To write bulk safe triggers we need to understand and utilize sets and maps. Sets are used to isolate distinct records, while maps are name-value pairs that hold the query results retrievable by record id. for more details refer the Apex developer guide.
Example:
======
trigger ChangeColor on Account (before insert, before update)
{
// create a set of all the unique ownerIds
Set<id> ownerIds = new Set<id>( );
for (Account a : Trigger.new)
{
ownerIds.add(a.OwnerId);
}
// query for all the User records for the unique userIds
Map<id, User> owners = new Map<id, User>([Select Favorite_Color__c from User Where Id in
:ownerIds]);
// iterate over the list of records being processed
for (Account a : Trigger.new)
{
a.Owner_Favorite_Color__c = owners.get(a.OwnerId).Favorite_Color__c;
}
}
