You can create a function and call it from a trigger on that column insert
event
Thanks
Dinesh Pandey
-----Original Message-----
From: pgsql-sql-owner@postgresql.org [mailto:pgsql-sql-owner@postgresql.org]
On Behalf Of Robert D. Kennedy
Sent: Sunday, September 04, 2005 12:20 AM
To: pgsql-sql@postgresql.org
Cc: rdkennedy@acm.org
Subject: [SQL] Table Constraint CHECK(SELECT()) alternative?
Hello,
I have seen in another thread that sub-queries in a CHECK constraint
have implementation ramifications that make them awkward to implement and
support. OK, fair enough, c'est la vie.
ERROR: cannot use subquery in check constraint
is the result. I have a model which seems to BEG for just such a feature
though. What alternatives are there to the use of CHECK(SELECT()) in this
model?
I am implementing a basic tree of nodes. There are leafs and
directories... node types. Each node in a directory has a unique name, and
the node type ids point to a table with node type names. Each node points to
a valid node (its parent). There is more... but now to my question. I wish
to constrain parent nodes to only be directories type nodes. Leafs cannot be
parents.
Table "public.raw_pnfs_nodes" Column | Type |
Modifiers
----------------+-----------------------+-----------------------------------
----------------------------------node_id | integer | not null default
nextval('public.raw_pnfs_nodes_node_id_seq'::text)parent_node_id | integer | not nullnode_name |
charactervarying(80) | not nullnode_type_id | smallint | not null
Indexes: "raw_pnfs_nodes_pkey" primary key, btree (node_id) "raw_pnfs_nodes_node_name_key" unique, btree
(node_name,parent_node_id)
Foreign-key constraints: "parent_refersto_node" FOREIGN KEY (parent_node_id) REFERENCES
raw_pnfs_nodes(node_id) ON UPDATE RESTRICT ON DELETE RESTRICT "raw_pnfs_nodes" FOREIGN KEY (node_type_id) REFERENCES
node_types(node_type_id) ON UPDATE RESTRICT ON DELETE RESTRICT
What I WANT to write is the table constraint:
CONSTRAINT parents_are_dirs_check CHECK( (SELECT A.node_type_id FROM
raw_pnfs_nodes A, raw_pnfs_nodes B WHERE A.node_id = B.parent_node_id) = 1)
where "1" is the id for directory node types (ok, this is not polished yet)
Hopefully this illustrates the idea. The node type of a node that is a
parent to another node must be the directory node type. Is there another way
to express this in SQL? I would like to avoid putting this into the business
logic layer.
Thanks much,
RDK
