dynamic soql for single picklist

Question

I have a dynamic soql as below.

string remainingbetquery ='select id,name,uw_Brand__r.Name,Market_Cluster__c,Product_Category_Sub_Division__c from uw_bet__c where (id not in :betfollowed) and (id not in :betowners) ';

query = query + 'and BET__r.Product_Category_Sub_Division__c ='+'\''+categories+'\'';

for some reason its not properly working and returning correct values .

categories is a dynamic picklist generated by code as well.so when user selects a value in front end it gets populated in controller

Updated with controller code for categories

public List<String> categories {
    get {
      if (categories == null) {

        categories = new List<String>();
        Schema.DescribeFieldResult field = uw_bet__C.Product_Category_Sub_Division__c .getDescribe();

        for (Schema.PicklistEntry f : field.getPicklistValues())
          categories.add(f.getLabel());

      }
      return categories;          
    }
    set;
  }

Pagecode for picklist
Category <select id="categories" >
            <option value=""></option>
            <apex:repeat value="{!categories}" var="tech">
              <option value="{!tech}">{!tech}</option>
            </apex:repeat>
          </select> 

Answer 1

Instead of the HTML element you'll want to make use of Visualforce's built in components, in this case the apex:selectList tag if you want user input to influence the controller's state.

You also don't seem to need to use dynamic SOQL. If you switch to using pre-compiled SOQL (the [] syntax) you'll gain some additional compile-time validations on your query and benefit from some extra security.

SOQL has a special syntax for binding a variable's value into a query without having to manually perform string manipulation: precede the name of the variable with a : character.

Rewriting your query with both of these changes would look like:

[select id,name,uw_Brand__r.Name,Market_Cluster__c,Product_Category_Sub_Division__c from uw_bet__c where (id not in :betfollowed) and (id not in :betowners) and BET__r.Product_Category_Sub_Division__c = :categories]