ORA-01731: circular view definition encountered - sql

Question

we are migrating over to oracle from sql server side.
on sqlserver we used to have a view like the following
create view blah
AS
Select column1,
column2
FROM blah;
but doing this on oracle produces circular view error.
is this not allowed on oracle side?

Answer 1

You cannot have a view reference itself. It logically does not make sense. A view is essentially a cached query whose results are displayed as a table. How can a query refer to itself?
Indeed, circular view definitions are not allowed in Oracle. If you have a circular view definition, then you likely have a bug in your database code that should be addressed. Perhaps the translation from SQL server to Oracle was flawed and accidentally introduced this circular definition?

Answer 2

You can actually do this in ORACLE, but it is more fragile, as you need to explicitly list the output columns of your CTE. So if you change the tables, you need to manually update the CTE.
Here is an example from our db, showing how to calculate the hierarchical depth of the a record...
CREATE OR REPLACE VIEW deploy.PHARMACYDISPENSE_EX
AS
WITH SRC (
PDID, WAREID, GCN_SEQNO, QTY, UOFM, XACTDTTM, CREATEDON, PROCESSEDON,
XACTTYPE, OPDID, CLOSEDON, BYPASSEDON, BYPASSEDBY, ITEMNO, LOTNO,
EXP_DATE, VOLUMETYPE, POTYPE, DEPTH
) AS (
SELECT D.PDID, D.WAREID, D.GCN_SEQNO, D.QTY, D.UOFM, D.XACTDTTM,
D.CREATEDON, D.PROCESSEDON, D.XACTTYPE, D.OPDID, D.CLOSEDON,
D.BYPASSEDON, D.BYPASSEDBY, D.ITEMNO, D.LOTNO, D.EXP_DATE,
D.VOLUMETYPE, D.POTYPE, 0 FROM deploy.PHARMACYDISPENSE D
WHERE OPDID IS NULL
UNION ALL
SELECT D.PDID, D.WAREID, D.GCN_SEQNO, D.QTY, D.UOFM, D.XACTDTTM,
D.CREATEDON, D.PROCESSEDON, D.XACTTYPE, D.OPDID, D.CLOSEDON,
D.BYPASSEDON, D.BYPASSEDBY, D.ITEMNO, D.LOTNO, D.EXP_DATE,
D.VOLUMETYPE, D.POTYPE, (S.DEPTH + 1)
FROM deploy.PHARMACYDISPENSE D JOIN SRC S ON S.PDID = D.OPDID
)
SELECT PD.*
FROM SRC PD;
The important part here is the WITH SRC (<output column list>) AS .... You need that output column list. So it is possible, and does work, it just takes a bit more code than in SQL Server.

Answer 3

Your example is incomplete - well at least doesn't show the pertinent parts.:
-- create a table
CREATE TABLE Scrap
(fieldName VARCHAR2(20));
-- create a view
CREATE VIEW ScrapVW1
AS
SELECT * FROM Scrap;
-- create a second view that uses the first view
CREATE VIEW ScrapVW2
AS
SELECT * FROM Scrap
UNION ALL
SELECT * FROM ScrapVW1;
-- recreate the first view that references the 2nd view which contains a reference to itself
CREATE OR REPLACE VIEW SCRAP_VW1
AS
SELECT * FROM ScrapVW2;
Gives a circular reference error when you try to recreate ScrapVW1. I would guess you have some unintentional name collision going on in your conversion. If it's quite complex I'd get rid of the 'CREATE OR REPLACE VIEW' syntax and just use CREATE VIEW which would then give you 'ORA-00955 Name already used' error.

Answer 4

Oracle deals with Hierarchical problems different than SQL apparently. Instead of self referring view, you can use connect by clause
SELECT employee_id, last_name, manager_id
FROM employees
CONNECT BY PRIOR employee_id = manager_id;