限制值在 table oracle sql 中出现的次数

Question

我有oracle sql table 同学，同学都分到组了。

我必须设置限制，所以一个组中的学生不超过 10 个（理想情况下一个组中不超过 N 个学生）。

基本上限制 table 具有相同 GROUP_ID 值的 N 行以上。

这里是table声明。

    CREATE TABLE  "students" 
       (    "STUDENT_ID" NUMBER(6,0), 
        "GROUP_ID" NUMBER(6,0), 
        "FULL_NAME" VARCHAR2(40), 
         CONSTRAINT "student_id_PK" PRIMARY KEY ("STUDENT_ID")
      USING INDEX  ENABLE
       )
    /
    ALTER TABLE  "students" ADD FOREIGN KEY ("GROUP_ID")
          REFERENCES  "groups" ("GROUP_ID") ENABLE
    /
    CREATE TABLE  "groups" 
       (    "GROUP_ID" NUMBER(6,0), 
        "NAME" VARCHAR2(40), 
         CONSTRAINT "group_id_PK" PRIMARY KEY ("GROUP_ID")
      USING INDEX  ENABLE
       )
    /

我尝试使用触发器，当一个组中有超过 10 个学生时会抛出错误。它适用于插入，但对 table 的更新毫无用处，因为我不断得到 error ORA-04091: table students is mutating

这里是触发器的简化版本：

SELECT count(*) INTO stud_in_g 
    FROM "students" 
    WHERE GROUP_ID = :NEW.GROUP_ID; 
    
IF stud_in_g>10 THEN 
    raise_application_error(-20001, 'to many students'); 
END IF;

我读到 oracle 在更新时禁止从 table 读取。如果是这样，那我怎么保证一个组不能超过10个学生呢？

Answer 1

触发器的“简化版本”没有显示它是什么类型的触发器 - 很可能是行级触发器。由于变异 table 触发器错误，这将不起作用。语句级触发器确实有效：

create table test_table (
    id                             number generated by default on null as identity 
                                   constraint test_table_id_pk primary key,
    name                           varchar2(100 char),
    the_group                      varchar2(100 char)
);

Table TEST_TABLE created.

BEGIN
insert into test_table (name, the_group ) values ('Chandler', 'A');
insert into test_table (name, the_group ) values ('Ross', 'A');
insert into test_table (name, the_group ) values ('Joey', 'A');
insert into test_table (name, the_group ) values ('Phoebe', 'B');
insert into test_table (name, the_group ) values ('Jennifer', 'B');
insert into test_table (name, the_group ) values ('Monica', 'B');
commit;
END;
/

PL/SQL procedure successfully completed.

CREATE OR REPLACE TRIGGER test_table_as 
    AFTER UPDATE OR INSERT ON test_table  
DECLARE
    l_max_members NUMBER := 3;
BEGIN
    FOR r IN (SELECT
                the_group,
                COUNT(*) AS cnt
                FROM
                test_table
               GROUP BY
                the_group
              HAVING
                COUNT(*) > l_max_members)
    LOOP
      IF r.cnt > l_max_members THEN
        raise_application_error(-20100,'Max '||l_max_members||' per group');
      END IF;
    END LOOP;
END;
/


Trigger TEST_TABLE_AS compiled

insert into test_table (name, the_group ) values ('Jill', 'B');

Error starting at line : 39 in command -
insert into test_table (name, the_group ) values ('Jill', 'B')
Error report -
ORA-20100: Max 3 per group
ORA-06512: at "KOEN.TEST_TABLE_AS", line 8
ORA-06512: at "KOEN.TEST_TABLE_AS", line 8
ORA-04088: error during execution of trigger 'KOEN.TEST_TABLE_AS'

限制值在 table oracle sql 中出现的次数

Limit number of occurrences of value in a table oracle sql

sql

oracle

triggers