position
orders rows based on their value for the expression
in this position of the select list.
ASC
DESC
specifies either ascending or descending order. ASC
is the default.
FOR UPDATE
locks the selected rows.
NOWAIT
returns control to you if the SELECT statement attempts to lock a
row that is locked by another user. If you omit this clause, ORACLE
waits until the row is available and then returns the results of the
SELECT statement.
PREREQUISITES:
For you to select data from a table or snapshot, the table or
snapshot must be in your own schema or you must have SELECT
privilege on the table or snapshot.
For you to select rows from the base tables of a view, the owner of
the schema containing the view must have SELECT privilege on the
base tables. Also, if the view is in a schema other than your own,
you must have SELECT privilege on the view.
The SELECT ANY TABLE system privilege also allows you to select data
from any table or any snapshot or any view"s base table.
If you are using Trusted ORACLE in DBMS MAC mode, your DBMS label
must dominate the creation label of each queried table, view, or
snapshot or you must have READUP system privileges.
SEE:
DELETE, UPDATE
CREATE TABLE command
PURPOSE:
To create a table, the basic structure to hold user data, specifying
this information:
* column definitions
* integrity constraints
* the table"s tablespace
* storage characteristics
* an optional cluster
* data from an arbitrary query
SYNTAX:
CREATE TABLE [schema.]table
( { column datatype [DEFAULT expr] [column_constraint] ...
| table_constraint}
[, { column datatype [DEFAULT expr] [column_constraint] ...
| table_constraint} ]...)
[ [PCTFREE integer] [PCTUSED integer]
[INITRANS integer] [MAXTRANS integer]
[TABLESPACE tablespace]
[STORAGE storage_clause]
| [CLUSTER cluster (column [, column]...)] ]
[ ENABLE enable_clause
| DISABLE disable_clause ] ...
[AS subquery]
where:
schema
is the schema to contain the table. If you omit schema, ORACLE
creates the table in your own schema.
table
is the name of the table to be created.
column
specifies the name of a column of the table. The number of columns
in a table can range from 1 to 254.
datatype
is the datatype of a column.
DEFAULT
specifies a value to be assigned to the column if a subsequent
INSERT statement omits a value for the column. The datatype of the
expression must match the datatype of the column. A DEFAULT
expression cannot contain references to other columns, the
pseudocolumns CURRVAL, NEXTVAL, LEVEL, and ROWNUM, or date constants
that are not fully specified.
column_constraint
defines an integrity constraint as part of the column definition.
table_constraint
defines an integrity constraint as part of the table definition.
PCTFREE
specifies the percentage of space in each of the table"s data blocks
reserved for future updates to the table"s rows. The value of
PCTFREE must be a positive integer from 1 to 99. A value of 0
allows the entire block to be filled by inserts of new rows. The
default value is 10. This value reserves 10% of each block for
updates to existing rows and allows inserts of new rows to fill a
maximum of 90% of each block.
PCTFREE has the same function in the commands that create and alter
clusters, indexes, snapshots, and snapshot logs. The combination of
PCTFREE and PCTUSED determines whether inserted rows will go into
existing data blocks or into new blocks.
PCTUSED
specifies the minimum percentage of used space that ORACLE maintains
for each data block of the table. A block becomes a candidate for
row insertion when its used space falls below PCTUSED. PCTUSED is
specified as a positive integer from 1 to 99 and defaults to 40.
PCTUSED has the same function in the commands that create and alter
clusters, snapshots, and snapshot logs.
The sum of PCTFREE and PCTUSED must be less than 100. You can use
PCTFREE and PCTUSED together use space within a table more
efficiently.
INITRANS
specifies the initial number of transaction entries allocated within
each data block allocated to the table. This value can range from 1
to 255 and defaults to 1. In general, you should not change the
INITRANS value from its default.
Each transaction that updates a block requires a transaction entry
in the block. The size of a transaction entry depends on your
operating system.
