How to retrieve a queue using SOQL query

                Queues are stored into Group Objects in Salesforce and Users that are present are being stored into GroupMember Object.

Use below SOQL Queries.

For All the Queues into Salesforce Org.

    select Id from Group where Type = 'Queue'

For Quering specific Queue.

          :select Id from Group where Type = 'Queue' AND NAME = 'QueueName'

// You can also use DeveloperName so that if you have used Queue Name Developer Name Will be same SOQL will be like

     select Id from Group where Type = 'Queue' AND DeveloperNAME = 'Test_Queue' 

// If QueueName is Test Queue the developer Name will be Test_Queue

If you want to query public groups/quques then use below SOQL.

select Id from Group where Type = 'Public' AND Developer_Name='Test_Group'