MySQL 8.0

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CREATE TABLE 7cn_invoice_totals  (
  `invoice_id` INTEGER,
  `code` VARCHAR(9),
  `amount` DECIMAL(10,4)
);

INSERT INTO 7cn_invoice_totals 
  (`invoice_id`, `code`, `amount`)
VALUES
  ('125', 'sub_total', '300.0000'),
  ('125', 'tax', '30.0000'),
  ('125', 'total', '330.0000'),
  ('125', 'sub_total', '300.0000'),
  ('125', 'tax', '30.0000'),
  ('125', 'total', '330.0000');
CREATE TABLE 7cn_invoices  (
  `id` INTEGER,
  `amount` INTEGER
);

INSERT INTO 7cn_invoices 
  (`id`, `amount`)
VALUES
  ('125', '330.0000');
select sum(a.amount) as total_sales, sum(c.amount) as total_GST
from  7cn_invoices a 
    INNER JOIN (SELECT DISTINCT `invoice_id`, `code`, `amount`  FROM 7cn_invoice_totals) c ON a.id = c.invoice_id
where c.code = 'tax';


total_sales
total_GST


330
30.0000