Question

我有以下三个表格，

select * from student_grade;
+--------------+----------+----------+-------+--------------------------+
| enrollmentid | courseid | personid | grade | credits_earned           |
+==============+==========+==========+=======+==========================+
|            1 |     1001 |        1 | A     |                     null |
|            2 |     1002 |        1 | B     |                     null |
|            3 |     1003 |        1 | A     |                     null |
|            4 |     3001 |        3 | A     |                     null |
|            5 |     3001 |        2 | B     |                     null |
|            6 |     4001 |        4 | A     |                     null |
|            7 |     4002 |        4 | A     |                     null |
+--------------+----------+----------+-------+--------------------------+
7 tuples
sql>select * from course;
+----------+-------------------+---------+--------------+
| courseid | title             | credits | departmentid |
+==========+===================+=========+==============+
|     1001 | Data structures   |      12 |          101 |
|     1002 | Algorithms        |      12 |          101 |
|     1003 | Graphics          |      20 |          101 |
|     2001 | DSP               |      20 |          102 |
|     2002 | Matlab            |      20 |          102 |
|     2003 | Maths             |      10 |          102 |
|     3001 | CAD               |      10 |          104 |
|     4001 | Power electronics |      10 |          103 |
|     4002 | Semi conductors   |      20 |          103 |
+----------+-------------------+---------+--------------+
9 tuples
sql>select * from grade_to_credits;
+-----------+--------------------------+
| gradechar | credits                  |
+===========+==========================+
| A         |                        1 |
| B         |                      0.9 |
+-----------+--------------------------+

我想做的是：我正在使用credits_earned表的student_grade和credits表的course的列更新credits表的grade_to_credits列。

就是这样

select c.credits * gc.credits 
from course c, grade_to_credits gc, student_grade sg 
where sg.courseid = c.courseid and sg.grade = gc.gradechar;
+--------------------------+
| L2                       |
+==========================+
|                       12 |
|                     10.8 |
|                       20 |
|                       10 |
|                        9 |
|                       10 |
|                       20 |
+--------------------------+

当我分别执行时，我得到了上述值，但是现在我想使用 update 查询来更新student_grade表中的这些值。

现在我将查询用作：

update student_grade 
set credits_earned = (select c.credits * gc.credits 
                      from course c, grade_to_credits gc, student_grade sg 
                      where sg.courseid = c.courseid 
                        and sg.grade = gc.gradechar);

但是上面的查询无法正常工作，我收到以下错误消息：

违反基数，期望标量值

我知道，我可以单独设置值，但是我想通过update命令来设置。请更正我要去哪里的地方。

Answer 1

无需太复杂，可以使用LEFT JOIN获得结果。设置操作通常比逐行操作更有效。

update student_grade 
SET  credits_earned = a.credits_earned
from (
select  s.enrollmentid,
        s.courseid,
        s.personid,
        s.grade,
        (g.credits * c.credits) as credits_earned
from student_grade s
LEFT OUTER JOIN @grade_to_credits g ON g.gradechar = s.grade
LEFT OUTER JOIN @course c           ON c.courseid  = s.courseid 
) as a
where student_grade.enrollmentid = a.enrollmentid

结果：

1   1001    1   A   12
2   1002    1   B   10,8
3   1003    1   A   20
4   3001    3   A   10
5   3001    2   B   9
6   4001    4   A   10
7   4002    4   A   20

Answer 2

注意：以下解决方案在MySQL中有效。（OP后来将问题从MySQL重新标记/更改为MonetDB）

您尝试执行的问题是内部子查询返回的行多。
即使只返回一行，它的另一个问题是用相同的值（内部子查询的输出）更新表credits_earned中的所有student_grade字段。
您应该避免使用隐式联接。他们很老，不是一个好习惯。

您宁愿INNER JOIN这些表，然后使用联接表的列以更新行数据。请尝试以下操作：

update student_grade sg 
inner join course  c on c.courseid = sg.courseid 
inner join grade_to_credits gc on gc.gradechar = sg.grade 
set sg.credits_earned = c.credits * gc.credits;

Answer 3

如果我理解正确，则只需要从子查询中删除student_grade：

update student_grade st
    set credits_earned = (select sc.credits * gc.credits
                          from course c cross join
                               grade_to_credits gc
                          where sg.courseid = c.courseid and
                                sg.grade = gc.gradechar
                         );

根据其他两个表更新列

3 个答案: