如何从表中创建以max值开头的Oracle序列？

Question

尝试在Oracle中创建一个以特定表的最大值开头的序列.为什么这不起作用？

CREATE SEQUENCE transaction_sequence
  MINVALUE 0
  START WITH (SELECT MAX(trans_seq_no)
     FROM TRANSACTION_LOG) 
  INCREMENT BY 1
  CACHE 20;

Answer 1

如果您可以使用PL/SQL,请尝试(编辑:将Neil的xlnt建议纳入下一个更高的值):

SELECT 'CREATE SEQUENCE transaction_sequence MINVALUE 0 START WITH '||MAX(trans_seq_no)+1||' INCREMENT BY 1 CACHE 20'
  INTO v_sql
  FROM transaction_log;

EXECUTE IMMEDIATE v_sql;

需要考虑的另一点:通过将CACHE参数设置为20,如果数据库出现故障,您将面临序列中最多丢失19个值的风险.数据库重新启动时CACHEd值将丢失.除非你经常按顺序进行,否则,你不关心差距,我会把它设置为1.

最后一项:您为CACHE和INCREMENT BY指定的值是默认值.您可以将它们关闭并获得相同的结果.

Answer 2

在这里,我的示例工作得很好:

declare
 ex number;
begin
  select MAX(MAX_FK_ID)  + 1 into ex from TABLE;
  If ex > 0 then
    begin
            execute immediate 'DROP SEQUENCE SQ_NAME';
      exception when others then
        null;
    end;
    execute immediate 'CREATE SEQUENCE SQ_NAME INCREMENT BY 1 START WITH ' || ex || ' NOCYCLE CACHE 20 NOORDER';
  end if;
end;

Answer 3

你可能想要开始 max(trans_seq_no) + 1.

看:

SQL> create table my_numbers(my_number number not null primary key);

Table created.

SQL> insert into my_numbers(select rownum from user_objects);

260 rows created.

SQL> select max(my_number) from my_numbers;

MAX(MY_NUMBER)
--------------
           260

SQL> create sequence my_number_sn start with 260;

Sequence created.

SQL> insert into my_numbers(my_number) values (my_number_sn.NEXTVAL);
insert into my_numbers(my_number) values (my_number_sn.NEXTVAL)
*
ERROR at line 1:
ORA-00001: unique constraint (NEIL.SYS_C00102439) violated

当您使用数字创建序列时,您必须记住,第一次选择序列时,Oracle将返回您为其分配的初始值.

SQL> drop sequence my_number_sn;

Sequence dropped.

SQL> create sequence my_number_sn start with 261;

Sequence created.

SQL>  insert into my_numbers(my_number) values (my_number_sn.NEXTVAL);

1 row created.

如果你想做'无间隙'的事情,我强烈建议你

1没有这样做,#2没有使用序列.

Answer 4

您不能在CREATE SEQUENCE语句中使用子选择.您必须事先选择该值.

Answer 5

承受中期,MAX值将只是承诺值的最大值.它可能会返回1234,您可能需要考虑某人已插入1235但未提交.

Answer 6

基于 Ivan Laharnar，代码更少，更简单：

declare
    lastSeq number;
begin
    SELECT MAX(ID) + 1 INTO lastSeq FROM <TABLE_NAME>;
    if lastSeq IS NULL then lastSeq := 1; end if;
    execute immediate 'CREATE SEQUENCE <SEQUENCE_NAME> INCREMENT BY 1 START WITH ' || lastSeq || ' MAXVALUE 999999999 MINVALUE 1 NOCACHE';
end;