通过字段组合将最后/最新记录留在表中

Question

如何为每个resource_owner_id和范围组合保留一个记录（最后/最新），并删除其他记录？

例如，对于resource_owner_id = 3和scopes = driver，它应该仅记录id为1357的记录。resource_owner_id= 5和scopes = driver-ID为1034的记录

我知道我可以使用function fb_signup(){ firebase.auth().createUserWithEmailAndPassword(email, password) .then(function success(userData){ var uid = userData.uid; var displayName = userData.displayName; var email = userData.email; var emailVerified = userData.emailVerified; var photoURL = userData.photoURL; var isAnonymous = userData.isAnonymous; var providerData = userData.providerData; $('#fbFormMessage').css({'display':'block','background-color':'#c9e9cc','color':'#347f3b'}); document.getElementById("fbFormMessage").innerHTML = 'Welcome Your registration was successful.'; console.log(uid + ' | ' + email + ' | ' + uName ) saveNewUser(uid, email, uName); return false; }) .catch(function failure(error) { var errorCode = error.code; var errorMessage = error.message; console.log(errorCode + " " + errorMessage); $('#fbFormMessage').css({'display':'block','background-color':'#FFEBEE','color':'#EF5350'}); document.getElementById("fbFormMessage").innerHTML = 'The email address is already in use by another account'; }); }; function saveNewUser(a,b,c){ console.log('4') var rootRef = firebase.database().ref(); var storesRef = rootRef.child('/users/'); var newStoreRef = storesRef.push(); newStoreRef.set({ uid: a, userName: c, email: b, }); }; ，然后按ID和范围（作用域为枚举-'driver'和'passenger'）获取所有记录，之后使用.ids并删除所有剩余的记录记录。

也许有一个更优雅的解决方案？

Answer 1

SQL Fiddle

ActiveRecord版本：

Model.select("DISTINCT ON(resource_owner_id, scopes) *")
     .order("resource_owner_id, scopes, id")

详细了解DISTINCT ON here。