Hi,

You have to use apex:param tag to pass the parameter inside the controller. So once you have the value inside the controller you can make the query accordingly.

Try the below code as reference:
/////////////////////// VF Page //////////////////////////////////////////////////////////////

<apex:page controller="PageBlockTableSortingCon" tabStyle="Account">
<apex:sectionHeader title="Accounts List with Sorting"></apex:sectionHeader>
<apex:form >
<apex:pageBlock title="" id="pageBlock">
<apex:pageBlockButtons location="top">
<apex:commandButton value="View" action="{!ViewData}" id="theButton" rerender="pageBlock"></apex:commandButton>
</apex:pageBlockButtons>
<apex:pageMessages ></apex:pageMessages>
<apex:pageBlockTable value="{!accounts}" var="a" rendered="{!NOT(ISNULL(accounts))}">
<apex:column >
<apex:facet name="header">
<apex:commandLink action="{!ViewData}" value="Account Name{!IF(sortExpression=='name',IF(sortDirection='ASC','?','?'),'')}" id="cmdSort">
<apex:param value="name" name="column" assignTo="{!sortExpression}" ></apex:param>
</apex:commandLink>
</apex:facet>
<apex:outputLink value="/{!a.Id}" target="_blank">{!a.Name}</apex:outputLink>
</apex:column>
<apex:column value="{!a.Phone}">
<apex:facet name="header">
<apex:commandLink action="{!ViewData}" value="Phone{!IF(sortExpression=='Phone',IF(sortDirection='ASC','?','?'),'')}">
<apex:param value="Phone" name="column" assignTo="{!sortExpression}" ></apex:param>
</apex:commandLink>
</apex:facet>
</apex:column>
<apex:column value="{!a.BillingCity}">
<apex:facet name="header">
<apex:commandLink action="{!ViewData}" value="Billing City{!IF(sortExpression=='BillingCity',IF(sortDirection='ASC','?','?'),'')}">
<apex:param value="BillingCity" name="column" assignTo="{!sortExpression}" ></apex:param>
</apex:commandLink>
</apex:facet>
</apex:column>
<apex:column value="{!a.BillingCountry}">
<apex:facet name="header">
<apex:commandLink action="{!ViewData}" value="Billing Country{!IF(sortExpression=='BillingCountry',IF(sortDirection='ASC','?','?'),'')}">
<apex:param value="BillingCountry" name="column" assignTo="{!sortExpression}" ></apex:param>
</apex:commandLink>
</apex:facet>
</apex:column>

</apex:pageBlockTable>
</apex:pageBlock>
</apex:form>
</apex:page>
///////////////// Controller ////////////////////////////////////////////////
public class PageBlockTableSortingCon {

private List<Account> accounts;
private String sortDirection = 'ASC';
private String sortExp = 'name';

public String sortExpression
{
get
{
return sortExp;
}
set
{
//if the column is clicked on then switch between Ascending and Descending modes
if (value == sortExp)
sortDirection = (sortDirection == 'ASC')? 'DESC' : 'ASC';
else
sortDirection = 'ASC';
sortExp = value;
}
}

public String getSortDirection()
{
//if not column is selected
if (sortExpression == null || sortExpression == '')
return 'ASC';
else
return sortDirection;
}

public void setSortDirection(String value)
{
sortDirection = value;
}

public List<Account> getAccounts() {
return accounts;
}

public PageReference ViewData() {
//build the full sort expression
string sortFullExp = sortExpression + ' ' + sortDirection;

//query the database based on the sort expression
accounts = Database.query('Select id, Name, BillingCity, BillingCountry, Phone from Account order by ' + sortFullExp + ' limit 1000');
return null;
}

}

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