Sequence serial number in Oracle

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; once emp_sequence is defined, you can use currval, nextval currval = to return the current Sequence Value nextval = to increase the sequence value, and then return the sequence value, for example, emp_sequence.currval emp_sequence.nextval, where sequence can be used: -subqueries that do not contain subqueries, snapshot, and view select statements-insert statements For example, insert into EMP values (em1_q. nextval, 'Lewis ', 'cler', 7902, sysdate, 1200, null, 20); select em1_q. currval from dual; but note:-the first nextval returns the initial value. The subsequent nextval automatically increases the value of your defined 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 order_seq;