Hi,

Try the below code snippet as reference:

<apex:page id="p1" controller="examples37">

<apex:form id="f1" >

<apex:pageBlock >

<apex:pageBlockSection >

 <apex:selectList value="{!countries}" onclick="check(this)" >

            <apex:selectOptions value="{!items}"/>

        </apex:selectList>

</apex:pageBlockSection>

</apex:pageBlock>

<apex:pageBlock id="pb1" >

    <apex:pageBlockSection columns="4">

        <apex:pageBlockTable value="{!Account}" var="a">

            <apex:column headerValue="test1"/>

        </apex:pageBlockTable>

        </apex:pageBlockSection>

</apex:pageBlock>

<apex:pageBlock id="pb2">

<apex:pageBlockSection >

        <apex:pageBlockTable value="{!Account}" var="b">

            <apex:column headerValue="test2"/>

        </apex:pageBlockTable>       

    </apex:pageBlockSection>

</apex:pageBlock>

<script>

function check(getpicklistval)

    {

    alert(getpicklistval.value);

    if(getpicklistval.value=='groupa')

    {

   document.getElementById('p1:f1:pb1').style.display='block';

    document.getElementById('p1:f1:pb2').style.display='none';

   }

   else

   {

   document.getElementById('p1:f1:pb1').style.display='none';

    document.getElementById('p1:f1:pb2').style.display='block';

   }

   return false;

    }

</script>

</apex:form>

</apex:page>

..............................

Controller

..............................

public class examples37 {

public List<Account>Account{get;set;}

String[] countries = new String[]{};

       public List<Account> getAccount() {

       Account acc=[select name from Account];

       return Account;

       }

        public List<SelectOption> getItems() {

            List<SelectOption> options = new List<SelectOption>();

            options.add(new SelectOption('groupa','groupa'));

            options.add(new SelectOption('groupb','groupb'));

            return options;

        }

        public String[] getCountries() {

            return countries;

        }

        public void setCountries(String[] countries) {

            this.countries = countries;

        }

}

Did this answer your question? If not, let me know what didn't work, or if so, please mark it solved.