oracle change a column to unique - oracle

Question

Table has been created in system this way
CREATE TABLE INSTANCES
(
DM INTEGER NOT NULL,
INSTANCEID VARCHAR2(512) NOT NULL,
INSTANCENAME VARCHAR2(64) NOT NULL UNIQUE,
HOSTNAME VARCHAR2(32) NOT NULL,
CONSTRAINT PK_INSTANCES PRIMARY KEY (INSTANCEID, HOSTNAME)
);
The new crete table statement is as below:
CREATE TABLE INSTANCES
(
DM INTEGER NOT NULL,
INSTANCEID VARCHAR2(512) NOT NULL UNIQUE,
INSTANCENAME VARCHAR2(64) NOT NULL UNIQUE,
HOSTNAME VARCHAR2(32) NOT NULL,
CONSTRAINT PK_INSTANCES PRIMARY KEY (INSTANCEID, HOSTNAME)
);
The differnce is INSTANCEID has UNIQUE in it. How do i Alter the table? I used the below statement and it did not work for me.
ALTER TABLE INSTANCES ADD CONSTRAINT ab UNIQUE ( INSTANCEID);
It gave an error:
ALTER TABLE INSTANCES ADD CONSTRAINT ab UNIQUE ( INSTANCEID)
Error report:
SQL Error: ORA-02261: such unique or primary key already exists in the table
02261. 00000 - "such unique or primary key already exists in the table"
*Cause: Self-evident.
*Action: Remove the extra key.
Please help me to Alter the table as required above. Thanks!
Here is the output of SELECT con.constraint_name, col.column_name, con.constraint_type
FROM user_cons_columns col
JOIN user_constraints con ON (col.constraint_name = con.constraint_name)
WHERE col.table_name = 'INSTANCES';
"CONSTRAINT_NAME","COLUMN_NAME","CONSTRAINT_TYPE"
"SYS_C0016531","DM","C"
"SYS_C0016532","INSTANCEID","C"
"SYS_C0016533","INSTANCENAME","C"
"SYS_C0016534","HOSTNAME","C"
"PK_INSTANCES","HOSTNAME","P"
"PK_INSTANCES","INSTANCEID","P"
"SYS_C0016536","INSTANCENAME","U"

Answer 1

You have already stated that INSTANCEID is supposed to be UNIQUE, so a constraint has been created.
CREATE TABLE INSTANCES
(
DM INTEGER NOT NULL,
INSTANCEID VARCHAR2(512) NOT NULL UNIQUE, -- UNIQUE constraint
INSTANCENAME VARCHAR2(64) NOT NULL UNIQUE,
HOSTNAME VARCHAR2(32) NOT NULL,
CONSTRAINT PK_INSTANCES PRIMARY KEY (INSTANCEID, HOSTNAME)
);
Edit: Ok, after reading your comment, try this:
SELECT con.constraint_name, col.column_name, con.constraint_type
FROM user_cons_columns col
JOIN user_constraints con ON (col.constraint_name = con.constraint_name)
WHERE col.table_name = 'INSTANCES'
AND con.constraint_type = 'U'
;
It will list UNIQUE constraints and associated columns for INSTANCE table. Please check if there is a unique constraint on the INSTANCEID column (and if that constraint has no other associated columns).
Example at SQLFiddle: http://sqlfiddle.com/#!4/43b43/6
Edit #2: creating named constraints, all options:
-- CREATE TABLE - "In Line" Constraints
CREATE TABLE ports (
ID NUMBER CONSTRAINT PORT_ID_PK PRIMARY KEY,
NAME VARCHAR2(20)
);
CREATE TABLE ports (
ID NUMBER,
NAME VARCHAR2(20) CONSTRAINT NAME_NN NOT NULL
);
CREATE TABLE ports (
ID NUMBER,
NAME VARCHAR2(20) CONSTRAINT NAME_UQ UNIQUE
);
CREATE TABLE ports (
ID NUMBER,
STATUS NUMBER CONSTRAINT PROPER_STATUS_CK
CHECK (STATUS IN (4, 5))
);
CREATE TABLE ships (
SHIP_ID NUMBER,
NAME VARCHAR2(20),
HOME_PORT_ID NUMBER CONSTRAINT SHIP_PORT_FK
REFERENCES PORTS (ID)
);
-- CREATE TABLE - "Out of Line" Constraints
CREATE TABLE ports (
ID NUMBER,
NAME VARCHAR2(20),
CONSTRAINT PORT_ID_PK PRIMARY KEY (ID)
);
-- NOT NULL constraints can not be created "Out of Line"!
CREATE TABLE ports (
ID NUMBER,
NAME VARCHAR2(20),
CONSTRAINT NAME_UQ UNIQUE (NAME)
);
CREATE TABLE ports (
ID NUMBER,
STATUS NUMBER,
CONSTRAINT PROPER_STATUS_CK
CHECK (STATUS IN (4, 5))
);
CREATE TABLE ships (
SHIP_ID NUMBER,
NAME VARCHAR2(20),
HOME_PORT_ID NUMBER,
CONSTRAINT SHIP_PORT_FK FOREIGN KEY
(HOME_PORT_ID) REFERENCES PORTS (ID)
);
-- ALTER TABLE - "In Line" Constraints
ALTER TABLE PORTS MODIFY ID
CONSTRAINT PORT_ID_PK PRIMARY KEY;
ALTER TABLE PORTS MODIFY NAME
CONSTRAINT NAME_NN NOT NULL;
ALTER TABLE PORTS MODIFY NAME
CONSTRAINT NAME_UQ UNIQUE;
ALTER TABLE SHIPS MODIFY HOME_PORT_ID
CONSTRAINT SHIP_PORT_FK REFERENCES PORTS (ID);
-- ALTER TABLE - "Out of Line" Constraints
ALTER TABLE PORTS ADD CONSTRAINT
PORT_ID_PK PRIMARY KEY (ID);
-- NOT NULL constraints can not be created "Out of Line"!
ALTER TABLE PORTS ADD CONSTRAINT
NAME_UQ UNIQUE (NAME);
ALTER TABLE PORTS ADD
CONSTRAINT PROPER_STATUS_CK
CHECK (STATUS IN (4, 5));
ALTER TABLE SHIPS ADD CONSTRAINT SHIP_PORT_FK
FOREIGN KEY (HOME_PORT_ID)
REFERENCES PORTS (ID);
NOT NULL constraints cannot be create of out line.