Sql 从两个不同的表中减去总计

我有两个不同的表，每个表都有一个金额相关字段和一个项目代码，每个项目代码有不同的金额

我想为两个表中的每个表添加每个项目代码的数量，然后得到差减法

如果我这样做：

SELECT codigo,
       ROUND(SUM(importe),2) AS 'resultado' 
FROM tabla1 
GROUP BY codigo

SELECT codigo,
       ROUND(SUM(importe),2) AS 'resultado' 
  FROM tabla2 
GROUP BY codigo

求和函数工作正常，但当我进行减法运算时，结果不正确：

SELECT t.codigo,
       ROUND(SUM(t.importe),2) - ROUND(SUM(d.importe),2) AS 'resultado' 
FROM tabla1 t
LEFT JOIN tabla2 d ON t.codigo=d.codigo 
GROUP BY t.codigo;

谢谢

试着这样做：

SELECT codigo,
       sum(resultado1 - resultado2) AS 'resultado'
FROM  (       
SELECT codigo,
       ROUND(SUM(importe),2) AS 'resultado1',
       0 AS 'resultado2'
FROM tabla1 
GROUP BY codigo
UNION ALL
SELECT codigo,
       0 AS 'resultado1',
       ROUND(SUM(importe),2) AS 'resultado2'
  FROM tabla2 
GROUP BY codigo
) tablatot
GROUP BY codigo

SELECT t.codigo,
       ROUND(SUM(ISNULL(t.importe, 0) - ISNULL(D.importe, 0)),2) AS 'resultado' 
FROM tabla1 t
LEFT JOIN tabla2 d ON t.codigo=d.codigo 
GROUP BY t.codigo;