This parameter ensures that a minimum number of concurrent
transactions can update the block and helps avoid the overhead of
dynamically allocating a transaction entry.
The INITRANS parameter serves the same purpose in clusters, indexes,
snapshots, and snapshot logs as in tables. The minimum and default
INITRANS value for a cluster or index is 2, rather than 1.
MAXTRANS
specifies the maximum number of concurrent transactions that can
update a data block allocated to the table. This limit does not
apply to queries. This value can range from 1 to 255 and the
default is a function of the data block size. You should not change
the MAXTRANS value from its default.
If the number concurrent transactions updating a block exceeds the
INITRANS value, ORACLE dynamically allocates transaction entries in
the block until either the MAXTRANS value is exceeded or the block
has no more free space.
The MAXTRANS parameter serves the same purpose in clusters,
snapshots, and snapshot logs as in tables.
TABLESPACE
specifies the tablespace in which ORACLE creates the table. If you
omit this option, then ORACLE creates the table in the default
tablespace of the owner of the schema containing the table.
STORAGE
specifies the storage characteristics for the table. This clause
has performance ramifications for large tables. Storage should be
allocated to minimize dynamic allocation of additional space.
CLUSTER
specifies that the table is to be part of the cluster. The columns
listed in this clause are the table columns that correspond to the
cluster"s columns. Generally, the cluster columns of a table are
the column or columns that comprise its primary key or a portion of
its primary key.
Specify one column from the table for each column in the cluster
key. The columns are matched by position, not by name. Since a
clustered table uses the cluster"s space allocation, do not use the
PCTFREE, PCTUSED, INITRANS, or MAXTRANS parameters, the TABLESPACE
option, or the STORAGE clause in conjunction with the CLUSTER
option.
ENABLE
enables an integrity constraint.
DISABLE
disables an integrity constraint.
Constraints specified in the ENABLE and DISABLE clauses of a CREATE
TABLE statement must be defined in the statement. You can also
enable and disable constraints with the ENABLE and DISABLE keywords
of the CONSTRAINT clause. If you define a constraint but do not
explicitly enable or disable it, ORACLE enables it by default.
You cannot use the ENABLE and DISABLE clauses in a CREATE TABLE
statement to enable and disable triggers.
AS subquery
inserts the rows returned by the subquery into the table upon its
creation.
If you include this clause, the column definitions can only specify
column names, default values, and integrity constraints, not
datatypes. ORACLE derives column datatypes and lengths from the
subquery. ORACLE also automatically defines NOT NULL constraints on
columns in the new table if they existed on the corresponding
columns of the selected table and the subquery does not modify the
column value with a SQL function or operator. A CREATE TABLE
statement cannot contain both the AS clause and a referential
integrity constraint definition.
The number of columns must equal the number of expressions in the
subquery. If all expressions in the subquery are columns, you can
omit the columns from the table definition entirely. In this case,
the names of the columns of table are the same as the columns in the
subquery.
PREREQUISITES:
To create a table in your own schema, you must have CREATE TABLE
system privilege. To create a table in another user"s schema, you
must have CREATE ANY TABLE system privilege. Also, the owner of the
schema to contain the table must have either space quota on the
tablespace to contain the table or UNLIMITED TABLESPACE system
privilege.
SEE:
ALTER TABLE, CONSTRAINT, CREATE CLUSTER, CREATE INDEX, CREATE
TABLESPACE, DISABLE, DROP TABLE, ENABLE, STORAGE
CONSTRAINT clause
PURPOSE:
To define an integrity constraint. An integrity constraint is a
rule that restricts the values for one or more columns in a table.
