name: The column name. Later in this chapter, we will come across situations where a column is not in the expected table. In these cases, you can specify the column name as <table-name>.<column-name>, or even <schema-name>.<table-name>.<column-name>.

The column name is required for runtime use. The reason the name attribute isn't marked #REQUIRED in the DTD is that some vendors may allow you to partially-specify column information during the mapping process, and have the rest filled in by a vendor-supplied tool.

jdbc-type: Determines what JDBC APIs the implementation uses to load and store data to the column. This attribute is only required when the column uses a non-default type for the associated data - for example, when a String is mapped to a CLOB column rather than a CHAR or VARCHAR.

Standard values for this attribute include all of the constant names from the java.sql.Types class, in either uppercase or lowercase: BIGINT, bigint, BLOB, blob, DECIMAL, decimal, VARCHAR, varchar, etc.

sql-type: The database-specific column type name. This attribute is only used by vendors that support creating tables from your mapping metadata. During table creation, the vendor will use the text of the sql-type attribute as the declared column type. If no sql-type is given, the vendor will choose an appropriate default based on the column's JDBC type, length, and scale.

length: The column length. This attribute is typically only used during table creation, though some vendors might use it to validate data before flushing. CHAR and VARCHAR columns typically default to a length of 255; other column types use the database default.

scale: The number of decimal digits a numeric column can hold. This attribute is often used in conjunction with length to form the proper column type name during table creation.

allows-null: Whether the column can store null values. Vendors may use this attribute both for table creation and at runtime; however, it is never required. Defaults to false for primary key columns and columns holding primitive field values, and true for all other columns.

default-value: A database-assigned default value for the column if no value is given in the INSERT SQL. Vendors will pass this default on to the database during table creation. When a new object is being persisted with Java default field values, vendors may use this attribute to decide whether to include the columns for those fields in the generated INSERT SQL.

	Note
	Kodo uses a combination of a column's default value and JDO metadata's null-value field attribute to decide whether to include a column in an INSERT statement. The following table describes what value is inserted for a field with a default value, based on whether the column has a default-value and on the field's null-value attribute:

target: In joins, the column that this column joins to. We examine joins in detail in the next section.

target-field: In joins, the primary key field of the related type that this column joins to. We examine joins in detail in the next section.