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A primary key column is identified by a primary key symbol in its row selector. If a primary key consists of more than one column, duplicate values are allowed in one column, but each combination of values from all the columns in the primary key must be unique.
In database relational modeling and implementation, a unique key (also known as a candidate key or just a key) is a set of attributes (columns) within a relational database table (also called a relation), such that:
A two-dimensional table-style collection of data in which all entries are single-valued, each column has a distinct name, all the values in a column are values of the attribute that is identified by the column name, the order of the columns is immaterial, each row is distinct, and the order of rows is immaterial. I am designing a table and I have decided to create an auto-generated primary key value as opposed to creating my own scheme or using natural keys. I see that SQL Server offers globally unique identifiers (GUIDs) as well as identities to create these valu. Mar 13, 2008 SQL Server worries about ensuring unique values so you don't have to. Avoid GUID (uniqueidentifier) datatypes since they are difficult to work with and are only necessary under certain replication scenarios. The Laws of Key Design Summary. Always add a primary key to all tables: A table without a key is not a table.
- the table does not have two distinct rows or records with the same values for these columns;
- this set of columns is minimal; i.e., removing any column from the key would result in duplicate values in the resulting subset.
When a column or set of columns is defined as unique to the database management system, the system verifies that each set of values is unique before assigning the constraint. After the columns are defined as unique, an error will occur if an insertion is attempted with values that already exist. Some systems will not allow key values to be updated, all systems will not allow duplicates. This ensures that uniqueness is maintained in both the primary table and any relations that are later bound to it.
Summary[edit]
Keys provide the means for database users and application software to identify, access and update information in a database table. There may be several keys in any given table. For example, in a table of employees, both employee number and login name are individually unique. The enforcement of a key constraint (i.e. a uniqueness constraint) in a table is also a data integrity feature of the database. The DBMS prevents updates that would cause duplicate key values and thereby ensures that tables always comply with the desired rules for uniqueness. Proper selection of keys when designing a database is therefore an important aspect of database integrity.
A relational database table may have one or more available unique keys (formally called candidate keys). One of those keys per table may be designated the primary key; other keys are called alternate keys.
Any key may consist of one or more attributes. For example, a Social Security Number might be a single attribute key for an employee; a combination of flight number and date might be a key consisting of two attributes for a scheduled flight.
There are several types of keys used in database modeling and implementations.
Key Name | Definition |
---|---|
Simple | A key made from only one attribute. |
Concatenated | A key made from more than one attribute joined together as a single key, such as part or whole name with a system generated number appended as often used for E-mail addresses. |
Compound | A key made from at least two attributes or simple keys, only simple keys exist in a compound key. |
Composite | A key containing at least one compound key with at least one other attribute or simple key (this is an extension of a compound key). |
Natural | A key made from data that exists outside the current database. In other words, the data is not system generated, such as a social security number imported from another system. |
Surrogate | An artificial key made from data that is system assigned or generated when another candidate key exists. Surrogate keys are usually numeric ID values and often used for performance reasons.[citation needed] |
Candidate | A key that may become the primary key. |
Primary | The key that is selected as the primary key. Only one key within an entity is selected to be the primary key. This is the key that is allowed to migrate to other entities to define the relationships that exist among the entities. When the data model is instantiated into a physical database, it is the key that the system uses the most when accessing the table, or joining the tables together when selecting data. |
Alternate | A non-primary key that can be used to identify only one row in a table. Alternate keys may be used like a primary key in a single-table select. |
Foreign | A key that has migrated to another entity. |
At the most basic definition, 'a key is a unique identifier',[1] so unique key is a pleonasm. Keys that are within their originating entity are unique within that entity. Keys that migrate to another entity may or may not be unique, depending on the design and how they are used in the other table. Foreign keys may be the primary key in another table; for example a PersonID may become the EmployeeID in the Employee table. In this case, the EmployeeID is both a foreign key and the unique primary key, meaning that the tables have a 1:1 relationship. In the case where the person entity contained the biological father ID, the father ID would not be expected to be unique because a father may have more than one child.
Here is an example of a primary key becoming a foreign key on a related table. ID migrates from the Author table to the Book table.
Here ID serves as the primary key in the table 'Author', but also as AuthorID serves as a Foreign Key in the table 'Book'. The Foreign Key serves as the link, and therefore the connection, between the two related tables in this sample database.
