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SQL-Schema Statements
SQL-Schema Statements provide maintenance of catalog objects for a schema --
tables, views and privileges.
A D V E R T I S E M E N T
This subset of SQL is also called the Data
Definition Language for SQL (SQL DDL).
There are 6 SQL-Schema Statements:
- CREATE TABLE
Statement -- create a new base table in the current schema
- CREATE VIEW
Statement -- create a new view table in the current schema
- DROP TABLE
Statement -- remove a base table from the current schema
- DROP VIEW
Statement -- remove a view table from the current schema
- GRANT Statement
-- grant access privileges for objects in the current schema to other users
- REVOKE Statement
-- revoke previously granted access privileges for objects in the current
schema from other users
Schema Overview
A relational database contains a catalog that describes the various
elements in the system. The catalog divides the database into sub-databases
known as schemas. Within each schema are database objects -- tables, views and
privileges.
The catalog itself is a set of tables with its own schema name -
definition_schema. Tables in the catalog cannot be modified directly. They
are modified indirectly with SQL-Schema statements.
Tables
The database table is the root structure in the relational model and in SQL. A
table (called a relation in relational) consists of rows and columns. In
relational, rows are called tuples and columns are called attributes.
Tables are often displayed in a flat format, with columns arrayed horizontally
and rows vertically:
Database tables are a logical structure with no implied physical
characteristics. Primary among the various logical tables is the base
table. A base table is persistent and self contained, that is, all data is part
of the table itself with no information dynamically derived from other
tables.
A table has a fixed set of columns. The columns in a base table are not
accessed positionally but by name, which must be unique among the columns of the
table. Each column has a defined
data type, and the value
for the column in each row must be from the defined data type or null.
The columns of a table are accessed and identified by name.
A table has 0 or more rows. A row in a base table has a value or null
for each column in the table. The rows in a table have no defined ordering and
are not accessed positionally. A table row is accessed and identified by the
values in its columns.
In SQL92, base tables can have duplicate rows (rows where each column has the
same value or null). However, the relational model does not recognize
tables with duplicate rows as valid base tables (relations). The
relational model requires that each base table have a unique identifier, known
as the Primary Key. The primary key for a table is a designated set of
columns which have a unique value for each table row. For a discussion of
Primary Keys, see Entity
Integrity under CREATE TABLE below.
A base table is defined using the
CREATE TABLE Statement.
This statement places the table description in the catalog and initializes an
internal entity for the actual representation of the base table.
Example base table - s:
| sno
|
name
|
city
|
| S1
|
Pierre
|
Paris
|
| S2
|
John
|
London
|
| S3
|
Mario
|
Rome
|
The s table records suppliers. It has 3 defined columns:
- sno -- supplier number, an unique identifier that is the primary key
- name -- the name of the supplier
- city -- the city where the supplier is located
At the current time, there are 3 rows.
Other types of tables in the system are derived tables. SQL-Data
statements use internally derived tables in computing results. A query is in
fact a derived table. For instance, the query operator - Union, combines two
derived tables to produce a third one. Much of the power of SQL comes from the
fact that its higher level operations are performed on tables and produce a
table as their result.
Derived tables are less constrained than base tables. Column names are not
required and need not be unique. Derived tables may have duplicate rows.
Views are a type of derived table that are cataloged in the database. See
Views below.
Views
A view is a derived table registered in the catalog. A view is defined using a
SQL query. The view is dynamically derived, that is, its contents are
materialized for each use. Views are added to the catalog with the
CREATE VIEW Statement.
Once defined in the catalog, a view can substitute for a table in SQL-Data
statements. A view name can be used instead of a base table name in the FROM
clause of a SELECT statement. Views can also be the subject of a modification
statement with some restrictions.
A SQL Modification Statement
can operate on a view if it is an updatable view. An updatable view has
the following restrictions on its defining query:
- The query FROM clause can reference a single table (or view)
- The single table in the FROM clause must be:
- a base table,
- a view that is also an updatable view, or
- a nested query that is updatable, that is, it follows the rules for
an updatable view query.
