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Sql使用SUM和UNION ALL 查詢兩張表中指定欄位的和

1、t_test1表

     

    t_test2表

     

2、查詢需求:求t_test1.num與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;