Oracle identity column examples. Letâs take some examples of using the Oracle identity columns. A) GENERATED ALWAYS example. The following statement creates a table named identitydemo that consists of an identity column: CREATE TABLE identitydemo ( id NUMBER GENERATED ALWAYS AS IDENTITY, description VARCHAR2 (100) NOT NULL). Jan 07, 2020 And then add primary key constraint. ALTER TABLE autoincrementtb ADD ( CONSTRAINT autoincrementtbpk PRIMARY KEY (id) ); Now we will create a sequence to generate unique auto incremented values. A sequence is a data object that can be used by multiple users to generate auto increment values(no duplicate values will be generated).
FOREIGN KEY, ADD PRIMARY KEY, ADD UNIQUE, DROP PRIMARY KEY, DROP FOREIGN KEY, or DROP CONSTRAINT is specified. The data type of a column that is added to the table is a distinct type. A fullselect is specified. A column is defined as a security label column. A column is defined as ROWID GENERATED BY DEFAULT. Dmitry Lenev sorry it is code bug because i tried to create foreign key on non generated column if i have three column col1 col2 col3 col3 is generated as concat(col1,col2) case 1 if i update col2 or col1 by update statement col3 value will change case 2 if i create foreign key on col1 on update cascade and the referred column value changed col1 value will change, so col3 value will change why.
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APPLIES TO: SQL Server 2016 and later Azure SQL Database Azure Synapse Analytics (SQL DW) Parallel Data Warehouse
Primary keys and foreign keys are two types of constraints that can be used to enforce data integrity in SQL Server tables. These are important database objects.
This topic contains the following sections.
Primary Key Constraints
A table typically has a column or combination of columns that contain values that uniquely identify each row in the table. This column, or columns, is called the primary key (PK) of the table and enforces the entity integrity of the table. Because primary key constraints guarantee unique data, they are frequently defined on an identity column.
When you specify a primary key constraint for a table, the Database Engine enforces data uniqueness by automatically creating a unique index for the primary key columns. This index also permits fast access to data when the primary key is used in queries. If a primary key constraint is defined on more than one column, values may be duplicated within one column, but each combination of values from all the columns in the primary key constraint definition must be unique.
As shown in the following illustration, the ProductID and VendorID columns in the Purchasing.ProductVendor table form a composite primary key constraint for this table. This makes sure that every row in the ProductVendor table has a unique combination of ProductID and VendorID. This prevents the insertion of duplicate rows.
Foreign Key Constraints
A foreign key (FK) is a column or combination of columns that is used to establish and enforce a link between the data in two tables to control the data that can be stored in the foreign key table. In a foreign key reference, a link is created between two tables when the column or columns that hold the primary key value for one table are referenced by the column or columns in another table. This column becomes a foreign key in the second table.
For example, the Sales.SalesOrderHeader table has a foreign key link to the Sales.SalesPerson table because there is a logical relationship between sales orders and salespeople. The SalesPersonID column in the SalesOrderHeader table matches the primary key column of the SalesPerson table. The SalesPersonID column in the SalesOrderHeader table is the foreign key to the SalesPerson table. By creating this foreign key relationship, a value for SalesPersonID cannot be inserted into the SalesOrderHeader table if it does not already exist in the SalesPerson table.
A table can reference a maximum of 253 other tables and columns as foreign keys (outgoing references). SQL Server 2016 (13.x) increases the limit for the number of other table and columns that can reference columns in a single table (incoming references), from 253 to 10,000. (Requires at least 130 compatibility level.) The increase has the following restrictions:
Indexes on Foreign Key Constraints
Unlike primary key constraints, creating a foreign key constraint does not automatically create a corresponding index. However, manually creating an index on a foreign key is often useful for the following reasons:
Referential Integrity
Although the main purpose of a foreign key constraint is to control the data that can be stored in the foreign key table, it also controls changes to data in the primary key table. For example, if the row for a salesperson is deleted from the Sales.SalesPerson table, and the salesperson's ID is used for sales orders in the Sales.SalesOrderHeader table, the relational integrity between the two tables is broken; the deleted salesperson's sales orders are orphaned in the SalesOrderHeader table without a link to the data in the SalesPerson table.
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A foreign key constraint prevents this situation. The constraint enforces referential integrity by guaranteeing that changes cannot be made to data in the primary key table if those changes invalidate the link to data in the foreign key table. If an attempt is made to delete the row in a primary key table or to change a primary key value, the action will fail when the deleted or changed primary key value corresponds to a value in the foreign key constraint of another table. To successfully change or delete a row in a foreign key constraint, you must first either delete the foreign key data in the foreign key table or change the foreign key data in the foreign key table, which links the foreign key to different primary key data.
Cascading Referential Integrity
By using cascading referential integrity constraints, you can define the actions that the Database Engine takes when a user tries to delete or update a key to which existing foreign keys point. The following cascading actions can be defined.
NO ACTION
The Database Engine raises an error and the delete or update action on the row in the parent table is rolled back.
CASCADE
Corresponding rows are updated or deleted in the referencing table when that row is updated or deleted in the parent table. CASCADE cannot be specified if a timestamp column is part of either the foreign key or the referenced key. ON DELETE CASCADE cannot be specified for a table that has an INSTEAD OF DELETE trigger. ON UPDATE CASCADE cannot be specified for tables that have INSTEAD OF UPDATE triggers.
SET NULL
All the values that make up the foreign key are set to NULL when the corresponding row in the parent table is updated or deleted. For this constraint to execute, the foreign key columns must be nullable. Cannot be specified for tables that have INSTEAD OF UPDATE triggers.
