你得到的是一个笛卡尔积 test1.id . 您需要在 JOIN 学生：

SELECT t1.id,Test1.name, t2.cnt_2,
       t3.sum_pos, t3.sum_neg
FROM Test1 t1 LEFT JOIN
     (SELECT ID_Test1, COUNT(*) as cnt_2
      FROM Test2
      GROUP BY ID_Test1
     ) t2
     ON t1.id = t2.ID_Test1 LEFT JOIN
     (SELECT ID_Test1, SUM(positive) as sum_pos, SUM(negative) as sum_neg
      FROM Test3
      GROUP BY ID_Test1
     ) t3
     ON t1.id = t3.ID_Test1
GROUP BY t1.id;

这是sql小提琴。