(4) Oracle ------ constraints, views, and the index sequence

1, constrained

• 1.1 What are the constraints?
• 1.2, table level and column-level constraint?
• 1.3, NOT NULL constraints
• 1.4, UNIQUE constraints
• 1.5, PRIMARY KEY constraint
• 1.6, FOREIGN KEY constraint
• 1.7, adding constraints
• 1.8, delete constraints
• 1.9, constraint invalidation
• 1.10, the activation constraint
• 1.11, query constraints

2, views

• 2.1, the concept of view
• 2.2 Why view
• 2.3, simple and complex view of view
• 2.4, create, query, modify, delete, view
• 2.5, using a predetermined view of DML
• 2.6 shield DML operation
• 2.7, Top-N analysis

3, views

• 3.1 What is the sequence?
• 3.2, create a sequence
• 3.3, the query sequence
• 3.4, delete sequence

4, Index

• 4.1, create an index
• 4.2, when you need to create an index
• 4.3, when you do not create an index
• 4.4, query index
• 4.5, delete the index
• 4.6, synonyms -synonym
• 4.7, create and delete synonyms

constraint

1. What is the constraint?

• Table-level constraints are mandatory for the
• There are five constraints:
  the NOT NULL
  UNIQUE
  PRIMARY KEY
  a FOREIGN KEY
  CHECK
• If you do not specify a constraint name, Oracle server automatically assigns the constraint name in accordance with the format SYS_Cn

2, table level and column-level constraint?

• Scope:
  ①, column-level constraints can only affect a column
  ②, table-level constraints may act on a plurality of columns (table-level constraints may of course also act in a column)

• Defined method: column constraint must, with table constraint is not defined in the column behind the column, but is defined independently.

• Non-empty (not null) constraint can only be defined in column

3, NOT NULL constraints

CREATE TABLE employees(
       employee_id  NUMBER(6),
       first_name VARCHAR2(20) NOT NULL,//系统命名
       job_id CONSTRAINT ji_nn NOT NULL VARCHAR2(10),//用户命名
       CONSTRAINT emp_emp_id_pk PRIMARY KEY (EMPLOYEE_ID)
);

4, UNIQUE constraints

CREATE TABLE employees(
    employee_id      NUMBER(6),
    last_name        VARCHAR2(25) UNIQUE,--系统命名
    email            VARCHAR2(25),
    salary           NUMBER(8,2),
    commission_pct   NUMBER(2,2),
    hire_date        DATE NOT NULL,
    CONSTRAINT emp_email_uk UNIQUE(email) --用户命名
    );

5, PRIMARY KEY constraint

CREATE TABLE departments(
    department_id        NUMBER(4),
    department_name      VARCHAR2(30) CONSTRAINT dept_name_nn NOT NULL,
    manager_id           NUMBER(6),
    location_id          NUMBER(4),
    CONSTRAINT dept_id_pk PRIMARY KEY(department_id));

6, FOREIGN KEY constraint

CREATE TABLE employees(
    employee_id      NUMBER(6),
    last_name        VARCHAR2(25) NOT NULL,
    email            VARCHAR2(25) UNIQUE,
    salary           NUMBER(8,2),
    commission_pct   NUMBER(2,2),
    hire_date        DATE NOT NULL,
    department_id    NUMBER(4),
    CONSTRAINT emp_dept_fk FOREIGN KEY (department_id)
    REFERENCES departments(department_id) on delete cascade
);

• FOREIGN KEY: table columns are designated at the table level
• REFERENCES: indicated in the parent table columns
• ON DELETE CASCADE (cascade delete): When the parent table column is deleted, the child table corresponding to the deleted columns are also
• ON DELETE SET NULL (blank cascade): sub-table corresponding column blanking

7, adding constraints

ALTER TABLE employees 
      ADD CONSTRAINT  emp_manager_fk 
      FOREIGN KEY(manager_id) REFERENCES employees(employee_id);

8, delete constraints

ALTER TABLE employees DROP CONSTRAINT  emp_manager_fk;

9, invalidation constraints

• Use the DISABLE clause to invalidate constraints in ALTER TABLE statement.

ALTER TABLE employees DISABLE CONSTRAINT emp_manager_fk;

10, the activation constraint

• ENABLE clause can be activated currently invalid constraints

ALTER TABLE	employees ENABLE CONSTRAINT	emp_manager_fk;

• When you define or activate UNIQUE or PRIMARY KEY constraint will be created automatically UNIQUE or PRIMARY KEY index

11, the query constraints

• Query the data dictionary view USER_CONSTRAINTS

SELECT constraint_name, constraint_type, search_condition
  FROM user_constraints
 WHERE table_name = 'EMPLOYEES';

view

1, the concept of view

• View is a virtual table.
• View of the existing table based on the table view is called a base table upon which to build.
• Provide data to view the content of the statement is a SELECT statement, the view can be understood as a SELECT statement stored up.
• Another manifestation of providing a base view table data to the user

2. Why view

• Control data access
• Simplify the query
• To avoid repeated access to the same data

3, a simple view and complex view

4, create, query, modify, delete, view

• Create a view

create or replace view empview 
as 
select employee_id emp_id,last_name name,department_name
from employees e,departments d
Where e.department_id = d.department_id

• Query View

SELECT * FROM	empview;

• Modifying the view (using CREATE OR REPLACE VIEW clause modified view)

CREATE OR REPLACE VIEW empview
  (emp_id, name,  department_id)
AS SELECT  employee_id, first_name || ' ' || last_name, 
           department_id
   FROM    employees
   WHERE   department_id = 80;