In a relational database, a candidate key uniquely identifies each row of data values in a database table. A candidate key comprises a single column or a set of columns in a single database table. No two distinct rows or data records in a database table can have the same data value (or combination of data values) in those candidate key columns since NULL values are not used. Depending on its design, a database table may have many candidate keys but at most one candidate key may be distinguished as the primary key.
A key constraint applies to the set of tuples in a table at any given point in time. A key is not necessarily a unique identifier across the population of all possible instances of tuples that could be stored in a table but it does imply a data integrity rule that duplicates should not be allowed in the database table. Some possible examples of keys are Social Security Numbers, ISBNs, vehicle registration numbers or user login names.
In principle any key may be referenced by foreign keys. Some SQL DBMSs only allow a foreign key constraint against a primary key but most systems will allow a foreign key constraint to reference any key of a table.
Defining keys in SQL[edit]
The definition of keys in SQL:
Likewise, keys can be defined as part of the
CREATE TABLE
SQL statement.Differences between Primary Key constraint and Unique constraint[edit]
Primary Key constraint
- A primary key cannot allow null (a primary key cannot be defined on columns that allow nulls).
- Each table cannot have more than one primary key.
- On some RDBMS a primary key generates a clustered index by default.
Unique constraint
- A unique constraint can be defined on columns that allow nulls, in which case rows that include nulls may not actually be unique across the set of columns defined by the constraint.
- Each table can have multiple unique constraints.
- On some RDBMS a unique constraint generates a nonclustered index by default.
Note that unlike the PRIMARY KEY constraint a UNIQUE constraint does not imply NOT NULL for the columns participating in the constraint. NOT NULL must be specified to make the column(s) a key. It is possible to put UNIQUE constraints on nullable columns but the SQL standard states that the constraint does not guarantee uniqueness of nullable columns (uniqueness is not enforced for rows where any of the columns contains a null).
According to the SQL[2] standard a unique constraint does not enforce uniqueness in the presence of nulls and can therefore contain several rows with identical combinations of nulls and non-null values — however not all RDBMS implement this feature according to the SQL standard.[3][4]
See also[edit]
References[edit]
- ^Awad, Elias (1985), Systems Analysis and Design, Second Edition, Richard D. Irwin, Inc., ISBN0-256-02824-9
- ^Summary of ANSI/ISO/IEC SQLArchived April 25, 2012, at the Wayback Machine
- ^'Constraints - SQL Database Reference Material - Learn sql, read an sql manual, follow an sql tutorial, or learn how to structure an SQL query!'. www.sql.org. Retrieved 16 August 2018.
- ^'Comparison of different SQL implementations'. troels.arvin.dk. Retrieved 16 August 2018.
External links[edit]
- Relation Database terms of reference, Keys: An overview of the different types of keys in an RDBMS
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Unique_key&oldid=948557350'
Oracle8 Concepts Release 8.0 A58227-01 |
|
It is not enough to have a good mind. The main thing is to use it well.
René Descartes, Le Discours de la Méthode
This chapter covers the main concepts you need to understand to use user-defined datatypes. It contains the following major sections:
References and Name Resolution
Oracle SQL is designed to be easy to use. For example, if
projects
is a table with a column called assignment
, and depts
is a table that does not contain a column called assignment
, you can write Oracle determines which table each column belongs to. You can, but don't have to, qualify the column names with table names:
You can, but don't have to, qualify the column names with table aliases:
Table Aliases
The first form of the SELECT statement above is the easiest to write and understand, but it can lead to undesired results if you later add an
assignment
column to the depts
table and forget to change the query. Oracle automatically recompiles the query and the new version uses the assignment
column from the depts
table. This situation is called inner capture. In order to avoid inner capture and similar misinterpretations of the intended meanings of SQL statements, Oracle requires you to use table aliases to qualify references to methods or attributes of objects. This also applies to attribute references via REFs. This requirement is called the capture avoidance rule.
For example, consider the following statements:
These define an object type
person
and two tables. The first is an object table for objects of type person
. The second has a single column of type person
. Now consider the following queries:
- In the first SELECT statement,
ssno
is the name of a column ofptab1
. No further qualification is required. - In the second SELECT statement,
ssno
is the name of an attribute of theperson
object in the column namedc1
. This reference requires a table alias. - The third SELECT statement is the same as the second, but contains the required table alias,
p
.
Qualifying references to object attributes with table names rather than table aliases, even if the table names are further qualified by schema names, does not satisfy this requirement.
For example, you cannot, in a query, use the expression
to refer to the
duedate
attribute of the assignment
column of the projects
table of the scott
schema. Table aliases should be unique throughout a query and should not be the same as schema names that could legally appear in the query.
