SPQuery LookUpFields

"UserDirectory” is the Lookup field for the following actions-

Adding a Lookup Item -

SPListItem listItem = myList.Items[0];
listItem["UserDirectory"] = new SPFieldLookupValue(16, “Details”); // Adding Detailsas text and 16 as ID
listItem.Update();

Retrieving Lookup field for an item-

SPListItem listItem = myList.Items[0];
SPFieldLookupValue lookupFieldText = new SPFieldLookupValue(listItem["UserDirectory"].ToString());
string lookUpValue = lookupFieldText .LookupValue;

Updating Lookup field for an item -

SPListItem listItem = myList.Items[0];
SPFieldLookupValue lookupFieldText = new SPFieldLookupValue(listItem ["UserDirectory"].ToString());
lookupFieldText .LookupValue = 17; -> updates the value of the Lookup field for listItem

Retrieving all the values in the Lookup Field -

DropDownList res = new DropDownList();
SPWeb web = SPControl.GetContextWeb(Context);
web = web.ParentWeb;
SPLookupField myLookup = (SPLookupField)web.Fields[str];
SPSite site = web.Site;
SPWeb lookupWeb = site.AllWebs[myLookup.LookupWebId];
SPList lookupList = lookupWeb.Lists[myLookup.LookupList];
foreach(SPListItem lookupItem in lookupList.Items)
{
res.Items.Add(lookupItem[myLookup.LookupField];
}

Finally, SPquery for Lookup field by ID in sharepoint -

<Query>
<Where>
<Eq>
<FieldRef Name='UserDirectory' LookupId=’TRUE’ />
<Value Type=’Lookup’>16</Value>
</Eq>
</Where>
</Query>