I want to design a VF Page. In which after selecting any object(lead/contact) all its fields will be displayed and based on operator and input value we can search data. But Database.query() in search is not working and also <apex:column value="{!a['name']}" Here is my code :

<apex:page controller="searchLeadorContact_task6_cntlr">
    <apex:form >
        <apex:pageBlock id="pbs1" title="Search Lead or Contact">
        <apex:pageMessages ></apex:pageMessages>
            <apex:pageBlockSection title="Select Object">
                <apex:outputLabel value="Select Object :"></apex:outputLabel>
                <apex:selectList value="{!sobj}" size="1" onchange="call_field()" >
                    <apex:selectOptions value="{!listObj}"></apex:selectOptions>
                </apex:selectList>
                <apex:actionFunction name="call_field" action="{!FieldSelect}" rerender="pbs2"/>
            </apex:pageBlockSection>
            
            <apex:pageBlockSection id="pbs2" title="Fields" rendered="true" columns="1" >
                <apex:pageBlockSectionItem >
                    <apex:outputLabel value="Select Fields:"></apex:outputLabel>
                    <apex:selectList value="{!sfield}" size="1">
                        <apex:selectOptions value="{!listFields}"></apex:selectOptions>
                    </apex:selectList>
                </apex:pageBlockSectionItem>
                
                <apex:pageBlockSectionItem >
                    <apex:outputLabel value=" Select Operator"></apex:outputLabel>
                    <apex:selectList value="{!soperator}" size="1">
                        <apex:selectOptions value="{!operators}"></apex:selectOptions>
                    </apex:selectList>    
                </apex:pageBlockSectionItem>
                
                <apex:pageBlockSectionItem >
                    <apex:outputLabel value="Enter data"></apex:outputLabel>
                    <apex:inputText value="{!data}"/>
                </apex:pageBlockSectionItem>
                
                <apex:pageBlockSectionItem >
                     <apex:outputLabel value=""></apex:outputLabel>
                    <apex:commandButton value="Search" action="{!Search}"/>
                </apex:pageBlockSectionItem>
                
                <apex:pageBlockTable value="{!tableData}" var="a">
                       <apex:column headerValue="ID" value="{!a}"/>
                       <apex:column headerValue="Name" value={!a['Name']}/>
                       <apex:column headerValue="{!sfield}"/>
                </apex:pageBlockTable>
                
            </apex:pageBlockSection>
        </apex:pageBlock>
    </apex:form>
</apex:page>


******************
public class searchLeadorContact_task6_cntlr {

    public String data { get; set; }
    public String soperator { get; set; }
    public Boolean showPBS2 { get; set; }
    public String sfield { get; set;}
    public String sobj { get; set; }
    public Set<String> setFields {get;set;}
    public Schema.SObjectType sobject1 {get;set;}
    public list<SelectOption> listObj { get; set; }
    public List<SelectOption> listFields { get; set; }
    public list<Schema.SObjectField> listSfields{get;set;}
    public Map<String,Schema.SObjectField> mobjField {get;set;}
    public list<sobject> tableData {get;set;}
    
    public searchLeadorContact_task6_cntlr ()
    { showPBS2= false;
      listObj = new List<SelectOption>();
      listObj.add(new SelectOption('None','None'));
      listObj.add(new SelectOption('Lead','Lead'));
      listObj.add(new SelectOption('Contact','Contact'));
      sobject1 = null;
      setFields = new Set<String>();
      listSfields = new List<Schema.SObjectField>();
      mobjField = new Map<String,Schema.SObjectField>();
      listFields = new List<SelectOption>();
      tableData = new List<sobject>();
    }
    
    public void FieldSelect()
    {   System.debug('Selected Sobject is'+sobj); 
        if(sobj=='None')
        { ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR,'Please Select an Object'));
        }
     else{
        showPBS2= true;
        System.debug('Field Select Function call');
        sobject1 = Schema.getGlobalDescribe().get(sobj);
        mobjField = sobject1.getDescribe().fields.getMap();
        setFields.addAll(mobjField.keySet());
        System.debug('Set Field values are'+setFields);
        listFields.add(new SelectOption('None','--None--'));
        for(String s:setFields)
        {
            listFields.add(new SelectOption(s,s));
        }
     }
    }
    
    
    public List<SelectOption> getoperators() {
        List<SelectOption> option=new List<SelectOption>();
        option.add(new SelectOption('none', '--None--'));
        option.add(new SelectOption('=','equals'));
        option.add(new SelectOption('!=','not equal to'));
        option.add(new SelectOption('<','less than'));
        option.add(new SelectOption('>','greater than'));
        option.add(new SelectOption('<=','less or equal'));
        option.add(new SelectOption('>=','greater or equal'));
        option.add(new SelectOption('Like','contains'));
        option.add(new SelectOption('NotLike','does not contain'));
        option.add(new SelectOption('Likestart','starts with'));
        return option;
    }
    
    
    public void Search() {
      
       String query = 'SELECT id,name,' +sfield+ ' from '+sobj+' WHERE '+sfield +' '+soperator+':' +data ;
      //  String company = 'American Banking Corp.';
      //  String query = 'select id , name from Lead where company = :company';
        System.debug('query is '+query);
        System.debug(Database.query(query));
        tableData = Database.query(query);
        System.debug('table data is '+tableData);
    }
}