SOQL

Get Ids

Use Map constructor Map<ID,sObject>(recordList) to get Ids from List of SObjects.

❌

Set<Id> accountIds = new Set<Id>();

for (Account acc : [SELECT Id FROM Account]) {
    accountIds.add(acc.Id);
}

List<Account> accounts = [SELECT Id FROM Account];

Set<Id> accountIds = new Set<Id>();

for (Account acc : accounts) {
    accountIds.add(acc.Id);
}

✅

Set<Id> accountIds = new Map<Id, Account>([SELECT Id FROM Account]).keySet();
List<Id> accountIds = new List<Id>(new Map<Id, Account>([SELECT Id FROM Account]).keySet());