一. 官網對Unique Constraints說明
http://download.oracle.com/docs/cd/E11882_01/server.112/e16508/datainte.htm#CNCPT1642
uniquekey constraint requires that every value in a column or set of columns beunique. No rows of a table may have duplicate values in a column (the uniquekey) or set of columns (the composite unique key) with a unique key constraint.
Note:
Theterm key refers only to the columns defined in the integrity constraint. Because the database enforces a unique constraint byimplicitly creating or reusing an index on the key columns, the term uniquekey is sometimes incorrectly used as a synonym for unique key constraint orunique index.
--數據庫在創建unique constraint的同時，強制創建或者重用列上的索引。如果之前列上沒有索引，那么強制創建的索引是unique index，如果列上已經存在索引，就重用之前的索引。
Uniquekey constraints are appropriate for any column where duplicate values are notallowed. Unique constraints differ from primary keyconstraints, whose purpose is to identify each table row uniquely, andtypically contain values that have no significance other than being unique.Examples of unique keys include:
（1）A customer phone number, where the primary key is the customernumber
（2）A department name, where the primary key is the department number
Asshown in Example2-1, a unique key constraint exists on the email column of the hr.employeestable. The relevant part of the statement is as follows:
CREATE TABLE employees ( ...
,email VARCHAR2(25)
CONSTRAINT emp_email_nn NOT NULL ...
,CONSTRAINT emp_email_uk UNIQUE (email) ... );
Theemp_email_uk constraint ensures that no two employees have the same emailaddress, as shown in Example5-1.
Example 5-1 Unique Constraint
SQL> SELECT employee_id, last_name,email FROM employees WHERE email = 'PFAY';
EMPLOYEE_ID LAST_NAME EMAIL
----------- --------------------------------------------------
202 Fay PFAY
SQL> INSERT INTO employees (employee_id,last_name, email, hire_date, job_id)
1 VALUES(999,'Fay','PFAY',SYSDATE,'ST_CLERK');
.
.
.
ERROR at line 1:
ORA-00001:unique constraint (HR.EMP_EMAIL_UK) violated
Unless a NOT NULLconstraint is also defined, a null always satisfies a unique key constraint. Thus,columns with both unique key constraints and NOT NULL constraints are typical.This combination forces the user to enter values in the unique key andeliminates the possibility that new row data conflicts with existing row data.
Note:
Because of the searchmechanism for unique key constraints on multiple columns, you cannot haveidentical values in the non-null columns of a partially null composite uniquekey constraint.
二. 相關測試
2.1 測試unique index 和 uniqueconstraint
SYS@anqing2(rac2)> create table ut(idnumber,phone varchar2(15),name varchar2(15));
Table created.
SYS@anqing2(rac2)> insert into utvalues(1,'13888888888','dave');
1 row created.
SYS@anqing2(rac2)> insert into utvalues(1,'13888888888','dave');
1 row created.
SYS@anqing2(rac2)> insert into utvalues(2,'13899999999','dba');
1 row created.
SYS@anqing2(rac2)> commit;
Commit complete.
--在phone 字段上，我們創建uniqueconstraint
SYS@anqing2(rac2)> alter table ut addconstraint uc_phone unique(phone);
alter table ut add constraint uc_phoneunique(phone)
*
ERROR at line 1:
ORA-02299: cannot validate (SYS.UC_PHONE) -duplicate keys found
--這里報錯，因為我們在插入數據的時候，有重復值，先刪除掉重復值
SYS@anqing2(rac2)> select * from ut;
ID PHONE NAME
---------- --------------- ---------------
1 13888888888 dave
2 13899999999 dba
1 13888888888 dave
SYS@anqing2(rac2)> delete from ut whererownum=1;
1 row deleted.
SYS@anqing2(rac2)> commit;
Commit complete.
SYS@anqing2(rac2)> select * from ut;
ID PHONE NAME
---------- --------------- ---------------
2 13899999999 dba
1 13888888888 dave
--唯一性約束創建成功
SYS@anqing2(rac2)> alter table ut addconstraint uc_phone unique(phone);
Table altered.
--查看約束
SYS@anqing2(rac2)> selectconstraint_name,constraint_type,table_name,index_owner,index_name fromuser_constraints where table_name = 'UT';
CONSTRAINT_NAME C TABLE_NAME INDEX_OWNER INDEX_NAME
--------------- - -------------------------- -------------
UC_PHONE U UT SYS UC_PHONE
--Oracle 自動創建了索引並關聯到約束, 索引名和約束名是相同的。
--驗證下索引
SYS@anqing2(rac2)> selectindex_name,index_type,uniqueness,generated from user_indexes wheretable_name='UT';
INDEX_NAME INDEX_TYPE UNIQUENES GENERATED
------------- ------------- -------------------
UC_PHONE NORMAL UNIQUE N
--我們並沒有創建索引，而是在創建unique constraint時，oracle 強制創建了uniqueindex。
--現在我們drop index 看看
SYS@anqing2(rac2)> drop index uc_phone;
drop index uc_phone
*
ERROR at line 1:
ORA-02429: cannot drop index used forenforcement of unique/primary key
--這里報錯，不能刪除unique/primary key 上的索引。在這種情況下，我們只有先刪除約束。
SYS@anqing2(rac2)> alter table ut dropconstraint uc_phone;
Table altered.