Note: Oracle recommends that you define table aliases in all UPDATE, DELETE, and SELECT statements and subqueries and use them to qualify column references, whether or not the columns contain object types. |
Method Calls without Arguments
Methods are functions or subroutines. The proper syntax for invoking them uses parentheses following the method name to enclose any calling arguments. In order to avoid ambiguities, Oracle requires empty parentheses for method calls that do not have arguments.
For example, if
tb
is a table with column c
of object type t
, and t
has a method m that does not take arguments, the following query illustrates the correct syntax: This differs from the rules for PL/SQL functions and procedures, where the parentheses are optional for calls that have no arguments.
Storage of User-Defined Types
Oracle stores and manages data of user-defined types in tables. It automatically and invisibly maps the complex structure of user-defined types into the simple rectangular structure of tables.
Leaf-Level Attributes
The structure of an object type is like a tree. The branches that grow from the trunk go to the attributes. If an attribute is of an object type, that branch sprouts subbranches for the attributes of the new object type.
Ultimately each branch comes to an end at an attribute that is of a built-in type or a collection type. These are called leaf-level attributes of the original object type. Oracle provides a table column for each leaf-level attribute.
The leaf-level attributes that not collection types are called the leaf-level scalar attributes of the object type.
Row Objects
In an object table, every leaf-level scalar or REF attribute has a column in which Oracle stores its actual data. This is also true of VARRAYs, unless they are too large (see 'VARRAYs' on page 12-5). Oracle stores leaf-level attributes of table types in separate tables associated with the object table. You must declare these tables as part of the object table declaration (see 'Nested Tables' on page 12-5).
Access to individual attributes of objects in an object table is simply access to columns of the table. Accessing the value of the object itself causes Oracle to invoke the default constructor for the type, using the columns of the object table as arguments. That is, Oracle supplies a copy of the object.
Oracle stores the system-generated object identifier in a hidden column. Oracle uses the object identifier to construct REFs to the object.
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Column Objects
When a table is defined with a column of an object type, Oracle invisibly adds columns to the table for the object type's leaf-level attributes. An additional column stores the NULL information of the object (that is, the atomic nulls of the top-level and the nested objects).
REFs
Oracle constructs a REF to a row object by invoking the built-in function REF on the row object. The constructed REF is made up of the object identifier, some metadata of the object table, and, optionally, the ROWID. An unscoped REF with ROWID to an object in an object table is 46 bytes in size. REFs to object views, REFs without ROWID, and scoped REFs are smaller.
The ROWID in a REF is used as a hint for efficient access. When Oracle dereferences a REF item, it uses the ROWID to choose a row; if the object identifier of the identified row matches the one in the REF, the access is successful. Otherwise, Oracle uses the index on the object identifier to identify the correct row.
The size of a REF in a column of REF type depends on the storage properties associated with the column. For example, if the column is declared as a REF WITH ROWID, Oracle stores the ROWID in the REF column; otherwise, it discards the ROWID.
If column is declared as REF with a SCOPE clause, then Oracle does not store the object table metadata and the ROWID in the column. A scoped REF is 16 bytes long.
Nested Tables
Sql System-generated Primary Key That Is Usually Hidden From Users Name
The rows of a nested table are stored in a separate storage table. You must supply a storage tablename when you define the table containing the nested table. If the table definition contains more than one table type - either in columns or in object types that appear in column definitions - you must supply a separate storage table for each.
For each nested table in the table definition, the associated storage table contains the rows of all instances of the given nested table in the rows of the parent table.
VARRAYs
All the elements of a VARRAY are stored in a single column. If the size of the array is smaller than 4000 bytes, Oracle stores it in line; if it is greater than 4000 bytes, Oracle stores it in a BLOB.
Properties of Object Attributes
Oracle allows you to specify some properties of object attributes:
Nulls
One possible property of a table column, object, object attribute, collection, or collection element is that it can be null. This means that the item has been initialized to NULL or has been left uninitialized. Usually this means that the value of the item is not yet known but might become available later.
An object whose value is NULL is called atomically null. In addition, attributes of an object can be null. These two uses of nulls are different.
For example, consider the
contacts
table defined as follows: The statement
gives a different result from
In both cases, Oracle allocates space in
contacts
for a new row and sets its date
column to the value given. In the first case, Oracle allocates space for an object in the external_person
column and sets each of its attributes to NULL. In the second case, it sets the external_person
column to NULL and does not allocate space for an object. A table row cannot be null. Therefore, Oracle does not allow you to set a row object to NULL. Similarly, a nested table of objects cannot contain an element whose value is NULL.
