It's most likely that you won't need to display a table inside another table, but there are cases when this workaround can be useful in obtaining a clear presentation of the data. For example, nested collections can be presented as nested tables as follows:

HashMap<Players, HashSet<Trophies>> dataHashMap = new HashMap<>();

Here, players are stored in HashMap as keys, and each player has a collection (HashSet) of trophies. Each HashSet value is a value in HashMap. Therefore, you need to display the table of players; however, you need to display each player's trophies. This can be achieved as shown in the following code:

<h:dataTable value="#{playersBean.dataHashMap.entrySet()}" var="t">
  <h:column>
    <f:facet name="header">Ranking</f:facet>
    #{t.key.ranking}
  </h:column>
  <h:column>
    <f:facet name="header">Name</f:facet>
    #{t.key.player}
  </h:column>
    ...
  <h:column>
    <f:facet...