- The query must be a basic query, not a:
- The select list cannot contain:
- the DISTINCT specifier,
- an Expression,
or
- duplicate column references
Subqueries are acceptable
in updatable views but cannot reference the underlying base table for the view's
FROM clause.
Privileges
SQL92 defines a SQL-agent as an implementation-dependent entity that
causes the execution of SQL statements. Prior to execution of SQL statements,
the SQL-agent must establish an authorization identifier for database
access. An authorization identifier is commonly called a user name.
A DBMS user may access database objects (tables, columns, views) as allowed
by the privileges assigned to that specific authorization identifier.
Access privileges may be granted by the system (automatic) or by other users.
System granted privileges include:
- All privileges on a table to the user that created the table.
This includes the privilege to grant privileges on the table to other
users.
- SELECT (readonly) privilege on the catalog (the tables in the schema -
definition_schema). This is granted to all users.
User granted privileges cover privileges to access and modify tables and their
columns. Privileges can be granted for specific SQL-Data Statements --
SELECT,
INSERT,
UPDATE,
DELETE.
CREATE TABLE Statement
The CREATE TABLE Statement creates a new base table. It adds the table
description to the catalog. A base table is a logical entity with persistence.
The logical description of a base table consists of:
- Schema -- the logical database schema the table resides in
- Table Name -- a name unique among tables and views in the Schema
- Column List -- an ordered list of column declarations (name,
data type)
- Constraints
-- a list of constraints on the contents of the table
The CREATE TABLE Statement has the following general format:
CREATE TABLE table-name ({column-descr|constraint} [,{column-descr|constraint}]...)
table-name is the new name for the table. column-descr is a column
declaration. constraint is a table constraint.
The column declaration can include optional column constraints. The
declaration has the following general format:
column-name is the name of the column and must be unique among the
columns of the table. data-type declares the type of the column. Data
types are described below.
column-constraints is an optional list of column constraints with no
separators.
Constraints
Constraint specifications add additional restrictions on the contents of the
table. They are automatically enforced by the DBMS. The column
constraints are:
- NOT NULL -- specifies that the column can't be set to null. If
this constraint is not specified, the column is nullable, that is, it
can be set to null. Normally, primary key columns are declared as NOT
NULL.
- PRIMARY KEY -- specifies that this column is the only column in the
primary key. There can be only one primary key declaration in a CREATE
TABLE. For primary keys with multiple columns, use the
PRIMARY KEY table
constraint. See Entity
Integrity below for a detailed description of primary keys.
- UNIQUE -- specifies that this column has a unique value or null
for all rows of the table.
- REFERENCES -- specifies that this column is the only column in a foreign
key. For foreign keys with multiple columns, use the
FOREIGN KEY table
constraint. See
Referential Integrity below for a detailed description of primary keys.
- CHECK -- specifies a user defined constraint on the table. See the table
constraint - CHECK,
below.
The table constraints are:
- PRIMARY KEY -- specifies the set of columns that
comprise the primary key. There can be only one primary key declaration in a
CREATE TABLE Statement. See
Entity Integrity
below for a detailed description of primary keys.
- UNIQUE -- specifies that a set of columns have unique values (or
nulls) for all rows in the table. The UNIQUE specifier is followed by a
parenthesized list of column names, separated by commas.
- FOREIGN KEY -- specifies the set of columns in a
foreign key. See
Referential Integrity below for a detailed description of foreign keys.
- CHECK -- specifies a user defined constraint, known
as a check condition. The CHECK specifier is followed by a predicate
enclosed in parentheses. For Intermediate Level SQL92, the CHECK predicate
can only reference columns from the current table row, with no subqueries.
Many DBMSs support subqueries in the check predicate.
The check predicate must evaluate to not False (that is, the
result must be True or Unknown) before a modification or addition of a row
takes place. The check is effectively made on the contents of the table
after the modification. For
INSERT Statements,
the predicate is evaluated as if the INSERT row were added to the table. For
UPDATE Statements,
the predicate is evaluated as if the row were updated. For
DELETE Statements,
the predicate is evaluated as if the row were deleted (Note: A check
predicate is only useful for DELETE if a self-referencing subquery is used.)
Data Type
This subsection describes data type specifications. The data type
categories are:
- Character (String) -- fixed or variable length character strings. The
character set is implementation defined but often defaults to ASCII.
- Numeric -- values representing numeric quantities. Numeric values are
divided into these two broad categories:
- Exact (also known as fixed-point) -- Exact numeric values
have a fixed number of digits to the left of the decimal point and a
fixed number of digits to the right (the scale). The total number of
digits on both sides of the decimal are the precision. A special subset
of exact numeric types with a scale of 0 is called integer.
- Approximate (also known as floating-point) -- Approximate
numeric values that have a fixed precision (number of digits) but a
floating decimal point.
All numeric types are signed.
- Datetime -- Datetime values include calendar and clock values (Date,
Time, Timestamp) and intervals. The datetime types are:
- Date -- calendar date with year, month and day
- Time -- clock time with hour, minute, second and fraction of second,
plus a timezone component (adjustment in hours, minutes)
- Timestamp -- combination calendar date and clock time with year,
month, day, hour, minute, second and fraction of second, plus a timezone
component (adjustment in hours, minutes)
- Interval -- intervals represent time and date intervals. They are
signed. An interval value can contain a subset of the interval fields,
for example - hour to minute, year, day to second. Interval types are
subdivided into:
- year-month intervals -- may contain years, months or combination
years/months value.
- day-time intervals -- days, hours, minutes, seconds, fractions
of second.
Data type declarations have the following general format:
Character (String)
CHAR [(length)]
CHARACTER [(length)]
VARCHAR (length)
CHARACTER VARYING (length)
length specifies the number of characters for fixed size
strings (CHAR, CHARACTER); spaces are supplied for shorter strings. If
length is missing for fixed size strings, the default length is
1. For variable size strings (VARCHAR, CHARACTER VARYING), length
is the maximum size of the string. Strings exceeding length are
truncated on the right.
Numeric
SMALLINT
INT
INTEGER
The integer types have default binary precision -- 15 for SMALLINT
and 31 for INT, INTEGER.
NUMERIC ( precision [, scale] )
DECIMAL ( precision [, scale] )
Fixed point types have a decimal precision (total number of digits)
and scale (which cannot exceed the precision). The default scale is 0.
NUMERIC scales must be represented exactly. DECIMAL values can be stored
internally with a larger scale (implementation defined).
FLOAT [(precision)]
REAL
DOUBLE
The floating point types have a binary precision (maximum significant
binary digits). Precision values are implementation dependent for REAL
and DOUBLE, although the standard states that the default precision for
DOUBLE must be larger than for REAL. FLOAT also uses an
implementation defined default for precision (commonly this is the same
as for REAL), but the binary precision for FLOAT can be explicit.
Datetime
DATE
TIME [(scale)] [WITH TIME ZONE]
TIMESTAMP [(scale)] [WITH TIME ZONE]
TIME and TIMESTAMP allow an optional seconds fraction (scale).
The default scale for TIME is 0, for TIMESTAMP 6. The optional
WITH TIME ZONE specifier indicates that the timezone adjustment is
stored with the value; if omitted, the current system timezone is
assumed.
INTERVAL
interval-qualifier
See below for a
description of the interval-qualifier.
Interval Qualifier
An interval qualifier defines the specific type of an interval value. The
qualifier for an interval type declares the sub-fields that comprise the
interval, the precision of the highest (left-most) sub-field and the scale of
the SECOND sub-field (if any).
Intervals are divided into sub-types -- year-month intervals and day-time
intervals. Year-month intervals can only contain the sub-fields - year and
month. Day-time intervals can contain day, hour, minute, second. The interval
qualifier has the following formats:
YEAR [(precision)] [ TO MONTH ]
MONTH [(precision)]
{DAY|HOUR|MINUTE} [(precision)] [ TO SECOND [(scale)] ]
DAY [(precision)] [ TO {HOUR|MINUTE} ]
HOUR [(precision)] [ TO MINUTE ]
SECOND [ (precision [, scale]) ]
The default precision is 2. The default scale is 6.
Entity Integrity
As mentioned earlier, the relational model requires that each base table have a
Primary Key. SQL92, on the other hand, allows a table to created without a
primary key. The advice here is to create all tables with primary keys.
A primary key is a constraint on the contents of a table. In relational
terms, the primary key maintains Entity Integrity for the table. It
constrains the table as follows,
- For a given row, the set of values for the primary key columns must be
unique from all other rows in the table,
- No primary key column can contain a null, and
- A table can have only one primary key (set of primary key columns).
Note: SQL92 does not require the second restriction on nulls in the
primary key. However, it is required for a relational system.
Entity Integrity (Primary Keys) is enforced by the DBMS and ensures
that every row has a proper unique identifier. The contents of any column in the
table with Entity Integrity can be uniquely accessed with 3 pieces of
information:
- table identifier
- primary key value
- column name
This capability is crucial to a relational system. Having a clear, consistent
identifier for table rows (and their columns) distinguishes relational systems
from all others. It allows the establishment of relationships between tables,
also crucial to relational systems. This is discussed below under
Referential Integrity.
The primary key constraint in the CREATE STATEMENT has two forms. When the
primary key consists of a single column, it can be declared as a column
constraint, simply - PRIMARY KEY, attached to the column descriptor. For
example:
sno VARCHAR(5) NOT NULL PRIMARY KEY
As a table constraint, it has the following format:
PRIMARY KEY ( column-1 [, column-2] ...)
column-1 and column-2 are the names of the columns of the primary
key. For example,
The order of columns in the primary key is not significant, except as the
default order for the FOREIGN KEY table constraint, See
Referential Integrity,
below.
Referential Integrity
Foreign keys provide relationships between tables in the database. In
relational, a foreign key in a table is a set of columns that reference the
primary key of another table. For each row in the referencing table, the foreign
key must match an existing primary key in the referenced table. The enforcement
of this constraint is known as Referential Integrity.
Referential Integrity requires that:
- The columns of a foreign key must match in number and type the columns
of the primary key in the referenced table.
- The values of the foreign key columns in each row of the referencing
table must match the values of the corresponding primary key columns for a
row in the referenced table.
The one exception to the second restriction is when the foreign key columns for
a row contain nulls. Since primary keys should not contain nulls,
a foreign key with nulls cannot match any row in the referenced table.
However, a row with a foreign key where any foreign key column contains null
is allowed in the referencing table. No corresponding primary key value
in the referenced table is required when any one (or more) of the foreign
key columns is null. Other columns in the foreign key may be null
or non-null. Such a foreign key is a null reference, because it does not
reference any row in the referenced table.
Like other constraints, the referential integrity constraint restricts
the contents of the referencing table, but it also may in effect restrict the
contents of the referenced table. When a row in a table is referenced
(through its primary key) by a foreign key in a row in another table, operations
that affect its primary key columns have side-effects and may restrict the
operation. Changing the primary key of or deleting a row which has referencing
foreign keys would violate the referential integrity constraints on the
referencing table if allowed to proceed. This is handled in two ways,
- The referenced table is restricted from making the change (and violating
referential integrity in the referencing table), or
- Rows in the referencing table are modified so the referential integrity
constraint is maintained.
These actions are controlled by the referential integrity effects
declarations, called referential triggers by SQL92. The referential integrity
effect actions defined for SQL are:
- NO ACTION -- the change to the referenced (primary key) table is not
performed. This is the default.
- CASCADE -- the change to the referenced table is propagated to the
referencing (foreign key) table.
- SET NULL -- the foreign key columns in the referencing table are set to
null.
Update and delete have separate action declarations. For CASCADE, update and
delete also operate differently:
- For update (the primary key column values have been modified), the
corresponding foreign key columns for referencing rows are set to the new
values.
- For delete (the primary key row is deleted), the referencing rows are
deleted.
