database, I can do something like:

SELECT * FROM users ORDER BY id WHERE vehicle = car

That should get me all the users that drives a car and not show users that drives a motorcycle for instance.

Is there something like this for Firebase? I can only retrieve specific data from one user?

My firebase database is like this: user -> user info (name, age, vehicle etc..)

I want to query every user that drives a car and display them in a row. How do I do that?

---

I have tried the following, but I didn't succeed with what I tried to do, since after users the users id is the next child. Is there a way to query past that?

var recents = firebase.database().child('users').orderByChild('department').equalTo(department);
            recents.on('child_added', function(snapshot) {
            var countOfUserInDepartment = snapshot.count;
            
                document.querySelector("#cphCount").innerHTML = countOfUserInDepartment;
            
            });

回答1:

There are no count queries (nor other aggregation queries) in the Firebase Database. Your options are:

1. Retrieve all data matching your query and count client-side
2. Keep a separate count-node that you update whenever you add/remove items.

For #2 you may find it convenient to use Cloud Functions, for which there an an example of keeping such a counter.

Also see:

• Firebase count group by
• Database-style Queries with Firebase
• In Firebase, is there a way to get the number of children of a node without loading all the node data?
• How to get size of an element/list in Firebase without get it all?

来源：https://stackoverflow.com/questions/46367231/retrieve-count-data-from-firebase-like-mysql