SET DEFAULT
All the values that make up the foreign key are set to their default values if the corresponding row in the parent table is updated or deleted. For this constraint to execute, all foreign key columns must have default definitions. If a column is nullable, and there is no explicit default value set, NULL becomes the implicit default value of the column. Cannot be specified for tables that have INSTEAD OF UPDATE triggers.
CASCADE, SET NULL, SET DEFAULT and NO ACTION can be combined on tables that have referential relationships with each other. If the Database Engine encounters NO ACTION, it stops and rolls back related CASCADE, SET NULL and SET DEFAULT actions. When a DELETE statement causes a combination of CASCADE, SET NULL, SET DEFAULT and NO ACTION actions, all the CASCADE, SET NULL and SET DEFAULT actions are applied before the Database Engine checks for any NO ACTION.
Triggers and Cascading Referential Actions
Cascading referential actions fire the AFTER UPDATE or AFTER DELETE triggers in the following manner:
Related Tasks
The following table lists the common tasks associated with primary key and foreign key constraints.
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MySQL supports foreign keys, which permit cross-referencing related data across tables, and foreign key constraints, which help keep the related data consistent.
A foreign key relationship involves a parent table that holds the initial column values, and a child table with column values that reference the parent column values. A foreign key constraint is defined on the child table.
The essential syntax for a defining a foreign key constraint in a
CREATE TABLE or ALTER TABLE statement includes the following:
Foreign key constraint usage is described under the following topics in this section:
Foreign key constraint naming is governed by the following rules:
Table and column identifiers in a
FOREIGN KEY .. REFERENCES clause can be quoted within backticks (` ). Alternatively, double quotation marks (' ) can be used if the ANSI_QUOTES SQL mode is enabled. The lower_case_table_names system variable setting is also taken into account.
Foreign key constraints are subject to the following conditions and restrictions:
For information about how the MySQL implementation of foreign key constraints differs from the SQL standard, see Section 1.8.2.3, âFOREIGN KEY Constraint Differencesâ.
When an
UPDATE or DELETE operation affects a key value in the parent table that has matching rows in the child table, the result depends on the referential action specified by ON UPDATE and ON DELETE subclauses of the FOREIGN KEY clause. Referential actions include:
For storage engines that support foreign keys, MySQL rejects any
INSERT or UPDATE operation that attempts to create a foreign key value in a child table if there is no matching candidate key value in the parent table.
For an
ON DELETE or ON UPDATE that is not specified, the default action is always RESTRICT .
Generated Always As Row Start
For
NDB tables, ON UPDATE CASCADE is not supported where the reference is to the parent table's primary key.
As of NDB 7.3.25 and NDB 7.4.24: For
NDB tables, ON DELETE CASCADE is not supported where the child table contains one or more columns of any of the TEXT or BLOB types. (Bug #89511, Bug #27484882)
InnoDB performs cascading operations using a depth-first search algorithm on the records of the index that corresponds to the foreign key constraint.
This simple example relates
parent and child tables through a single-column foreign key:
Generated Always As Foreign Keyboard
This is a more complex example in which a
product_order table has foreign keys for two other tables. One foreign key references a two-column index in the product table. The other references a single-column index in the customer table:
You can add a foreign key constraint to an existing table using the following
ALTER TABLE syntax:
The foreign key can be self referential (referring to the same table). When you add a foreign key constraint to a table using
ALTER TABLE , remember to first create an index on the column(s) referenced by the foreign key.
Oracle Generated Always As
You can drop a foreign key constraint using the following
ALTER TABLE syntax:
If the
FOREIGN KEY clause defined a CONSTRAINT name when you created the constraint, you can refer to that name to drop the foreign key constraint. Otherwise, a constraint name was generated internally, and you must use that value. To determine the foreign key constraint name, use SHOW CREATE TABLE :
Adding and dropping a foreign key in the same
ALTER TABLE statement is supported for ALTER TABLE .. ALGORITHM=INPLACE . It is not supported for ALTER TABLE .. ALGORITHM=COPY .
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Foreign key checking is controlled by the
foreign_key_checks variable, which is enabled by default. Typically, you leave this variable enabled during normal operation to enforce referential integrity. Beginning with MySQL NDB Cluster 7.3.2, the foreign_key_checks variable has the same effect on NDB tables as it does for InnoDB tables. Previously, the setting was ignored for NDB tables and all such checks were enforced (Bug #14095855).
The
foreign_key_checks variable is dynamic and supports both global and session scopes. For information about using system variables, see Section 5.1.8, âUsing System Variablesâ.
Disabling foreign key checking is useful when:
Db2 Generated Always As Identity
When
foreign_key_checks is disabled, foreign key constraints are ignored, with the following exceptions:
Disabling
foreign_key_checks has these additional implications:
To view a foreign key definition, use
SHOW CREATE TABLE :
You can obtain information about foreign keys from the
INFORMATION_SCHEMA.KEY_COLUMN_USAGE table. An example of a query against this table is shown here:
You can obtain information specific to
InnoDB foreign keys from the INNODB_SYS_FOREIGN and INNODB_SYS_FOREIGN_COLS tables. Example queries are show here:
In the event of a foreign key error involving
InnoDB tables (usually Error 150 in the MySQL Server), information about the latest foreign key error can be obtained by checking SHOW ENGINE INNODB STATUS output.
ER_NO_REFERENCED_ROW_2 and ER_ROW_IS_REFERENCED_2 error messages for foreign key operations expose information about parent tables, even if the user has no parent table access privileges. To hide information about parent tables, include the appropriate condition handlers in application code and stored programs.
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