A referential integrity constraint in the CREATE STATEMENT has two forms. When
the foreign key consists of a single column, it can be declared as a column
constraint, like:
column-descr REFERENCES references-specification
As a table constraint, it has the following format:
FOREIGN KEY (column-list) REFERENCES references-specification
column-list is the referencing table columns that comprise the foreign
key. Commas separate column names in the list. Their order must match the
explicit or implicit column list in the references-specification.
The references-specification has the following format:
table-2 [ ( referenced-columns ) ]
[ ON UPDATE { CASCADE | SET NULL | NO ACTION }]
[ ON DELETE { CASCADE | SET NULL | NO ACTION }]
The order of the ON UPDATE and ON DELETE clauses may be reversed. These
clauses declare the effect action when the referenced primary key is updated or
deleted. The default for ON UPDATE and ON DELETE is NO ACTION.
table-2 is the referenced table name (primary key table). The optional
referenced-columns list the columns of the referenced primary key. Commas
separate column names in the list. The default is the primary key list in
declaration order.
Contrary to the relational model, SQL92 allows foreign keys to reference any
set of columns declared with the UNIQUE constraint in the referenced
table (even when the table has a primary key). In this case, the
referenced-columns list is required.
Example table constraint for referential integrity (for the sp table):
FOREIGN KEY (sno)
REFERENCES s(sno)
ON DELETE NO ACTION
ON UPDATE CASCADE
CREATE TABLE Examples
Creating the example tables:
CREATE TABLE s
(sno VARCHAR(5) NOT NULL PRIMARY KEY,
name VARCHAR(16),
city VARCHAR(16)
)
CREATE TABLE p
(pno VARCHAR(5) NOT NULL PRIMARY KEY,
descr VARCHAR(16),
color VARCHAR(8)
)
CREATE TABLE sp
(sno VARCHAR(5) NOT NULL REFERENCES s,
pno VARCHAR(5) NOT NULL REFERENCES p,
qty INT,
PRIMARY KEY (sno, pno)
)
Create for sp with a constraint that the qty column can't be negative:
CREATE TABLE sp
(sno VARCHAR(5) NOT NULL REFERENCES s,
pno VARCHAR(5) NOT NULL REFERENCES p,
qty INT CHECK (qty >= 0),
PRIMARY KEY (sno, pno)
)
CREATE VIEW Statement
The CREATE VIEW statement creates a new database view. A view is effectively a
SQL query stored in the catalog. The CREATE VIEW has the following general
format:
CREATE VIEW view-name [ ( column-list ) ] AS query-1
[ WITH [CASCADED|LOCAL] CHECK OPTION ]
view-name is the name for the new view. column-list is an optional
list of names for the columns of the view, comma separated. query-1 is
any SELECT statement without an ORDER BY clause. The optional WITH CHECK OPTION
clause is a constraint on updatable views.
column-list must have the same number of columns as the select list in
query-1. If column-list is omitted, all items in the select list
of query-1 must be named. In either case, duplicate column names are not
allowed for a view.
The optional WITH CHECK OPTION clause only applies to updatable views.
It affects SQL INSERT and UPDATE statements. If WITH CHECK OPTION is specified,
the WHERE predicate for query-1 must evaluate to true for the added row
or the changed row.
The CASCADED and LOCAL specifiers apply when the underlying table for
query-1 is another view. CASCADED requests that WITH CHECK OPTION apply to
all underlying views (to any level.) LOCAL requests that the current WITH
CHECK OPTION apply only to this view. LOCAL is the default.
