how to find duplicates and gaps in this scenario in mysql

Question

Hi I have a table that looks like

-----------------------------------------------------------
|  id  |  group_id | source_id | target_id | sortsequence |
-----------------------------------------------------------
|  2   |    1      |    2      |   4       |     1        |   
-----------------------------------------------------------
|  4   |    1      |    20     |   2       |     1        |   
-----------------------------------------------------------
|  5   |    1      |    2      |   14      |     1        |   
-----------------------------------------------------------
|  7   |    1      |    2      |   7       |     3        |   
-----------------------------------------------------------
|  20  |    2      |    20     |   4       |     3        |   
-----------------------------------------------------------
|  21  |    2      |    20     |   4       |     1        |   
-----------------------------------------------------------

Scenario

There are two scenarios that needs to be handled.

1. Sortsequence column value should be unique against one source_id and group_id. For example if all the records having group_id = 1 AND source_id = 2 should have sortsequence unique. In above example records having id= and 5 which are having group_id = 1 and source_id = 2 have same sortsequence which is 1. This is faulty record. I need to find out these records.
2. If group_id and source_id is same. The sortsequence columns value should be continous. There should be no gap. For example in above table records having id = 20, 21 having same group_id and source_id and sortsequence value is 3 and 1. Even this is unique but there is a gap in sortsequence value. I need to also find out these records.

MY So Far Effort

I have written a query

SELECT source_id,`group_id`,GROUP_CONCAT(id) AS children 
FROM
    table 
GROUP BY source_id,
  sortsequence,
  `group_id` 
 HAVING COUNT(*) > 1 

This query only address the scenario 1. How to handle scenario 2? Is there any way to do it in same query or I have to write other to handle second scenario.

By the way query will be dealing with million of records in table so performance must be very good.

Answer 1

Got answer from Tere J Comments. Following query covers above mentioned both criteria.

 SELECT 
     source_id, `group_id`, GROUP_CONCAT(id) AS faultyIDS    
 FROM
     table
 GROUP BY
     source_id,group_id 
 HAVING
     COUNT(DISTINCT sortsequence) <> COUNT(sortsequence) OR COUNT(sortsequence) <> MAX(sortsequence) OR MIN(sortsequence) <> 1

May be it can help others.