how we can use proper soql query

Question

I have created a costum object, email. In this object I am storing date and sender. Examples:

Date               sender
8/2/2013 4:00 AM   Amit
8/3/2013 4:00 AM   Jayant
8/3/2013 4:10 AM   Jony

I want to display the data in Visualforce page like this:

Date   8/3/2013
sender : Jony , Jayant //Here i want to display Jony 1st bcz same date but latest time.
---------------------------
Date   8/2/2013
sender : Amit
---------------------------

How can I write a SOQL query for this?

Answer 1

Assuming that your sobject is defined as Email__c and that the fields are Date__c and Sender__c.

The query would be:

SELECT Date__c, Sender__c FROM Email__c ORDER BY Date__c DESC

You can then use visualforce to format your datetime value accordingly:

<apex:outputText value="{0,date,dd.MM.yyyy}">
    <apex:param value="{!object.dateValue}" /> 
</apex:outputText>

More info on visualforce outputText component can be found in the documentation.

EDIT: My mistake, I didn't realise it was really a question about aggregating values.

As user e-bacho 2.0 pointed out in the comments below, the best way to solve this is with a map:

Map<Date, List <Sender__c>>

Answer 2

I believe that Sender__c is a lookup to another object. That said here is an example code (not tested, there might be some syntax issues maybe):

Apex:

// In your class
Map <Date, List <Sender__c>> dateMap {get; set;}
List <Email__c> emails {get; set;}

// Inside your constructor or action method
dateMap = new Map <Date, List <Sender__c>> ();
List <Email__c> emails = [SELECT Date__c, Sender__c, Sender__r.Name FROM Email__c ORDER BY Date__c DESC];

for (Email__c email : emails)
{
    if (dateMap.get(email.Date__c) == null)
    {
        dateMap.put(email.Date__c, new List <Sender__c> ());
    }

    dateMap.get(email.Date__c).add(new Sender__c(Id = email.Sender__c, Name = email.Sender__r.Name));
}



VisualForce:

<apex:repeat value="{!emails}" var="email">
    Date:
    <apex:outputText value="{0,date,dd.MM.yyyy}">
        <apex:param value="{!email.Date__c}" /> 
    </apex:outputText>

    Sender:
    <apex:repeat value="{!dateMap[email.Date__c]}" var="sender">
        <apex:outputText value="{!sender.Name}" /><br/>
    </apex:repeat>

    ----------------
</apex:repeat>

I hope that's what you're after.