CREATE VIEW Examples
Parts with suppliers:
CREATE VIEW supplied_parts AS
SELECT *
FROM p
WHERE pno IN (SELECT pno FROM sp)
WITH CHECK OPTION
Access example:
SELECT * FROM supplied_parts
| pno
|
descr
|
color
|
| P1
|
Widget
|
Red
|
| P2
|
Widget
|
Blue
|
Joined view:
CREATE VIEW part_locations (part, quantity, location) AS
SELECT pno, qty, city
FROM sp, s
WHERE sp.sno = s.sno
Access examples:
SELECT * FROM part_locations
| part
|
quantity
|
location
|
| P1
|
NULL
|
Paris
|
| P1
|
200
|
London
|
| P1
|
1000
|
Rome
|
| P2
|
200
|
Rome
|
SELECT part, quantity
FROM part_locations
WHERE location = 'Rome'
| part
|
quantity
|
| P1
|
1000
|
| P2
|
200
|
DROP TABLE Statement
The DROP TABLE Statement removes a previously created table and its description
from the catalog. It has the following general format:
DROP TABLE table-name {CASCADE|RESTRICT}
table-name is the name of an existing base table in the current schema.
The CASCADE and RESTRICT specifiers define the disposition of other objects
dependent on the table. A base table may have two types of dependencies:
- A view whose query specification references the drop table.
- Another base table that references the drop table in a constraint
- a CHECK constraint or REFERENCES constraint.
RESTRICT specifies that the table not be dropped if any dependencies exist. If
dependencies are found, an error is returned and the table isn't dropped.
CASCADE specifies that any dependencies are removed before the drop is
performed:
- Views that reference the base table are dropped, and the sequence is
repeated for their dependencies.
- Constraints in other tables that reference this table are dropped; the
constraint is dropped but the table retained.
DROP VIEW Statement
The DROP VIEW Statement removes a previously created view and its description
from the catalog. It has the following general format:
DROP VIEW view-name {CASCADE|RESTRICT}
view-name is the name of an existing view in the current schema. The
CASCADE and RESTRICT specifiers define the disposition of other objects
dependent on the view. A view may have two types of dependencies:
- A view whose query specification references the drop view.
- A base table that references the drop view in a constraint - a
CHECK constraint.
RESTRICT specifies that the view not be dropped if any dependencies exist. If
dependencies are found, an error is returned and the view isn't dropped.
CASCADE specifies that any dependencies are removed before the drop is
performed:
- Views that reference the drop view are dropped, and the sequence
is repeated for their dependencies.
- Constraints in base tables that reference this view are dropped; the
constraint is dropped but the table retained.
GRANT Statement
The GRANT Statement grants access privileges for database objects to other
users. It has the following general format:
GRANT privilege-list ON [TABLE] object-list TO user-list
privilege-list is either ALL PRIVILEGES or a comma-separated list of
properties: SELECT, INSERT, UPDATE, DELETE. object-list is a
comma-separated list of table and view names. user-list is either PUBLIC
or a comma-separated list of user names.
The GRANT statement grants each privilege in privilege-list for each
object (table) in object-list to each user in user-list. In
general, the access privileges apply to all columns in the table or view, but it
is possible to specify a column list with the UPDATE privilege specifier:
UPDATE [ ( column-1 [, column-2] ... ) ]
If the optional column list is specified, UPDATE privileges are granted for
those columns only.
The user-list may specify PUBLIC. This is a general grant, applying to
all users (and future users) in the catalog.
Privileges granted are revoked with the
REVOKE Statement.
The optional specificier WITH GRANT OPTION may follow user-list in the
GRANT statement. WITH GRANT OPTION specifies that, in addition to access
privileges, the privilege to grant those privileges to other users is granted.
GRANT Statement Examples
GRANT SELECT ON s,sp TO PUBLIC
GRANT SELECT,INSERT,UPDATE(color) ON p TO art,nan
GRANT SELECT ON supplied_parts TO sam WITH GRANT OPTION
REVOKE Statement
The REVOKE Statement revokes access privileges for database objects previously
granted to other users. It has the following general format:
REVOKE privilege-list ON [TABLE] object-list FROM user-list
The REVOKE Statement revokes each privilege in privilege-list for each
object (table) in object-list from each user in user-list. All
privileges must have been previously granted.
The user-list may specify PUBLIC. This must apply to a previous GRANT TO
PUBLIC.
REVOKE Statement Examples
REVOKE SELECT ON s,sp FROM PUBLIC
REVOKE SELECT,INSERT,UPDATE(color) ON p FROM art,nan
REVOKE SELECT ON supplied_parts FROM sam
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