A nested table or array can be null. A null collection is different from an empty one, that is, a collection containing no elements.
Defaults
When you declare a table column to be of an object type or collection type, you can include a DEFAULT clause. This provides a value to use in cases where you do not explicitly specify a value for the column. The default clause must contain a literal invocation of the constructor method for that object or collection.
A literal invocation of a constructor method is defined recursively to be an invocation of the constructor method in which any arguments are either literals or literal invocations of constructor methods.
For example, consider the following statements:
The following is a literal invocation of the constructor method for the nested table type
people
: The following example shows how to use literal invocations of constructor methods to specify defaults:
Note that the term
people( )
is a literal invocation of the constructor method for an empty people
table. Constraints
You can define constraints on an object table just as you can on other tables.
You can define constraints on the leaf-level scalar attributes of a column object, with the exception of REFs that are not scoped (see 'Scoped REFs' on page 11-8).
The following examples illustrate the possibilities.
The first example places a primary key constraint on the
ssno
column of the object table person_extent
: The
department
table in the next example has a column whose type is the object type location
defined in the previous example. The example defines constraints on scalar attributes of the location
objects that appear in the dept_loc
column of the table. Indexes
You can define indexes on an object table or on the storage table for a nested table column or attribute just as you can on other tables.
You can define indexes on leaf-level scalar attributes of column objects, except that you can only define indexes on REF attributes or columns if the REF is scoped (see 'Scoped REFs' on page 11-8).
The following example defines an index on an attribute of an object column:
This code creates an index on the city attribute of the department address.
Wherever Oracle expects a column name in an index definition, you can also specify a scalar attribute of an object column.
Triggers
You can define triggers on an object table just as you can on other tables. You cannot define a trigger on the storage table for a nested table column or attribute.
You cannot modify the values of collections (or LOBs) in the code that defines a trigger action. Otherwise there are no special restrictions on using user-defined types with triggers.
The following example defines a trigger on the person_extent table defined in an earlier section:
Privileges on User-Defined Types and Their Methods
Privileges for user-defined types exist at the system level and schema object level.
System Privileges
Oracle defines the following system privileges for user-defined types:
- CREATE TYPE allows you to create user-defined types in your own schema.
- CREATE ANY TYPE allows you to create user-defined types in any schema.
- ALTER ANY TYPE allows you to alter user-defined types in any schema.
- DROP ANY TYPE allows you to drop named types in any schema.
- EXECUTE ANY TYPE allows you to use and reference named types in any schema.
The CONNECT and RESOURCE roles include the CREATE TYPE system privilege. The DBA role includes all of the above privileges.
Schema Object Privileges
The only schema object privilege that applies to user-defined types is EXECUTE.
EXECUTE on a user-defined type allows you to use the type to:
- Define a table.
- Define a column in a relational table.
- Declare a variable or parameter of the named type.
EXECUTE lets you invoke the type's methods, including the constructor.
Method execution and the associated permissions are the same as for stored PL/SQL procedures.
Using Types in New Types or Tables
In addition to the permissions detailed in the previous sections, you need specific privileges to:
- Create types or tables that use types created by other users.
- Grant use of your new types or tables to other users.
You must have the EXECUTE ANY TYPE system privilege, or you must have the EXECUTE object privilege for any type you use in defining a new type or table. You must have received these privileges explicitly, not through roles.
If you intend to grant access to your new type or table to other users, you must have either the required EXECUTE object privileges with the GRANT option or the EXECUTE ANY TYPE system privilege with the option WITH ADMIN OPTION. You must have received these privileges explicitly, not through roles.
Example
Assume that three users exist with the CONNECT and RESOURCE roles: user1, user2, and user3
User1 performs the following DDL in the user1 schema:
User2 performs the following DDL in the user2 schema:
The following statements succeed, because user2 has EXECUTE on user1's type2 with the GRANT option:
However, the following grant fails, because user2 does not have EXECUTE on user1.type1 with the GRANT option:
User3 can successfully perform the following actions:
Privileges on Type Access and Object Access
The privileges that regulate use of tables apply equally to object tables:
- SELECT lets you access an object and its attributes from the table.
- UPDATE lets you modify attributes of objects in the table.
- INSERT lets you add new objects to the table.
- DELETE lets you delete objects from the table.
Similar table and column privileges regulate the use of table columns of user-defined types.
