| < Day Day Up > |
|
Tables in a database management system often have primary keys, unique keys, and foreign keys.
A primary key is one or more columns that are used to uniquely identify a row in a table. A table can have only one primary key. The column(s) in a primary key cannot contain null values.
A unique key is also one or more columns that can be used to uniquely identify a row in a table. A table can have one or more unique keys. Unlike a primary key, a unique key can contain null values.
A foreign key is one or more columns that are associated with a primary key or unique key in another table. A table might have one or more foreign keys. A foreign key is dependent upon its associated primary or unique key. In other words, a foreign key cannot exist without that primary or unique key.
Note | When specifying metadata for a DBMS table with foreign keys, if you want to preserve the foreign key, you must specify metadata for all of the tables that are referenced by the foreign keys. |
For example, suppose that Table 1 had foreign keys that referenced primary keys in Table 2 and Table 3. To preserve the foreign keys in Table 1, you could use the Metadata Importer wizard or a source designer wizard to import metadata for Tables 1, 2, and 3.
| < Day Day Up > |
|