Sql System-generated Primary Key That Is Usually Hidden From Users

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In this article we learn the difference between a primary and unique key, and why both are important to maintaining a relational database structure. All the examples for this lesson are based on Microsoft SQL Server Management Studio and the AdventureWorks2012 database. At the most basic definition, 'a key is a unique identifier', 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. A is a system-generated primary key that is usually hidden from users surrogate key when a DBMS creates a key, it is usually an automatic numbering data type, such as the Access AutoNumber data type.

Entity Integrity No field that is part of primary key may accept null values To from BCIS 4660 at University of North Texas. A foreign key is a column or collection of columns in one table that is required to match the value of the primary key for some row in another table, or be null. 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.

APPLIES TO: SQL Server 2016 and later Azure SQL Database Azure Synapse Analytics (SQL DW) Parallel Data Warehouse

You can define a primary key in SQL Server by using SQL Server Management Studio or Transact-SQL. Creating a primary key automatically creates a corresponding unique clustered index, or a nonclustered index if specified as such.

Before You Begin

Limitations and Restrictions

A table can contain only one PRIMARY KEY constraint.
All columns defined within a PRIMARY KEY constraint must be defined as NOT NULL. If nullability is not specified, all columns participating in a PRIMARY KEY constraint have their nullability set to NOT NULL.

Security

Permissions

Creating a new table with a primary key requires CREATE TABLE permission in the database and ALTER permission on the schema in which the table is being created.

Creating a primary key in an existing table requires ALTER permission on the table.

Using SQL Server Management Studio

To create a primary key

In Object Explorer, right-click the table to which you want to add a unique constraint, and click Design.
In Table Designer, click the row selector for the database column you want to define as the primary key. If you want to select multiple columns, hold down the CTRL key while you click the row selectors for the other columns.
Right-click the row selector for the column and select Set Primary Key.

Caution

If you want to redefine the primary key, any relationships to the existing primary key must be deleted before the new primary key can be created. A message will warn you that existing relationships will be automatically deleted as part of this process.

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.

If you define a compound key, the order of columns in the primary key matches the order of columns as shown in the table. However, you can change the order of columns after the primary key is created. For more information, see Modify Primary Keys.

Using Transact-SQL

To create a primary key in an existing table

The following example creates a primary key on the column TransactionID in the AdventureWorks database.

To create a primary key in a new table

The following example creates a table and defines a primary key on the column TransactionID in the AdventureWorks database.

To create a primary key with clustered index in a new table

The following example creates a table and defines a primary key on the column CustomerID and a clustered index on TransactionID in the AdventureWorks database.

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 of ptab1. No further qualification is required.
In the second SELECT statement, ssno is the name of an attribute of the person object in the column named c1. 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.

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

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.

Sql System-generated Primary Key That Is Usually Hidden From Users List

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.

Sql System-generated Primary Key That Is Usually Hidden From Users Guide

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:

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.

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.

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

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

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.

Sql System-generated Primary Key That Is Usually Hidden From Users 2017

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