Sql使用SUM和UNION ALL 查询两张表中指定字段的和

1、t_test1表

     

    t_test2表

     

2、查询需求：求t_test1.num与t_test2.amount之和

  方法一：

SELECT   SUM(test.num)   AS count  FROM(SELECT  *   FROM  t_test1  UNION  ALL SELECT *  FROM  t_test2) AS test;

方法二：

SELECT   SUM(test.num)   AS count  FROM(SELECT   t_test1.num   FROM  t_test1  UNION  ALL SELECT t_test2.amount  FROM  t_test2) AS test;