JPA Column annotation

[pwgit-create]

The @Column annotation in JPA offers several attributes that allow for detailed customization of how
a field is mapped to a database column. Here are some of the commonly used parameters besides name and length:

1. nullable:

   • Indicates whether the column can include NULL values.

   @Column(nullable = false)
   private String firstName;

2. unique:

   • Specifies that the value of this column must be unique across all rows in the table.

   @Column(unique = true)
   private String email;

3. insertable:

   • Indicates whether the column is included in SQL INSERT statements generated by JPA.

   @Column(insertable = false)
   private String createdBy;

4. updatable:

   • Specifies whether the column is included in SQL UPDATE statements generated by JPA.

   @Column(updatable = false)
   private Timestamp creationTime;

5. columnDefinition:

   • Provides a complete database-specific definition for the column, overriding all other settings.

   @Column(columnDefinition = "VARCHAR(255) DEFAULT 'default_value'")
   private String customField;

6. table:

   • Specifies the table name that contains the column (typically used in conjunction with @SecondaryTable).

   // Assuming you have a secondary table annotated like this:
   // @Entity
   // @SecondaryTable(name = "address", pkJoinColumns = @PrimaryKeyJoinColumn(name = "user_id"))
   // public class User {
   //     ...
   // }
   
   // The field mapped to the secondary table would be specified as follows:
   @Column(table = "address")
   private String street;

7. precision and scale:

   • Specify precision and scale for decimal values.

   @Column(precision = 10, scale = 2)
   private BigDecimal balance;

8. columnDefinition (for complex database-specific constraints):

   • Specifies a complete column definition in the form of a SQL fragment.

   @Column(columnDefinition = "INT DEFAULT 0")
   private Integer statusCode;

Here's an example entity using multiple @Column attributes:

@Entity
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "username", length = 50, nullable = false, unique = true)
    private String username;

    @Column(length = 100)
    private String email;

    @Column(nullable = false, columnDefinition = "TIMESTAMP DEFAULT CURRENT_TIMESTAMP")
    private Timestamp createdAt;

    // Getters and setters
}

Using these attributes, you can finely control the behavior of each field in your entity classes when they are
mapped to database columns.