Insert characters into entire row and union with continue of row_number I have first query 

IF OBJECT_ID('tempdb..#tmp1') IS NOT NULL DROP TABLE #tmp1
select 
    IDX=ROW_NUMBER() over(order by A_ID)
    ,A = A_ID
    ,Number = COUNT(*)
    ,Value = sum(ammount)
into #tmp1
from #t
group by A_ID


Need to do union with 

   select   
    IDX  = ''
    ,A = 'Total'
    ,SUM (Number)
    ,SUM(Value)
   from #tmp1


After UNION ALL I get dataset: 

+--------+------------+-------------+--------+
|  IDX   |     A      |  Number     |  Value |
+--------+------------+-------------+--------+
|      1 |  PCX       |      12     |  22.00 |
|      2 |  TOC       |       3     |  89.00 |
|      0 | Total      |      15     | 111.00 |
+--------+------------+-------------+--------+


But I need:

    +--------+------------+-------------+--------+
    |  IDX   |     A      |  Number     |  Value |
    +--------+------------+-------------+--------+
    |      1 |  PCX       |      12     |  22.00 |
    |      2 |  TOC       |       3     |  89.00 |
    |============================================|
    |      3 | Total      |      15     | 111.00 |
    +--------+------------+-------------+--------+


How to insert entire row of '=' and continue row number of IDX ? 

Version is 2014 Enterprise.
You could use GROUP BY GROUPING SETS and the GROUPING() function:

SELECT 
    idx = ROW_NUMBER() OVER (ORDER BY GROUPING(a_id), a_id),
    a = CASE WHEN GROUPING(a_id) = 0 THEN a_id ELSE 'Total' END, 
    number = COUNT(*),
    value = SUM(amount)
FROM #t
GROUP BY GROUPING SETS ((a_id), ()) 
ORDER BY idx ; 


In older versions (that don't have GROUPING SETS) you could use ROLLUP instead. In even older versions (or any version if you prefer a simpler query), you could split the query into 2 parts and UNION ALL with:

SELECT 
    idx = ROW_NUMBER() OVER (a_id),
    a = a_id, 
    number = COUNT(*),
    value = SUM(amount)
FROM #t
GROUP BY a_id

UNION ALL

SELECT 
    1 + COUNT(DISTINCT a_id),
    a_id, 
    COUNT(*),
    SUM(amount)
FROM #t

ORDER BY idx ; 


Tested at rextester.com