• Delete View (delete view just delete the definition of the view, and does not delete the data base table)

drop view empview

5, a predetermined view of DML

• DML operations may be performed in a simple view,
• When the view definition comprises one element can not use the delete:
  Group functions
  GROUP BY clause
  DISTINCT keyword
  ROWNUM pseudo-column
• E.g:

create or replace view sal_view
as select
avg(salary) avg_sal from employees
group by department_id

• When the view definition comprises one element can not use the update:
  Group functions
  GROUP BY clause
  DISTINCT keyword
  ROWNUM pseudo-column
  column is defined as the expression

• When the view definition comprises one of the following elements can not insert:
  Group functions
  GROUP BY clause
  DISTINCT keyword
  ROWNUM pseudo-column
  is defined as an expression column
  in Table non-null columns not included in the view definition

6, the shield DML operation

• Options may be used WITH READ ONLY DML operations on the view screen
• Any DML operation will return an Oracle server error

CREATE OR REPLACE VIEW empvu10
    (employee_number, employee_name, job_title)
AS SELECT	employee_id, last_name, job_id
   FROM     employees
   WHERE    department_id = 10
   WITH READ ONLY;

7, Top-N analysis

`

• Discover some of the largest value of Top-N analysis:

SELECT ROWNUM, e.empno, e.ename
  FROM emp e
 WHERE ROWNUM <= 5
 ORDER BY e.empno;

• Note:
  for ROWNUM (pseudo-column) using only <or <=, and a =,>,> = will not return any data.

sequence

1. What is the sequence?

• Sequence:
  for a plurality of users for generating database objects unique value
  automatically provides unique value
  shared object
  primarily used to provide primary key
  sequence value into memory access efficiency can be improved
• Create a sequence of CREATE SEQUENCE statement

CREATE SEQUENCE sequence
       [INCREMENT BY n]  --每次增长的数值
       [START WITH n]    --从哪个值开始
       [{MAXVALUE n | NOMAXVALUE}]
       [{MINVALUE n | NOMINVALUE}]
       [{CYCLE | NOCYCLE}]     --是否需要循环
       [{CACHE n | NOCACHE}];  --是否缓存登录

2, create a sequence

• Create a sequence EMP_SEQ provide the primary key for the table emp

CREATE SEQUENCE EMP_SEQ
                INCREMENT BY 10
                START WITH 120
                MAXVALUE 9999
                NOCACHE
                NOCYCLE;

3, the query sequence

• Query the data dictionary view USER_SEQUENCES obtain sequence information is defined

SELECT	sequence_name, min_value, max_value, 
	increment_by, last_number
FROM	user_sequences;

• Display sequence if the next valid value NOCACHE option is specified, the column LAST_NUMBER
• And dummy column NEXTVAL CURRVAL
• NEXTVAL return sequence next valid value, any user can reference
• The current value is stored in the sequence CURRVAL
• NEXTVAL should be specified before CURRVAL, otherwise it will report an error CURRVAL not yet defined in this session.

Insert into emp(empno,ename) values(seq.nextval,’c’);

4, delete sequence

• DROP SEQUENCE statement to remove the use of sequence
• After deletion, the sequence can not be referenced again

DROP SEQUENCE emp_seq;

index

• Mode that is independent of the object table, the table may be stored in a different table or disk space
• The index is deleted or damaged, will not have an impact on the table, its impact is only on the speed
• Index Once established, Oracle management system will be maintained automatically, and decide when to use Oracle Management index. Users do not have to specify which index to use in the query
• When you delete a table, all will be automatically deleted based index of the table
• By pointer accelerate query speed Oracle server
• By the method of rapid positioning data, reduce disk I / O

1, to create an index

• Automatically created: the system automatically creates a unique index on the corresponding column in the definition of PRIMARY KEY constraints or UNIQUE
• Created manually: Users can create nonunique indexes on other columns to speed up queries
• Create an index on LAST_NAME column of the EMP table

CREATE INDEX 	emp_last_name_idx
ON 		emp(last_name);

2, when you need to create an index

• You can create the following indexes:
  the value of the distribution range is very wide data columns
  often appear in the WHERE clause or join condition columns
  table is accessed frequently and large amount of data, data access would account for about 2% to 4% of the total data

3, when not to create an index

• Do not create the following indexes:
  the table is small
  columns do not often occur as the join condition in the WHERE clause or
  data queries than 2% to 4%
  for the regular updates

• Index do not need, just say when we query with a name, the speed will be faster. Of course, check the faster, the speed will slow insertion. Because while inserting data, but also need to maintain an index.

4, query index

• Information can use the data dictionary views USER_INDEXES and USER_IND_COLUMNS View index

SELECT	ic.index_name, ic.column_name,
	    ic.column_position col_pos,ix.uniqueness
FROM	user_indexes ix, user_ind_columns ic
WHERE	ic.index_name = ix.index_name
AND	ic.table_name = 'EMP';

5, remove the index

• Use DROP INDEX command to delete the index

DROP INDEX emp_last_name_idx;

• Only the owner of the index or have DROP ANY INDEX user privileges can delete the index, the delete operation can not be rolled back

6, synonyms -synonym

• Access the same object using synonyms:
  easy access to other user objects
  shortening the length of the name of the object

CREATE SYNONYM e FOR emp;
select * from e;

7, create and delete synonyms

• Create a synonym for the view DEPT_SUM_VU

CREATE SYNONYM  d_sum
FOR  dept_sum_vu;

• Delete Synonym

DROP SYNONYM d_sum;