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| -rw-r--r-- | doc/src/sgml/ddl.sgml | 28 |
1 files changed, 26 insertions, 2 deletions
diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml index 23fa832742f..605027e8535 100644 --- a/doc/src/sgml/ddl.sgml +++ b/doc/src/sgml/ddl.sgml @@ -916,6 +916,11 @@ CREATE TABLE orders ( </para> <para> + You can assign your own name for a foreign key constraint, + in the usual way. + </para> + + <para> A foreign key can also constrain and reference a group of columns. As usual, it then needs to be written in table constraint form. Here is a contrived syntax example: @@ -931,9 +936,28 @@ CREATE TABLE t1 ( match the number and type of the referenced columns. </para> + <indexterm> + <primary>foreign key</primary> + <secondary>self-referential</secondary> + </indexterm> + <para> - You can assign your own name for a foreign key constraint, - in the usual way. + Sometimes it is useful for the <quote>other table</quote> of a + foreign key constraint to be the same table; this is called + a <firstterm>self-referential</firstterm> foreign key. For + example, if you want rows of a table to represent nodes of a tree + structure, you could write +<programlisting> +CREATE TABLE tree ( + node_id integer PRIMARY KEY, + parent_id integer REFERENCES tree, + name text, + ... +); +</programlisting> + A top-level node would have NULL <structfield>parent_id</structfield>, + but non-NULL <structfield>parent_id</structfield> entries would be + constrained to reference valid rows of the table. </para> <para> |