Find The Number Of Foriegn Keys That Generated

• Primary And Foreign Keys In Database
• Find The Number Of Foreign Keys That Generated Work
• Mysql Foreign Keys
• Find The Number Of Foreign Keys That Generated People
• Find The Number Of Foreign Keys That Generated Number
• Find The Number Of Foreign Keys That Generated Mean

Summary: in this tutorial, you will learn how to use the Db2 foreign key constraint to enforce the referential integrity between the data across tables.

Introduction to the Db2 foreign key

Let’s take a look at the contacts and phones tables:

In this diagram, each contact may have zero or many phones such as home phone, work phone, and emergency phone. However, each phone must belong to one and only one contact. The relationship between the contacts and phones is one-to-many.

List Dependencies for SQL Server Foreign Keys. By: Rick Dobson. Some different approaches to developing this kind of information as well as ways of validating the information generated by the different approaches? When the primary key for the referenced table is a 'natural' value that is not an arbitrary number but has some meaning for. To list all the Foreign keys in a table in a SQL Server database run: USE Databasename SELECT. FROM informationschema.tableconstraints WHERE constrainttype = 'Foreign Key' and TableName =.

Primary And Foreign Keys In Database

For each row in the phones table, you can always find a corresponding row in the contacts table. But the current setup does not ensure this relationship. It means you can insert a new row into the phones table with the contact identification (contact_id) that does not exist in the contacts table.

Furthermore, if you delete a contact, all the phones of the deleted contact will remain in the phones table. The rows in the phones table that does not have corresponding rows in the contacts table are called orphaned rows.

In order to enforce the relationship between contacts and phones tables, you need to use a foreign key constraint.

What is a foreign key?

A foreign key is a column or group of columns in a table that uniquely identifies a row in another table. The foreign key constraints define foreign keys.

Back to our example, the contact_id in the phones table should be the foreign key of the phones table. Because for each phone in the phones table, you can find a corresponding contact in the contacts table.

To add a foreign key constraint to the phones table, you use the following ALTER TABLE statement:

The contacts table is called the parent table to which the foreign key references. The phones table is called the child table (or dependent table) to which the foreign key constraint is applied.

The contact_id column in the contacts table is called the parent key and the contact_id column in the phones table is called the foreign key or foreign key column.

In the database world, referential integrity is a mechanism to ensure that the relationship of data between tables remains consistent. And to enforce the referential integrity, you use foreign key constraints. Therefore, foreign key constraints are also known as referential integrity constraints or referential constraints.

Db2 FOREIGN KEY constraint syntax

Find The Number Of Foreign Keys That Generated Work

The following illustrates the syntax of defining a foreign key constraint:

In this syntax:

First, specify a constraint name in the CONSTRAINT clause. The CONSTRAINT clause is optional. If you omit it, Db2 will generate a name for the foreign key constraint.

Second, specify a list of comma-separated foreign key columns enclosed by parentheses in the FOREIGN KEY clause.

Third, specify the name of the parent table and a list of comma-separated columns to which the foreign key columns reference.

ON UPDATE rules

Db2 triggers the ON UPDATE rule when you update a row in either parent or child table. The update rule has two options NO ACTION and RESTRICT.

When you update the row in the parent key column of the parent table, Db2 rejects the update if there is the corresponding row exists in the child table for both RESTRICT and NO ACTION option.

When you update the row in the foreign key column of the child table, Db2 rejects the update for RESTRICT option and allows the update for the NO ACTION, with the condition that the new value of the foreign key column exists in the parent table.

ON DELETE rules

Mysql Foreign Keys

Db2 triggers the ON DELETE rule when you delete a row in the parent table. Db2 determines whether or not to delete the rows in the child table based on the following options:

• NO ACTION or RESTRICT does not delete any row in both tables and issues an error.
• CASCADE deletes the row in the parent table and all related rows in the child table.
• SET NULL deletes the row in the parent table and updates values in the foreign key columns in the child table to NULL only if these columns are not nullable columns.

You can use the foreign key constraint to define foreign keys in the CREATE TABLE or ALTER TABLE statement.

Db2 FOREIGN KEY constraint examples

Let’s take some example of using the foreign key constraint to understand it better.

1) Creating a table which has a single foreign key example

First, insert a new contact into the contacts table:

The contact John Doe has the contact id 1:

Next, add two phones for the contact John Doe:

Then, delete the contact id 1 from the contacts table. Because we declare the ON DELETE rule with the CASCADE action, Db2 will delete all phones of John Doe from the phones table:

After, verify the deletion by querying data from the contacts table:

It returned no row.

Finally, view data in the contacts table by using the following SELECT statement:

It also returns an empty set.

