Get list of one column SOQL

How to Obtain Unique Field Values Without Using a For Loop?

The simplest way to retrieve record IDs without a for loop is by using a Map and the keySet() method.

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

But how can you obtain values for other fields?

You can accomplish this using field aliasing.

As shown on previous slide, a Map will automatically use the Id as a Map key.

Set<String> industries = new Set<String>();

for (Account acc : [SELECT Industry FROM Account]) {
    industries.add(acc.Industry);
}

Set<String> industries = new Map<String, SObject>([
    SELECT Industry Id
    FROM Account
    WHERE Industry != NULL
    GROUP BY Industry
]).keySet();

To avoid Row with null Id at index: 0 always add null check (!= NULL)