- Sql Server Default Foreign Key Generation Logic 2017
- Sql Server Default Foreign Key Generation Logic Free
- Primary Key
- Sql Server Default Foreign Key Generation Logical
Dec 30, 2015 For example, they can be used to facilitate automated generation and validation of foreign key constraints. I don't always remember to create foreign key constraints when I add new tables and columns to a database. And even if I do remember, I'm not always the only person who makes schema changes to a database, and others can forget too. New relationship diagram: Add surrogate key using ApexSQL Refactor. To replace a natural key with a surrogate key using ApexSQL Refactor, a free SSMS and VS SQL database refactoring tool, choose the table in the Object Explorer panel and from the Apex SQL Refactor menu choose the Add surrogate key command, under the Other refactors sub-menu. SQL Constraint Foreign Key. A foreign key is a column (or columns) that references a column (most often the primary key) of another table. The purpose of the foreign key is to ensure referential integrity of the data. In other words, only values that are supposed to appear in the database are permitted.
SQL FOREIGN KEY Constraint
A FOREIGN KEY is a key used to link two tables together.
A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table.
The table containing the foreign key is called the child table, and the table containing the candidate key is called the referenced or parent table.
Look at the following two tables:
'Persons' table:
PersonID | LastName | FirstName | Age |
---|---|---|---|
1 | Hansen | Ola | 30 |
2 | Svendson | Tove | 23 |
3 | Pettersen | Kari | 20 |
'Orders' table:
OrderID | OrderNumber | PersonID |
---|---|---|
1 | 77895 | 3 |
2 | 44678 | 3 |
3 | 22456 | 2 |
4 | 24562 | 1 |
Notice that the 'PersonID' column in the 'Orders' table points to the 'PersonID' column in the 'Persons' table.
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The 'PersonID' column in the 'Persons' table is the PRIMARY KEY in the 'Persons' table.
The 'PersonID' column in the 'Orders' table is a FOREIGN KEY in the 'Orders' table.
The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.
The FOREIGN KEY constraint also prevents invalid data from being inserted into the foreign key column, because it has to be one of the values contained in the table it points to.
SQL FOREIGN KEY on CREATE TABLE
The following SQL creates a FOREIGN KEY on the 'PersonID' column when the 'Orders' table is created:
MySQL:
CREATE TABLE Orders (
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);
SQL Server / Oracle / MS Access:
CREATE TABLE Orders (
OrderID int NOT NULL PRIMARY KEY,
OrderNumber int NOT NULL,
PersonID int FOREIGN KEY REFERENCES Persons(PersonID)
);
OrderID int NOT NULL PRIMARY KEY,
OrderNumber int NOT NULL,
PersonID int FOREIGN KEY REFERENCES Persons(PersonID)
);
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
CREATE TABLE Orders (
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID)
REFERENCES Persons(PersonID)
);
OrderID int NOT NULL,
OrderNumber int NOT NULL,
PersonID int,
PRIMARY KEY (OrderID),
CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID)
REFERENCES Persons(PersonID)
);
SQL FOREIGN KEY on ALTER TABLE
To create a FOREIGN KEY constraint on the 'PersonID' column when the 'Orders' table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Orders
ADD FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
ADD FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Orders
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
Sql Server Default Foreign Key Generation Logic 2017
DROP a FOREIGN KEY Constraint
To drop a FOREIGN KEY constraint, use the following SQL:
MySQL:
ALTER TABLE Orders
DROP FOREIGN KEY FK_PersonOrder;
DROP FOREIGN KEY FK_PersonOrder;
SQL Server / Oracle / MS Access:
ALTER TABLE Orders
DROP CONSTRAINT FK_PersonOrder;
DROP CONSTRAINT FK_PersonOrder;
-->
This article describes how to create foreign key relationships in SQL Server 2019 (15.x) by using SQL Server Management Studio or Transact-SQL. You create a relationship between two tables when you want to associate rows of one table with rows of another.
