I have a db table with a column that is a String. I do not consider the case to be significant (e.g. "TEST == "test"). Unfortunately, it appears that JPA2 does, because both values are inserted into my table; I would like the second one to be rejected.
Is there a generic way to annotate an "ignore-case" unique constraint on a string column?
As an alternative, I could also consider putting a unique "ignore-case" constraint on the actual db column. Is that possible in Oracle 10?
What I don't want to do is write code, because this occurs often in this particular db.
All help is greatly appreciated.
you can achieve this with a function-based unique index
create unique index <index_name> on <table_name> (UPPER(<column_name>));
for Example
create table t111( col varchar2(10));
create unique index test_idx on t111 (UPPER(col));
insert into t111 values('test');
insert into t111 values ('TEST');
Related
I have a programming situation in which I have to check the uniqueness of a column of the table.
Say My table is employee table and I have added a column that is code. Now I have to prevent the insertion of MGR and mgr.
So that means I can not have 2 rows having case insensitive values.
How to add the check constraints ?
If you are using Oracle, you may add a unique index on the lowercase of your column.
create unique index <index_name> on <tablename>(lower(<column_name>))
I understand I can't do this straightforward from studying similar questions on stackoverflow and other sites.
However, I need to do this and I'm willing to go with workarounds.
I tried to create a non-unique index with online and parallel, and then drop the old unique index. However, it fails saying ORA-01408: such column list already indexed.
How to convert an unique index to a non-unique one?
If you don't want to drop the old index before creating the new one, you can cheat a bit by creating the new index with an additional useless column, e.g.:
Assuming a table with the following configuration:
create table mytable (id number);
create unique index myunique on mytable (id);
To convert the index to non unique:
create index temp on mytable (id, 1);
drop index myunique;
create index mynonunique on mytable (id);
drop index temp;
In practice I'm not sure how necessary this is - generally I'd just drop and recreate the index in some low-activity period, preferably take the application down.
Oracle now supports multiple indexes applied to the same set of columns as long as they differ in uniqueness (and/or in some other properties such as bitmap vs. btree and partitioning) and
as long as only one of them is visible.
Therefore you can alter the existing index - changing it to invisible without dropping it and then create a new non-unique index.
CREATE TABLE mytable (id NUMBER);
CREATE UNIQUE INDEX mytable_unique_idx ON mytable(id);
ALTER INDEX mytable_unique_idx INVISIBLE;
CREATE INDEX mytable_nonunique_idx ON mytable(id);
Note that invisible indexes are still maintained by the database and you can change between them by turning one of them to invisible and the second one to visible.
I have a few non-unique constraints that I want to alter into unique constraints ( business rules have changed since the data model was made ). Is there any way to do it with out dropping and recreating as a unique constraint? I was thinking there would be an option in the alter constraint command, but I have not found anything.
Thanks!!
You cannot convert a non-unique index into a unique index.
(It's difficult to say what cannot be done. I'm basing this answer on looking at the ALTER INDEX page of the SQL Language Reference, searching for the word UNIQUE, and not finding any relevant hints. I looked at 11g instead of 10g, but that's probably better in this case because there are a few features that exist in 10g but are only documented in 11g.)
However, you can use a non-unique index for a unique constraint. But there are some performance considerations: a unique index would be smaller and faster.
create table my_table(a number);
create index my_table_index on my_table(a);
alter table my_table add constraint my_table_unique unique (a)
using index my_table_index;
You can't modify a constraint in the way you wish you can only drop and recreate it. If you want to do this with no downtime then look into the DBMS_REDEFINITION package.
In my case, just do drop and re-create the index:
DROP INDEX index_name;
CREATE UNIQUE INDEX index_name ON table_name (col01,col02) TABLESPACE indx;
Could someone clarify what is the purpose of having unique index without unique constraint (Oracle)?
For example,
create table test22(id int, id1 int, tmp varchar(20));
create unique index idx_test22 on test22(id);
insert into test22(id, id1, tmp) values (1, 2, 'aaa'); // ok
insert into test22(id, id1, tmp) values (1, 2, 'aaa'); // fails, ORA-00001: unique
// constraint (TEST.IDX_TEST22) violated
So far it looks like there is a constraint. But
create table test33(id int not null primary key,
test22_id int not null,
foreign key(test22_id) references test22(id) );
also fails with "ORA-02270: no matching unique or primary key for this column-list".
I'm totally confused by this behaviour. Is there a constraint or not?
There are many articles that explain why it's possible to have a unique constraint without unique index; that is clear and makes perfect sense. However, I don't understand the reason for unique index without constraint.
A constraint and an index are separate logical entities. A unique constraint, for example, is visible in USER_CONSTRAINTS (or ALL_CONSTRAINTS or DBA_CONSTRAINTS). An index is visible in USER_INDEXES (or ALL_INDEXES or DBA_INDEXES).
A unique constraint is enforced by an index though it is possible (and sometimes necessary) to enforce a unique constraint using a non-unique index. A deferrable unique constraint, for example, is enforced using a non-unique index. If you create a non-unique index on a column and subsequently create a unique constraint, you can also use that non-unique index to enforce the unique constraint.
In practice, a unique index acts very much like a unique, non-deferrable constraint in that it raises the same error that a unique constraint raises since the implementation of unique constraints uses the index. But it is not quite the same because there is no constraint. So, as you've seen, there is no unique constraint so you cannot create a foreign key constraint that references the column.
