Oracle序列nextval和currval令人困惑

Question

在我的程序中,我写这个

INSERT INTO questions(id, value) VALUES(my_seq.NEXTVAL, p_question);

INSERT INTO DEPENDENCIES(parent_question_id, child_question_id)
VALUES (my_seq.CURRVAL, my_seq.NEXTVAL);

假设序列的最后一个值等于1.在这种情况下,我期望这个结果:

 my_seq.NEXTVAL = 2;
 my_seq.CURRVAL = 2;
 my_seq.NEXTVAL = 3;

但它插入了这个:

 my_seq.NEXTVAL = 2;
 my_seq.CURRVAL = 3;
 my_seq.NEXTVAL = 3;

我想知道为什么oracle以这种方式检索序列值？

Answer 1

如果你有三个单独的陈述:

1. NEXTVAL

2. CURRVAL

3. NEXTVAL

那你就是对的.但你只有两个

1. NEXTVAL

2. currval和nextval

Currval和nextval是单个原子步骤的一部分 - 在nextval之前不会处理currval.

您需要为此使用变量:

DECLARE
  l_parent_question_id     NUMBER ;
  l_child_question_id      NUMBER ;
BEGIN
  l_parent_question_id := my_seq.NEXTVAL ;

  INSERT INTO questions(id, value) VALUES(l_parent_question_id, p_question);

  l_child_question_id := my_seq.NEXTVAL ;

  INSERT INTO DEPENDENCIES(parent_question_id, child_question_id)
  VALUES (l_parent_question_id, l_child_question_id); 

END ;