How to create an auto-increment field in Oracle-A Brief Introduction to ORACLE SEQUENCE

The method for creating an auto-increment field in Oracle-A Brief Introduction to oracle sequence first assumes that there is such a table:

create  table S_Depart  (    DepartId              INT                              not  null,    DepartName           NVARCHAR2( 40)                    not  null,    DepartOrder           INT                             default  0,     constraint PK_S_DEPART  primary  key (DepartId) ); 

 

In oracle, sequence is the so-called serial number, which is automatically increased every time it is obtained. It is generally used in places where the sequence numbers need to be sorted. 1. Create Sequence you must first have the create sequence or create any sequence permission, create sequence emp_sequence increment by 1 -- add several start with 1 each time -- count NOMAXvalue from 1 -- do not set the maximum value NOCYCLE -- accumulate all the time, do not loop CACHE 10; -- set cache cache sequence, if the system is down or in other cases, the sequence is not continuous. You can also set it to --------- NOCACHE. The sequence created for S_Depart is as follows:

Create sequence s_depart minvalue 1 maxvalue 999999999999999999999999999 start with 1 increment by 1 nocache; once emp_sequence is defined, you can use CURRVAL, nextval currval = the current value of returned sequence NEXTVAL = increases the sequence value, and then returns the sequence value, for example, where emp_sequence.CURRVAL emp_sequence.NEXTVAL can use sequence: -the SET of-UPDATE in the valueS of-NSERT statement in the subquery that does not contain the SELECT statement-INSERT statement of the subquery, snapshot, and VIEW can be seen as follows: insert into S_Depart (departId, partition name, Departorder) values (S_S_Depart.Nextval, '000000', 1); SELECT em1_q. currval from dual;

 

Note:-the first NEXTVAL returns the initial value. The subsequent NEXTVAL automatically increases the value of your increment by and then returns the added value. CURRVAL always returns the value of the current SEQUENCE, but CURRVAL can be used only after the first NEXTVAL initialization; otherwise, an error will occur. NEXTVAL increases the SEQUENCE value once. Therefore, if you use multiple NEXTVAL values in the same statement, their values are different. Understand? -If the CACHE value is specified, ORACLE can place some sequence in the memory in advance, so that the access speed is faster. After the cache is obtained, oracle automatically retrieves another group to the cache. The cache may be skipped. For example, if the database suddenly fails to be shut down (shutdown abort), the sequence in the cache will be lost. Therefore, nocache can be used to prevent this situation when creating sequence. 2. Alter Sequence: you are the owner of the sequence, or you have the alter any sequence permission to modify the sequence. you can alter all sequence parameters except start. if you want to change the start value, you must drop sequence and re-create.

Alter sequence example alter sequence emp_sequence increment by 10 MAXvalue 10000 CYCLE -- To NOCACHE from the beginning after 10000; affects the initialization parameters of Sequence: SEQUENCE_CACHE_ENTRIES = sets the number of sequence that can be simultaneously cached. You can easily Drop Sequence drop sequence order_seq; a simple example: create sequence SEQ_IDminvalue 1 maxvalue 99999999 start with 1 increment by 1 nocacheorder;

 

The code of the producer is:

Create or replace trigger tri_test_id before insert on S_Depart -- S_Depart is the table name for each rowdeclare nextid number; begin IF: new. departId IS NULLor: new. departId = 0 THEN -- DepartId is the column name select SEQ_ID.nextval -- SEQ_ID, which is exactly the into nextid from sys. dual;: new. departId: = nextid; end if; end tri_test_id;

 

OK. The above code can be used to implement the auto-increment function.