SYNTAX:
Column constraint:
[CONSTRAINT constraint]
{ [NOT] NULL
| {UNIQUE | PRIMARY KEY}
| REFERENCES [schema.]table [(column)]
[ON DELETE CASCADE]
| CHECK (condition) }
{ [ USING INDEX [PCTFREE integer]
[INITRANS integer] [MAXTRANS integer]
[TABLESPACE tablespace]
[STORAGE storage_clause] ]
[ EXCEPTIONS INTO [schema.]table
| DISABLE }
Table constraint:
[CONSTRAINT constraint]
{ {UNIQUE | PRIMARY KEY} (column [,column] ...)
| FOREIGN KEY (column [,column] ...)
REFERENCES [schema.]table [(column [,column] ...)]
[ON DELETE CASCADE]
| CHECK (condition) }
{ [ USING INDEX [PCTFREE integer]
[INITRANS integer] [MAXTRANS integer]
[TABLESPACE tablespace]
[STORAGE storage_clause] ]
[ EXCEPTIONS INTO [schema.]table[@dblink]
| DISABLE }
where:
CONSTRAINT
identifies the integrity constraint by the name constraint. ORACLE
stores this name in the data dictionary along with the definition of
the integrity constraint. If you omit this identifier, ORACLE
generates a name with this form:
SYS_Cn
where
n
is an integer that makes the name unique
within the database.
For the names and definitions of integrity constraints, query the
data dictionary.
NULL
specifies that a column can contain null values.
NOT NULL
specifies that a column cannot contain null values.
If you do not specify NULL or NOT NULL in a column definition, NULL
is the default.
UNIQUE
designates a column or combination of columns as a unique key.
PRIMARY KEY
designates a column or combination of columns as the table"s primary
key.
FOREIGN KEY
designates a column or combination of columns as the foreign key in
a referential integrity constraint.
REFERENCES
identifies the primary or unique key that is referenced by a foreign
key in a referential integrity constraint.
ON DELETE CASCADE
specifies that ORACLE maintains referential integrity by
automatically removing dependent foreign key values if you remove a
referenced primary or unique key value.
CHECK
specifies a condition that each row in the table must satisfy.
USING INDEX
specifies parameters for the index ORACLE uses to enforce a UNIQUE
or PRIMARY KEY constraint. The name of the index is the same as the
name of the constraint. You can choose the values of the INITRANS,
MAXTRANS, TABLESPACE, STORAGE, and PCTFREE parameters for the index.
For information on these parameters, see the CREATE TABLE command.
Only use this clause when enabling UNIQUE and PRIMARY KEY
constraints.
EXCEPTIONS INTO
identifies a table into which ORACLE places information about rows
that violate an enabled integrity constraint. This table must exist
before you use this option. If you omit schema, ORACLE assumes the
exception table is in your own schema. The exception table must be
on your local database.
DISABLE
disables the integrity constraint. If an integrity constraint is
disabled, ORACLE does not enforce it.
If you do not specify this option, ORACLE automatically enables the
integrity constraint.
You can also enable and disable integrity constraints with the
ENABLE and DISABLE clauses of the CREATE TABLE and ALTER TABLE
commands.
PREREQUISITES:
CONSTRAINT clauses can appear in either CREATE TABLE or ALTER TABLE
commands. To define an integrity constraint, you must have the
privileges necessary to issue one of these commands. See the CREATE
TABLE and ALTER TABLE commands.
Defining a constraint may also require additional privileges or
preconditions that depend on the type of constraint.
SEE:
ALTER TABLE, CREATE TABLE, DISABLE, ENABLE
Example: To define the table staff, as user scott, you could enter:
CREATE TABLE staff (
empno NUMBER NOT NULL PRIMARY KEY,
ename CHAR(20) NOT NULL CHECK (ename = UPPER),
job CHAR(10),
mgr NUMBER REFERENCES scott.staff(empno),
hiredate DATE CHECK (hiredate >= SYSDATE - 7),
sal NUMBER(10,2) CHECK (sal > 800),
comm NUMBER(9,2) DEFAULT NULL,
deptno NOT NULL REFERENCES scott.dept(deptno)
)
PCTFREE 5 PCTUSED 75
نوشته شده توسط
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94/10/16:: 3:27 عصر
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