根据其他两个表更新列

Question

我有如下三个table，

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 course table 的 credits 列和 credits 的 credits 列更新 student_grade table 的 credits_earned 列18=] table.

就这样，

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 |
+--------------------------+

我在单独执行时得到了上述值，但现在我想使用 update 查询在 student_grade table 中更新这些值。

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);

但是上面的查询不起作用，我得到的错误是：

Cardinality violation, scalar value expected

我知道，我可以单独设置值，但我想通过更新命令来设置。不对的地方请指正

Answer 1

注意： 下面的解决方案适用于 MySQL。（OP 稍后 retagged/changed 从 MySQL 到 MonetDB 的问题）

您尝试的问题是内部子查询返回多行。
即使它 returns 只有一行，它的另一个问题是用相同的值更新 table student_grade 中的所有 credits_earned 字段（输出内部子查询）。
您应该避免使用隐式联接。他们太老了，不是一个好习惯。

你应该 INNER JOIN 这些 table，然后使用加入 table 的列以更新按行数据。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 2

如果我没理解错的话，你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
                         );

Answer 3

无需使事情过于复杂，使用 LEFT JOIN 即可实现结果。集合运算通常比逐行运算更有效。

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

根据其他两个表更新列

Updating column based on other two tables

sql

monetdb

sql-update