• Hibernate Generated Key And Foreign Key As Primary Key Examples
• Hibernate Generated Key And Foreign Key As Primary Keys
• Foreign Key In Mysql

SQL FOREIGN KEY. In the relational databases, a foreign key is a field or a column that is used to establish a link between two tables. In simple words you can say that, a foreign key in one table used to point primary key in another table. Hibernate annotations with composite primary key I want to know how to use hibernate annotations in case of composite primary key in one table. I tried like this, I have created a table without primary key.

• • Constructor Summary

    Constructors

    Constructor and Description
    ForeignKey()

  • Method Summary

    All MethodsInstance MethodsConcrete Methods

    Modifier and Type	Method and Description
    void	addReferencedColumns(Iterator referencedColumnsIterator)
    void	alignColumns() Validates that columnspan of the foreignkey and the primarykey is the same.
    void	disableCreation()
    String	generatedConstraintNamePrefix()
    String	getExportIdentifier() Get a unique identifier to make sure we are not exporting the same database structure multiple times.
    String	getKeyDefinition()
    List	getReferencedColumns() Returns the referenced columns if the foreignkey does not refer to the primary key
    String	getReferencedEntityName()
    Table	getReferencedTable()
    boolean	isCascadeDeleteEnabled()
    boolean	isCreationEnabled()
    boolean	isPhysicalConstraint()
    boolean	isReferenceToPrimaryKey() Does this foreignkey reference the primary key of the reference table
    void	setCascadeDeleteEnabled(boolean cascadeDeleteEnabled)
    void	setKeyDefinition(String keyDefinition)
    void	setName(String name)
    void	setReferencedEntityName(String referencedEntityName)
    void	setReferencedTable(Table referencedTable)
    String	sqlConstraintString(Dialect dialect, String constraintName, String defaultCatalog, String defaultSchema)
    String	sqlDropString(Dialect dialect, String defaultCatalog, String defaultSchema)
    String	toString()

    • Methods inherited from class org.hibernate.mapping.Constraint

      addColumn, addColumns, columnIterator, containsColumn, generateName, generateName, getColumn, getColumnIterator, getColumns, getColumnSpan, getName, getTable, hashedName, isGenerated, setTable, sqlCreateString

    • Methods inherited from class java.lang.Object

      clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

In the relational databases, a foreign key is a field or a column that is used to establish a link between two tables.

In simple words you can say that, a foreign key in one table used to point primary key in another table.

Let us take an example to explain it:

Here are two tables first one is students table and second is orders table.

Here orders are given by students.

First table:

Second table:

Hibernate Generated Key And Foreign Key As Primary Key Examples

Here you see that 'S_Id' column in the 'Orders' table points to the 'S_Id' column in 'Students' table.

• The 'S_Id' column in the 'Students' table is the PRIMARY KEY in the 'Students' table.
• The 'S_Id' column in the 'Orders' table is a FOREIGN KEY in the 'Orders' table.

The foreign key constraint is generally prevents action that destroy links between tables.

It also prevents invalid data to enter in foreign key column.

SQL FOREIGN KEY constraint ON CREATE TABLE:

Vmware workstation 8 license key generator. (Defining a foreign key constraint on single column)

To create a foreign key on the 'S_Id' column when the 'Orders' table is created:

MySQL:

Hibernate Generated Key And Foreign Key As Primary Keys

SQL Server /Oracle / MS Access:

SQL FOREIGN KEY constraint for ALTER TABLE:

If the Order table is already created and you want to create a FOREIGN KEY constraint on the 'S_Id' column, you should write the following syntax:

Defining a foreign key constraint on single column:

MySQL / SQL Server / Oracle / MS Access:

DROP SYNTAX for FOREIGN KEY COSTRAINT:

If you want to drop a FOREIGN KEY constraint, use the following syntax:

MySQL:

SQL Server / Oracle / MS Access:

Difference between primary key and foreign key in SQL:

These are some important difference between primary key and foreign key in SQL-

Primary key cannot be null on the other hand foreign key can be null.

Primary key is always unique while foreign key can be duplicated.

Primary key uniquely identify a record in a table while foreign key is a field in a table that is primary key in another table.

There is only one primary key in the table on the other hand we can have more than one foreign key in the table.

By default primary key adds a clustered index on the other hand foreign key does not automatically create an index, clustered or non-clustered. You must manually create an index for foreign key.

Foreign Key In Mysql