Need help in writing sql query

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PostgreSQL

Hi all

tax type   date                  taxname    a1    a2    a3   a4 
--------- -------------------- ----------- ---- ------ ---- ----  
"No Tax";"2014-01-28 15:17:14";"Inclusive";0.00;132.00;0.00;132.00
"No Tax";"2014-01-29 12:45:02";"Inclusive";0.00;148.00;0.00;148.00
"No Tax";"2014-01-29 14:07:07";"Inclusive";0.00;180.00;0.00;180.00
"No Tax";"2014-01-29 14:17:42";"Inclusive";0.00;50.00;0.00;50.00
"No Tax";"2014-01-29 14:49:39";"Inclusive";0.00;185.00;0.00;185.00
"No Tax";"2014-02-01 11:00:19";"Inclusive";0.00;165.00;0.00;165.00
"No Tax";"2014-02-03 11:52:26";"Inclusive";0.00;66.00;0.00;66.00
"No Tax";"2014-02-03 13:37:09";"Inclusive";0.00;185.00;0.00;185.00
"No Tax";"2014-03-30 17:14:36";"Inclusive";0.00;37.00;0.00;37.00
"VAT 1";"2014-01-07 11:01:01";"Inclusive";1.00;29.70;0.30;30.00
"VAT 1";"2014-01-08 18:44:07";"Inclusive";1.00;178.22;1.78;180.00
"VAT 1";"2014-01-15 15:48:23";"Inclusive";1.00;69.50;0.70;70.20
"VAT 1";"2014-01-16 13:15:37";"Inclusive";1.00;24.75;0.25;25.00
"VAT 1";"2014-01-18 17:35:19";"Inclusive";1.00;247.52;2.48;250.00
"VAT 1";"2014-03-30 13:04:24";"Inclusive";1.00;49.50;0.50;50.00
"VAT 2";"2014-01-01 11:13:54";"Inclusive";2.00;233.33;4.67;238.00
"VAT 2";"2014-01-01 11:55:34";"Inclusive";2.00;98.04;1.96;100.00
"VAT 2";"2014-01-01 12:23:34";"Inclusive";2.00;490.20;9.80;500.00
"VAT 2";"2014-01-01 13:13:19";"Inclusive";2.00;26.47;0.53;27.00
"VAT 2";"2014-01-01 14:16:49";"Inclusive";2.00;93.33;1.87;95.20
"VAT 2";"2014-03-30 19:44:09";"Inclusive";2.00;284.31;5.69;290.00
"VAT 3";"2014-01-01 11:02:49";"Inclusive";5.50;246.45;13.55;260.00
"VAT 3";"2014-01-01 11:04:28";"Inclusive";5.50;184.83;10.17;195.00
"VAT 3";"2014-01-01 11:13:54";"Inclusive";5.50;227.49;12.51;240.00
"VAT 3";"2014-01-01 11:13:54";"Inclusive";5.50;244.93;13.47;258.40

I have this table.
As you can see tax type is having more then one value.
so what i want to do is i have to display the sum(a2) for each tax type
means i want the output like this

"No Tax";"2014-01-28 15:17:14";"Inclusive";0.00;50.000;0.00;700.00
"VAT 2";"2014-01-01 11:13:54";"Inclusive";200.00;244.93;13.47;258.40
"VAT 3";"2014-01-01 11:13:54";"Inclusive";800.00;244.93;13.47;258.40

Please tell me how to write the query

This is what i tried

select to_char(td.bill_date,'MM-dd-yyyy') as bill_date,td.tax_name,td.tax_type,td.tax_value, ((td.item_total) -(td.tax_amount)) as SaleValue, (td.tax_amount) as tax_amount, (td.item_total) as Total from tbl_transaction_details td join tbl_transaction_master tm on td.bill_no=tm.bill_no Where to_char(td.bill_date,'MM-dd-yyyy')>='03-01-2014' and to_char(td.bill_date,'MM-dd-yyyy')<='03-02-2014' and td.account='ABCD'  and td.menu='ABCD' and tm.card>0  group by td.tax_amount,td.item_total,td.bill_date,td.tax_name,td.tax_type,td.tax_value order by td.bill_date,td.tax_value ASC;

Please guys, need your help..