Package org.hibernate.dialect.unique

Interface UniqueDelegate

All Known Implementing Classes:
AlterTableUniqueDelegate, AlterTableUniqueIndexDelegate, CreateTableUniqueDelegate, DefaultUniqueDelegate, SkipNullableUniqueDelegate
public interface UniqueDelegate
Dialect-level delegate responsible for applying unique constraints in DDL. Uniqueness can be specified in any of three ways:
  1. For single-column constraints, by adding unique to the column definition. See getColumnDefinitionUniquenessFragment(org.hibernate.mapping.Column, org.hibernate.boot.model.relational.SqlStringGenerationContext)
  2. By inclusion of the unique constraint in the create table statement. See getTableCreationUniqueConstraintsFragment(org.hibernate.mapping.Table, org.hibernate.boot.model.relational.SqlStringGenerationContext)
  3. By creation of a unique constraint using separate alter table statements. See getAlterTableToAddUniqueKeyCommand(org.hibernate.mapping.UniqueKey, org.hibernate.boot.Metadata, org.hibernate.boot.model.relational.SqlStringGenerationContext). Also, see getAlterTableToDropUniqueKeyCommand(org.hibernate.mapping.UniqueKey, org.hibernate.boot.Metadata, org.hibernate.boot.model.relational.SqlStringGenerationContext).
The first two options are generally preferred, and so we use CreateTableUniqueDelegate where possible. However, for databases where unique constraints may not contain a nullable column, and unique indexes must be used instead, we use AlterTableUniqueIndexDelegate.

Hibernate specifies that a unique constraint on a nullable column considers null values to be distinct. Some databases default to the opposite semantic, where null values are considered equal for the purpose of determining uniqueness. This is almost never useful, and is the opposite of what we want when we use a unique constraint on a foreign key to map an optional OneToOne association. Therefore, our UniqueDelegates must jump through hoops to emulate the sensible semantics specified by ANSI, Hibernate, and common sense, namely, that two null values are distinct. A particularly egregious offender is Sybase, where we must simply skip creating the unique constraint.

  • Method Details

    • getColumnDefinitionUniquenessFragment

      String getColumnDefinitionUniquenessFragment(Column column, SqlStringGenerationContext context)
      Get the SQL fragment used to make the given column unique as part of its column definition, usually just " unique", or return an empty string if uniqueness does not belong in the column definition.

      This is for handling single columns explicitly marked unique, not for dealing with unique keys.

      Parameters:
      column - The column to which to apply the unique
      context - A context for SQL string generation
      Returns:
      The fragment (usually "unique"), empty string indicates the uniqueness will be indicated using a different approach

    • getTableCreationUniqueConstraintsFragment

      String getTableCreationUniqueConstraintsFragment(Table table, SqlStringGenerationContext context)
      Get the SQL fragment used to specify the unique constraints on the given table as part of the create table command, with a leading comma, usually something like: 
      , unique(x,y), constraint abc unique(a,b,c)

      or return an empty string if there are no unique constraints or if the unique constraints do not belong in the table definition.

      The implementation should iterate over the unique keys of the given table by calling Table.getUniqueKeys() and generate a fragment which includes all the unique key declarations.

      Parameters:
      table - The table for which to generate the unique constraints fragment
      context - A context for SQL string generation
      Returns:
      The fragment, typically in the form ", unique(col1, col2), unique(col20)". The leading comma is important!

    • getAlterTableToAddUniqueKeyCommand

      String getAlterTableToAddUniqueKeyCommand(UniqueKey uniqueKey, Metadata metadata, SqlStringGenerationContext context)
      Get the alter table command used to create the given unique key constraint, or return the empty string if the constraint was already included in the create table statement by getTableCreationUniqueConstraintsFragment(org.hibernate.mapping.Table, org.hibernate.boot.model.relational.SqlStringGenerationContext).
      Parameters:
      uniqueKey - The UniqueKey instance. Contains all information about the columns
      metadata - Access to the bootstrap mapping information
      context - A context for SQL string generation
      Returns:
      The alter table command

    • getAlterTableToDropUniqueKeyCommand

      String getAlterTableToDropUniqueKeyCommand(UniqueKey uniqueKey, Metadata metadata, SqlStringGenerationContext context)
      Get the alter table command used to drop the given unique key which was previously created by getAlterTableToAddUniqueKeyCommand(org.hibernate.mapping.UniqueKey, org.hibernate.boot.Metadata, org.hibernate.boot.model.relational.SqlStringGenerationContext).
      Parameters:
      uniqueKey - The UniqueKey instance. Contains all information about the columns
      metadata - Access to the bootstrap mapping information
      context - A context for SQL string generation
      Returns:
      The alter table command