插入表需要来自另一个表的特定条件

Posted 2023-02-22

我有两个具有以下架构的SQL表：

• road_test（test_ID，考官ID，student_ID，vin，test_date）
• lessons_count（学生ID，课程）

我正在寻找某种方法，要求学生至少有5个lessons_taken才能插入road_test表。

是否存在某种允许这样做的触发器或约束条件？

答案

不存储课程数量；在需要时进行计算。

这是我的建议：

SQL> -- the final table SQL> create table road_test 2 (test_id number, student_id number, vin number); Table created. SQL> -- table that shows which lessons were taken by which student SQL> create table lesson 2 (student_id number, lesson_id number); Table created. SQL>

应该控制您是否允许将学生的记录插入road_test表中的触发器：对所上课的数量进行计数，如果它太低（为了简单起见，我将其设置为3）：]]
SQL> create or replace trigger trg_bi_road
  2    before insert on road_test
  3    for each row
  4  declare
  5    l_cnt number;
  6  begin
  7    select count(*)
  8      into l_cnt
  9      from lesson
 10      where student_id = :new.student_id;
 11    if l_cnt < 3 then
 12       raise_application_error(-20001,
 13         'You have to take at least 3 lessons');
 14    end if;
 15  end;
 16  /

Trigger created.

SQL>
测试（正如我所说：为简单起见，仅限于3节课：]
SQL> -- initial record
SQL> insert into lesson(student_id, lesson_id) values (1, 100);

1 row created.

SQL> -- can I enter that student into the ROAD_TEST table? Nope
SQL> insert into road_test (test_id, student_id, vin) values (555, 1, 123456);
insert into road_test (test_id, student_id, vin) values (555, 1, 123456)
            *
ERROR at line 1:
ORA-20001: You have to take at least 3 lessons
ORA-06512: at "SCOTT.TRG_BI_ROAD", line 9
ORA-04088: error during execution of trigger 'SCOTT.TRG_BI_ROAD'


SQL> -- Let's insert 2 more lessons for the same student
SQL> insert into lesson(student_id, lesson_id) values (1, 200);

1 row created.

SQL> insert into lesson(student_id, lesson_id) values (1, 300);

1 row created.

SQL> -- New attempt for the ROAD_TEST table:
SQL> insert into road_test (test_id, student_id, vin) values (555, 1, 123456);

1 row created.

SQL> select * From lesson;

STUDENT_ID  LESSON_ID
---------- ----------
         1        100
         1        200
         1        300

SQL> select * from road_test;

   TEST_ID STUDENT_ID        VIN
---------- ---------- ----------
       555          1     123456

SQL>