There are cases where you can create a unique index that you cannot create a unique constraint. A function-based index, for example, that enforces conditional uniqueness. If I wanted to create a table that supported logical deletes but ensure that COL1 is unique for all non-deleted rows
SQL> ed
Wrote file afiedt.buf
1 CREATE TABLE t (
2 col1 number,
3 deleted_flag varchar2(1) check( deleted_flag in ('Y','N') )
4* )
SQL> /
Table created.
SQL> create unique index idx_non_deleted
2 on t( case when deleted_flag = 'N' then col1 else null end);
Index created.
SQL> insert into t values( 1, 'N' );
1 row created.
SQL> insert into t values( 1, 'N' );
insert into t values( 1, 'N' )
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.IDX_NON_DELETED) violated
SQL> insert into t values( 1, 'Y' );
1 row created.
SQL> insert into t values( 1, 'Y' );
1 row created.
But if we're talking about a straight unique non-function based index, there are probably relatively few cases where it really makes more sense to create the index rather than creating the constraint. On the other hand, there are relatively few cases where it makes much difference in practice. You'd almost never want to declare a foreign key constraint that referenced a unique constraint rather than a primary key constraint so you rarely lose something by only creating the index and not creating the constraint.
As was already explained in other answers: constraints and the indexes are different entities. But they lack precise definitions and official comments on the topic. Before we discuss the relationship between these two entities lets take a look at their purpose independent of each other.
Purpose of a constraint1:
Use a constraint to define an integrity constraint-- a rule that restricts the values in a database.
The purposes of an index2:
You can create indexes on columns to speed up queries. Indexes provide faster access to data for operations that return a small portion of a table's rows.
In general, you should create an index on a column in any of the following situations:
The column is queried frequently.
A referential integrity constraint exists on the column.
A UNIQUE key integrity constraint exists on the column.
Now we know what constraints and indexes are, but what is the relationship between them?
The relationship between indexes and constraints is3:
a constraint MIGHT create an index or use an existing index to efficient enforce itself. For example, a PRIMARY KEY constraint will either create an index (unique or non-unique depending) or it will find an existing suitable index and use it.
an index has nothing to do with a constraint. An index is an index.
So, a constraint MIGHT create/use and index. An INDEX is an INDEX, nothing more, nothing less.
So sum this up and directly address the following sentence from your question:
However, I don't understand the reason for unique index without constraint.
Indexes speed up queries and integrity checks (constraints). Also for conditional uniqueness a unique (functional) index is used as this cannot be achieved with a constraint.
Hopefully this brings a little bit more clarification to the whole topic, but there is one aspect of the original question that remains unanswered:
Why did the following error occur when no constraint existed:
ORA-00001: unique constraint (TEST.IDX_TEST22) violated
The answer is simple: there is no constraint and the error message misnames it!
See the official "Oracle Ask TOM" comment 4 on the same problem:
It isn't a constraint. the error message "misnames" it.
If it were a constraint, you could create a foreign key to it -- but you cannot.
Hope it helps.
Links:
1 Oracle 10g Documentation on Constraints
2 Oracle 10g Documentation on Selecting an Index Strategy
3 4 "Oracle Ask TOM" answer to a similar problem
Another point which may be useful in this context is :
Disabling/Dropping an existing unique constraint do not drop the underlying unique index. You got to drop the unique index explicitly.
You can not make conditional uniqueness by declaring a unique constraint, But you can do it by declaring a unique index.
Supporse if you try to execute below:
alter table test22
add constraint test22_u
unique (id, case when tmp = 'aaa' then null else tmp end);
ORA-00904: : invalid identifier
But if you can do it by using the unique index
create unique index test22_u
on test22 ( customer_id,
case when is_default = 'Y' then null else address_id end)
Using Oracle, if a column value can be 'YES' or 'NO' is it possible to constrain a table so that only one row can have a 'YES' value?
I would rather redesign the table structure but this is not possible.
[UDPATE] Sadly, null values are not allowed in this table.
Use a function-based index:
create unique index only_one_yes on mytable
(case when col='YES' then 'YES' end);
Oracle only indexes keys that are not completely null, and the CASE expression here ensures that all the 'NO' values are changed to nulls and so not indexed.
This is a kludgy hack, but if the column allows NULLs, then you could use NULL in place of "NO" and use "YES" just as before. Apply a unique key constraint to that column, and you'll never get two "YES" values, but still have many NOs.
Update: #Nick Pierpoint: suggested adding a check constraint so that the column values are restricted to just "YES" and NULL. The syntax is all worked out in his answer.
You will want to check a Tom Kyte article with exactly this question being asked and his answer:
http://tkyte.blogspot.com/2008/05/another-of-day.html
Summary: don't use triggers, don't use autonomous transactions, use two tables.
If you use an Oracle database, then you MUST get to know AskTom and get his books.
It doesn't work on the table definition.
However, if you update the table using a trigger calling a stored procedure, you could make sure that only one row contains "YES".
Set all rows to "NO"
Set the row you want to YES
Following on from my comment to a previous answer by yukondude, I'd add a unique index and a check constraint:
create table mytest (
yesorno varchar2(3 char)
);
create unique index uk_mytest_yesorno on mytest(yesorno);
alter table mytest add constraint ck_mytest_yesorno check (yesorno is null or yesorno = 'YES');
Does Oracle support something like filtered indices (last week I heard that e.g. MSSQL2008 does)? Maybe you can define a unique key which applies only to rows with the value "Yes" in your column.
I guess I'd use a second table to point to the appropriate row in your current table. That other table could be used to store values of other variables too too.