SYS@anqing2(rac2)> drop index uc_phone;
drop index uc_phone
*
ERROR at line 1:
ORA-01418: specified index does not exist
--再次drop 索引時，提示索引已經不存在，說明已經在刪除約束的同時，把索引刪掉了。
SYS@anqing2(rac2)> selectconstraint_name,constraint_type,table_name,index_owner,index_name fromuser_constraints where table_name = 'UT';
no rows selected
SYS@anqing2(rac2)> selectindex_name,index_type,uniqueness,generated from user_indexes wheretable_name='UT';
no rows selected
結論：
當約束列上沒有索引時，在創建unique constraint 時，oracle 會自動創建unique index，並且該索引不能刪除，當刪除unique constraint 時，unique index 會自動刪除。
2.2 測試unique constraint 和non-unique index
--現在字段phone上創建B-Tree索引
SYS@anqing2(rac2)> create indexidx_ut_phone on ut(phone);
Index created.
--查看索引
SYS@anqing2(rac2)> selectindex_name,index_type,uniqueness,generated from user_indexes wheretable_name='UT';
INDEX_NAME INDEX_TYPE UNIQUENES GENERATED
------------- ------------- -------------------
IDX_UT_PHONE NORMAL NONUNIQUE N
--創建unique constraint
SYS@anqing2(rac2)> alter table ut add constraint uc_phoneunique(phone);
Table altered.
--查看約束和索引信息
SYS@anqing2(rac2)> selectconstraint_name,constraint_type,table_name,index_owner,index_name fromuser_constraints where table_name = 'UT';
CONSTRAINT_NAME C TABLE_NAME INDEX_OWNER INDEX_NAME
--------------- - -------------------------- -------------
UC_PHONE U UT SYS IDX_UT_PHONE
--這里重用了已經存在的索引
SYS@anqing2(rac2)> selectindex_name,index_type,uniqueness,generated from user_indexes wheretable_name='UT';
INDEX_NAME INDEX_TYPE UNIQUENES GENERATED
------------- ------------- -------------------
IDX_UT_PHONE NORMAL NONUNIQUE N
--刪除索引
SYS@anqing2(rac2)> drop indexIDX_UT_PHONE;
drop index IDX_UT_PHONE
*
ERROR at line 1:
ORA-02429: cannot drop index used forenforcement of unique/primary key
--這個提示和之前的一樣，我們先刪除約束，在來查看
SYS@anqing2(rac2)> alter table ut dropconstraint uc_phone;
Table altered.
SYS@anqing2(rac2)> select constraint_name,constraint_type,table_name,index_owner,index_namefrom user_constraints where table_name = 'UT';
no rows selected
--這里約束已經刪除掉了。
SYS@anqing2(rac2)> selectindex_name,index_type,uniqueness,generated from user_indexes wheretable_name='UT';
INDEX_NAME INDEX_TYPE UNIQUENES GENERATED
------------- ------------- -------------------
IDX_UT_PHONE NORMAL NONUNIQUE N
--但是我們的索引並在刪除約束時刪除掉
--在手工刪除索引，成功
SYS@anqing2(rac2)> drop indexIDX_UT_PHONE;
Index dropped.
SYS@anqing2(rac2)> selectindex_name,index_type,uniqueness,generated from user_indexes wheretable_name='UT';
no rows selected
--重新把約束和索引加上，然后一次刪除
SYS@anqing2(rac2)> create indexidx_ut_phone on ut(phone);
Index created.
SYS@anqing2(rac2)> alter table ut addconstraint uc_phone unique(phone);
Table altered.
SYS@anqing2(rac2)> selectconstraint_name,constraint_type,table_name,index_owner,index_name fromuser_constraints where table_name = 'UT';
CONSTRAINT_NAME C TABLE_NAME INDEX_OWNER INDEX_NAME
--------------- - -------------------------- -------------
UC_PHONE U UT SYS IDX_UT_PHONE
SYS@anqing2(rac2)> selectindex_name,index_type,uniqueness,generated from user_indexes wheretable_name='UT';
INDEX_NAME INDEX_TYPE UNIQUENES GENERATED
------------- ------------- -------------------
IDX_UT_PHONE NORMAL NONUNIQUE N
SYS@anqing2(rac2)> alter table ut drop constraint uc_phone drop index;
Table altered.
SYS@anqing2(rac2)> selectconstraint_name,constraint_type,table_name,index_owner,index_name fromuser_constraints where table_name = 'UT';
no rows selected
SYS@anqing2(rac2)> selectindex_name,index_type,uniqueness,generated from user_indexes wheretable_name='UT';
no rows selected
--索引和約束一次刪除
小結：
當我們的列上有索引時，在創建unique constraint時，Oracle 會重用之前的索引，並且不會改變索引的類型，在第一個測試里，Oracle 自動創建的索引是unique index。
當我們刪除約束時，關聯的索引不會自動刪除。 這個問題的MOS 上有說明。 參考MOS [ID309821.1]。
我們可以分兩步，先刪除約束，在刪除索引。 MOS 提供了方法，就是在刪除約束時，加上drop index，這樣就能一次搞定。
SQL>altertable ut drop constraint uc_phone drop index;
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