new第7章基本数据对象的管理.ppt

NEXTVAL 和 CURRVAL 伪列 只能在使用NEXTVAL后才能使用CURRVAL NEXTVAL 返回序列中下一个有效的值，任何用户都可以引用 CURRVAL 中存放序列的当前值 NEXTVAL 应在 CURRVAL 之前指定 ，二者应同时有效 7.12.2 使用序列 序列 DEPT_DEPTID_SEQ 的当前值 INSERT INTO departments(department_id, department_name, location_id) VALUES (dept_deptid_seq.NEXTVAL, Support, 2500); 1 row created. SELECT dept_deptid_seq.CURRVAL FROM dual; 7.12.3 删除序列 删除序列需要使用DROP SEQUENCE语句，其语法如下： DROP SEQUENCE sequence_name; 7.12.4 查询序列 查询数据字典视图 USER_SEQUENCES获取序列定义信息 如果指定NOCACHE 选项，则列LAST_NUMBER 显示序列中下一个有效的值 SELECT sequence_name, min_value, max_value, increment_by, last_number FROM user_sequences; * Lesson Aim Functions make the basic query block more powerful and are used to manipulate data values. This is the first of two lessons that explore functions. It focuses on single-row character, number, and date functions, as well as those functions that convert data from one type to another, for example, character data to numeric data. * Confirming Indexes Confirm the existence of indexes from the USER_INDEXES data dictionary view. You can also check the columns involved in an index by querying the USER_IND_COLUMNS view. The example on the slide displays all the previously created indexes, with the names of the affected column, and the index’s uniqueness, on the EMPLOYEES table. * Removing an Index You cannot modify indexes. To change an index, you must drop it and then re-create it. Remove an index definition from the data dictionary by issuing the DROP INDEX statement. To drop an index, you must be the owner of the index or have the DROP ANY INDEX privilege. In the syntax: index is the name of the index Note: If you drop a table, indexes and constraints are automatically dropped, but views and sequences remain. * Creating a Sequence Automatically generate sequential numbers by using the CREATE SEQUENCE statement. In the syntax: sequence is the name of the sequence generator INCREMENT BY n specifies the interval be