Retrieving data of user-defined types does not require type information. Interpreting the data, however, does require such information. When Oracle receives requests for type information, it verifies that the requestor has EXECUTE privilege on the type before supplying the requested information.
Consider the following schema:
Sql System-generated Primary Key That Is Usually Hidden From Users In Word
and the following two queries:
For either query, Oracle checks the user's SELECT privilege for the emp table. For the first query, the user needs to obtain the emp_type type information to interpret the data. When the query accesses the emp_type type, Oracle checks the user's EXECUTE privilege.
Execution of the second query, however, does not involve named types, so Oracle does not check type privileges.
Additionally, using the schema from the previous section, user3 can perform the following queries:
Note that in both selects by user3, user3 does not have explicit privileges on the underlying types, but the statement succeeds because the type and table owners have the necessary privileges with the GRANT option.
Sql System-generated Primary Key That Is Usually Hidden From Users List
Oracle checks privileges on the following requests, and returns an error if the requestor does not have the privilege for the action:
- Pinning an object in the object cache using its REF value causes Oracle to check SELECT privilege on the containing object table.
- Modifying an existing object or flushing an object from the object cache, causes Oracle to check UPDATE privilege on the destination object table. Flushing a new object causes Oracle to check INSERT privilege on the destination object table.
- Deleting an object causes Oracle to check DELETE privilege on the destination table. Pinning an object of named type causes Oracle to check EXECUTE privilege on the object type.
- Invoking a method causes Oracle to check EXECUTE privilege on the corresponding object type.
Oracle does not provide column level privileges for object tables.
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Dependencies and Incomplete Types
Types can depend upon each other for their definitions. For example, you might want to define object types
employee
and department
in such a way that one attribute of employee
is the department the employee belongs to and one attribute of department
is the employee who manages the department. Types that depend on each other in this way, either directly or via intermediate types, are called mutually dependent. A diagram of mutually dependent types, with arrows representing the dependencies, always reveals a path of arrows starting and ending at one of the types.
Oracle allows such cyclic dependencies only when at least one branch of the cycle uses REFs.
For example, you can define the following types:
This is a legal set of mutually dependent types and a legal sequence of SQL DDL statements. Oracle compiles it without errors. The first statement
is optional. It makes the compilation proceed without errors. It establishes
department
as an incomplete object type. A REF to an incomplete object type compiles without error, so the compilation of employee
proceeds. When Oracle reaches the last statement, which completes the definition of
department
, all of the components of department
have compiled successfully, so the compilation finishes without errors. Without the optional declaration of
department
as an incomplete type, employee
compiles with errors. Oracle then automatically adds employee
to its library of schema objects as an incomplete object type. This makes the declarations of emp_list
and department
compile without errors. When employee
is recompiled after emp_list
and department
are complete, employee
compiles without errors and becomes a complete object type. Completing Incomplete Types
Sql System-generated Primary Key That Is Usually Hidden From Users 2017
Once you have declared an incomplete object type, you must complete it as an object type. You cannot, for example, declare it to be a table type or an array type. The only alternative is to drop the type.
This is also true if Oracle has made the type an incomplete object type for you - as it did when
employee
failed to compile in the previous section. This restriction applies even if there are no REFs to the incomplete object type anywhere in the schema. The rule Oracle follows is that once it flags a type as the potential target of REFs, that type must remain a potential REF target until it is dropped.
Oracle recognizes only object types as potential REF targets.
Type Dependencies of Tables
If a table contains data that relies on a type definition for access, any change to the type causes the table's data to become inaccessible. This happens if privileges required by the type are revoked or if the type or a type it depends on is dropped. The table then becomes invalid and cannot be accessed.
A table that is invalid because of missing privileges automatically becomes valid and accessible if the required privileges are re-granted.
A table that is invalid because a type it depends on has been dropped can never be accessed again. The only permissible action is to drop the table.
The SQL commands REVOKE and DROP TYPE return an error and abort if the type referred to in the command has tables or other types that depend on it.
The FORCE option with either of these commands overrides that behavior. The command succeeds and the affected tables or types become invalid.
Import/Export of User-Defined Types
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The Export and Import utilities move data into and out of Oracle databases. They are also back up or archive data and aid migration to different releases of the Oracle RDBMS.
Export and Import support user-defined types. Export writes user-defined type definitions, foreign function library definitions, directory alias definitions, and all of the associated data to the dump file. Import then recreates these items from the dump file.
Additional Information: See Oracle8 Utilities for more information about Export and Import. |
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