Why Prime Faces Data Table sorting not working - primefaces

Question

I am new to prime faces and want to sort a data table using sort by and attributes but the table is not being sorted. I am using 5.3 PF.
<p:dataTable id="myTable"
styleClass="standardTable marginTopFive paddingBottom"
value="#{User.users}" paginator="true"
paginatorTemplate="{CurrentPageReport} {FirstPageLink} {PreviousPageLink} {PageLinks} {NextPageLink} {LastPageLink} {RowsPerPageDropdown}"
paginatorPosition="bottom" rows="10" var="user"
emptyMessage="Error"
sortBy="#{user.userId}" sortOrder="descending">
<f:facet name="spacer">
<f:verbatim>
</f:verbatim>
</f:facet>
<f:facet name="footer">
</f:facet>
<p:column width="8%">
<f:facet name="header">
</f:facet>
<h:column>
<h:selectBooleanCheckbox
value="#{user.checked[user.userId]}" />
</h:column>
</p:column>
<p:column width="20%" sortBy="#{mpc.name}">
<f:facet name="header">
<h:outputText
value="User Name" />
</f:facet>
<h:outputText value="#{user.user_name}" />
</p:column>
<p:column width="30%" sortBy="#{mpc.email}">
<f:facet name="header">
<h:outputText
value="Email" />
</f:facet>
<h:outputText value="#{user.user_email}" />
</p:column>
</p:dataTable>

Answer 1

You set the name of the iterator variable as var="user", but here <p:column sortBy="#{mpc.name}"/> you called it as mpc.
So you must replace <p:column sortBy="#{mpc.name}"/> with <p:column sortBy="#{user.user_name}"/> and <p:column sortBy="#{mpc.email}"> with <p:column sortBy="#{user.user_email}">