Oracle - views and sequences

View definition

It is a virtual table exported from one or more data tables （ Virtual table ）, The data corresponding to the view is not really stored in the database , For example, define the result of a query as a view , Then you can query the view directly

advantage ：

（1） Simplify the operation of data ： Views can simplify the way users work with data

（2） Focus on specific data ： Unnecessary or sensitive data may not appear in the view

（3） Security mechanism , Different users have different access rights to data

（4） Provide backward monitoring , Table change , The view can remain unchanged

View operation

characteristic ：

（1） Views are not actually stored , When the view allows modification , Modify the data in the view , The data in the table is also modified

（2） Read only view does not allow data modification

（3） Views with constraints

（4） Multi table association can also modify data , The data of the base table will be modified

create VIEW classstudent AS SELECT * FROM TBL_STUDENTS

// or replace When the view exists , Replace with a new ,with check option Is a view with check constraints 
create OR REPLACE VIEW classstudent AS SELECT * FROM TBL_STUDENTS WITH CHECK OPTION

//  Create a read-only view 
create OR REPLACE VIEW classstudent AS SELECT * FROM TBL_STUDENTS WITH READ ONLY

//  Force view , That is, the view created when the table does not exist , Scenarios for dynamically generating tables 
CREATE force VIEW view_test as SELECT * from tbl_test

//  Complex view    Multi-table query , Allow to modify data , Key retention table 
CREATE VIEW view_student_class AS
SELECT * FROM TBL_STUDENTS JOIN TBL_CLASSES ON TBL_STUDENTS.STU_CID = TBL_CLASSES.CLASS_ID

//  Aggregate view , Aggregate functions must have column names 
CREATE VIEW view_student as 
	SELECT COUNT(*) as count,STU_AGE as age FROM TBL_STUDENTS GROUP BY STU_AGE

notes ：

（1） Key retention table , For updatable views , There must be a one-to-one relationship between the rows in the view and the rows in the base table , Because only in this way can we reverse map to the original base table

（2） The aggregate function must have column names when making views

（3） The view of the aggregate function cannot be modified

Materialized view

Materialized view is to create a copy of the query results , Occupied storage space , Query efficiency will be high , But it will take up space , The permission problem is not solved , Follow up learning .

Update the way ：

                COMPLETE  Completely updated

                FAST  Incremental updating , According to the update log of the base table , Determine whether the view needs to be updated

                ON COMMIT Is to refresh the materialized view when the base table changes

                ON COMMON It's a manual refresh

                BUILD IMMEDIATE  Generate data when creating views

ORA-01031: insufficient privileges

Sequence

Create a series of numbers ,mysql The primary key autoincrement of is written directly ,oracle You need to create a sequence

-- Create a simple sequence 
CREATE sequence seq1;

-- The next value of the query sequence , Execute one increment at a time 
SELECT seq1.NEXTval	from dual;

-- The current value of the query sequence 
select seq1.currval from dual;

CREATE sequence seq_withMax
	maxvalue 20; -- Create a 
	
SELECT seq_withMax.nextval FROM dual; --  from the beginning , Each increment 1, Maximum to 20


--  Jumping sequence 
CREATE sequence seq3
	INCREMENT BY 2 --  The default is 1
	START WITH 1000 -- The default is MaxValue, Decreasing is minvalue
	maxvalue	1002
	minvalue 100
	cycle -- Whether to circulate  nocycle
	cache 10; --  Put it into memory every time 10 It's worth , By default cache 20, Don't use cache Is configured as nocache, Save on disk to 10 了 , The cache to 20 了 , At this time, restarting the service will start from 21 Start , Cause punctuation , There must be a cycle with cache less than once , namely cache Default 20*INCREMENT by = 200 individual , Debi cache yes 10, Less than 200
	
	
	select seq3.nextval from dual; --  from 1000 Start , Each increment 2, To 1002 after , From again 100 Start 


	CREATE sequence seq4
		INCREMENT BY -2
		START WITH 1000
		maxvalue 1001;
		
	SELECT seq4.nextval FROM dual; --  from 1000 Start , Decrease each time 2

alter sequenct You can modify the sequence , Do not modify start with