Before You Begin! Limits and Restrictions
Sql Server Default Foreign Key Generation Logic Free
- A foreign key constraint does not have to be linked only to a primary key constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table.
- When a value other than NULL is entered into the column of a FOREIGN KEY constraint, the value must exist in the referenced column; otherwise, a foreign key violation error message is returned. To make sure that all values of a composite foreign key constraint are verified, specify NOT NULL on all the participating columns.
- FOREIGN KEY constraints can reference only tables within the same database on the same server. Cross-database referential integrity must be implemented through triggers. For more information, see CREATE TRIGGER.
- FOREIGN KEY constraints can reference another column in the same table. This is referred to as a self-reference.
- A FOREIGN KEY constraint specified at the column level can list only one reference column. This column must have the same data type as the column on which the constraint is defined.
- A FOREIGN KEY constraint specified at the table level must have the same number of reference columns as the number of columns in the constraint column list. The data type of each reference column must also be the same as the corresponding column in the column list.
- The Database Engine does not have a predefined limit on either the number of FOREIGN KEY constraints a table can contain that reference other tables, or the number of FOREIGN KEY constraints that are owned by other tables that reference a specific table. Nevertheless, the actual number of FOREIGN KEY constraints that can be used is limited by the hardware configuration and by the design of the database and application. 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 supported for DELETE and UPDATE DML operations. 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, or Stretch Database.
- FOREIGN KEY constraints are not enforced on temporary tables.
- If a foreign key is defined on a CLR user-defined type column, the implementation of the type must support binary ordering. For more information, see CLR User-Defined Types.
- A column of type varchar(max) can participate in a FOREIGN KEY constraint only if the primary key it references is also defined as type varchar(max).
Permissions
![Foreign Foreign](/uploads/1/3/3/2/133275730/802777810.gif)
Creating a new table with a foreign key requires CREATE TABLE permission in the database and ALTER permission on the schema in which the table is being created.
Creating a foreign key in an existing table requires ALTER permission on the table.
Primary Key
Create a foreign key relationship in Table Designer
Using SQL Server Management Studio
- In Object Explorer, right-click the table that will be on the foreign-key side of the relationship and click Design.The table opens in Table Designer.
- From the Table Designer menu, click Relationships.
- In the Foreign-key Relationships dialog box, click Add.The relationship appears in the Selected Relationship list with a system-provided name in the format FK_<tablename>_<tablename>, where tablename is the name of the foreign key table.
- Click the relationship in the Selected Relationship list.
- Click Tables and Columns Specification in the grid to the right and click the ellipses (..) to the right of the property.
- In the Tables and Columns dialog box, in the Primary Key drop-down list, choose the table that will be on the primary-key side of the relationship.
- In the grid beneath, choose the columns contributing to the table's primary key. In the adjacent grid cell to the left of each column, choose the corresponding foreign-key column of the foreign-key table.Table Designer suggests a name for the relationship. To change this name, edit the contents of the Relationship Name text box.
- Choose OK to create the relationship.
Create a foreign key in a new table
Using Transact-SQL
Sql Server Default Foreign Key Generation Logical
The following example creates a table and defines a foreign key constraint on the column
TempID
that references the column SalesReasonID
in the Sales.SalesReason
table in the AdventureWorks database. The ON DELETE CASCADE and ON UPDATE CASCADE clauses are used to ensure that changes made to Sales.SalesReason
table are automatically propagated to the Sales.TempSalesReason
table.Create a foreign key in an existing table
Using Transact-SQL
The following example creates a foreign key on the column
TempID
and references the column SalesReasonID
in the Sales.SalesReason
table in the AdventureWorks database.![Sql Server Default Foreign Key Generation Logic Sql Server Default Foreign Key Generation Logic](/uploads/1/3/3/2/133275730/590731935.png)
For more information, see:
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- table_constraint.