2) Creating a table with multiple foreign keys example

The following statement creates a new table called members:

Suppose each member can have one or many favorite books and each book may belong to favorite lists of many users. The relationship between members and books are many-to-many.

The following favorite_books table stores the favorite books of members:

The favorite_books table has two foreign keys. The first one refers to the book_id column of the books table and the second one references to the member_id of the members table.

The favorite_books table is known as an associative table, pivot table, or mapping table. We often use these kinds of tables to manage the many-to-many relationship.

Find The Number Of Foreign Keys That Generated People

In this tutorial, you have learned about Db2 foreign key and how to use the foreign key constraint to enforce referential integrity.

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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.

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• A table can contain only one primary key constraint.

• A primary key cannot exceed 16 columns and a total key length of 900 bytes.

• The index generated by a primary key constraint cannot cause the number of indexes on the table to exceed 999 nonclustered indexes and 1 clustered index.

• If clustered or nonclustered is not specified for a primary key constraint, clustered is used if there no clustered index on the table.

• 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.

• If a primary key is defined on a CLR user-defined type column, the implementation of the type must support binary ordering.

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:

• Greater than 253 foreign key references are only supported for DELETE DML operations. UPDATE and MERGE operations are not supported.

• A table with a foreign key reference to itself is still limited to 253 foreign key references.

• Greater than 253 foreign key references are not currently available for columnstore indexes, memory-optimized tables, Stretch Database, or partitioned foreign key tables.

Indexes on Foreign Key Constraints

Find The Number Of Foreign Keys That Generated Number

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:

• Foreign key columns are frequently used in join criteria when the data from related tables is combined in queries by matching the column or columns in the foreign key constraint of one table with the primary or unique key column or columns in the other table. An index enables the Database Engine to quickly find related data in the foreign key table. However, creating this index is not required. Data from two related tables can be combined even if no primary key or foreign key constraints are defined between the tables, but a foreign key relationship between two tables indicates that the two tables have been optimized to be combined in a query that uses the keys as its criteria.

• Changes to primary key constraints are checked with foreign key constraints in related tables.

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.

Find The Number Of Foreign Keys That Generated Mean

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:

• All the cascading referential actions directly caused by the original DELETE or UPDATE are performed first.

• If there are any AFTER triggers defined on the affected tables, these triggers fire after all cascading actions are performed. These triggers fire in opposite order of the cascading action. If there are multiple triggers on a single table, they fire in random order, unless there is a dedicated first or last trigger for the table. This order is as specified by using sp_settriggerorder.

• If multiple cascading chains originate from the table that was the direct target of an UPDATE or DELETE action, the order in which these chains fire their respective triggers is unspecified. However, one chain always fires all its triggers before another chain starts firing.

• An AFTER trigger on the table that is the direct target of an UPDATE or DELETE action fires regardless of whether any rows are affected. There are no other tables affected by cascading in this case.

• If any one of the previous triggers perform UPDATE or DELETE operations on other tables, these actions can start secondary cascading chains. These secondary chains are processed for each UPDATE or DELETE operation at a time after all triggers on all primary chains fire. This process may be recursively repeated for subsequent UPDATE or DELETE operations.

• Performing CREATE, ALTER, DELETE, or other data definition language (DDL) operations inside the triggers may cause DDL triggers to fire. This may subsequently perform DELETE or UPDATE operations that start additional cascading chains and triggers.

• If an error is generated inside any particular cascading referential action chain, an error is raised, no AFTER triggers are fired in that chain, and the DELETE or UPDATE operation that created the chain is rolled back.

• A table that has an INSTEAD OF trigger cannot also have a REFERENCES clause that specifies a cascading action. However, an AFTER trigger on a table targeted by a cascading action can execute an INSERT, UPDATE, or DELETE statement on another table or view that fires an INSTEAD OF trigger defined on that object.

Related Tasks

The following table lists the common tasks associated with primary key and foreign key constraints.

Task	Topic
Describes how to create a primary key.	Create Primary Keys
Describes how to delete a primary key.	Delete Primary Keys
Describes how to modify a primary key.	Modify Primary Keys
Describes how to create foreign key relationships	Create Foreign Key Relationships
Describes how to modify foreign key relationships.	Modify Foreign Key Relationships
Describes how to delete foreign key relationships.	Delete Foreign Key Relationships
Describes how to view foreign key properties.	View Foreign Key Properties
Describes how to disable foreign key constraints for replication.	Disable Foreign Key Constraints for Replication
Describes how to disable foreign key constraints during an INSERT or UPDATE statement.	Disable Foreign Key Constraints with INSERT and UPDATE Statements