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 confl...
Theterm key refers only to the columns defined in the integrity constraint. Because the database enforces a unique constraint byimplicitly creating or reusing anindexon the key columns, the term uniquekey is sometimes incorrectly used as a synonym for unique key constraint orunique index. --数据...
另: UNIQUE Constraints vs UNIQUE Indexes Many database administrators ask about the difference between a UNIQUE constraint and a UNIQUE index. While you may use different Transact-SQL commands to create them (ALTER TABLE…ADD CONSTRAINT for constraints and CREATE UNIQUE INDEX for indexes), they hav...
Theterm key refers only to the columns defined in the integrity constraint.Because the database enforces a unique constraint byimplicitly creating or reusing anindexon the key columns,the term uniquekey is sometimes incorrectly used as a synonym for unique key constraint orunique index. --数据库...
ADD CONSTRAINT 约束名 PRIMARY KEY (列名1, 列名2, ...); 2、唯一约束(Unique) 唯一约束用于确保表中的某一列或多列的组合具有唯一的值,与主键约束不同,唯一约束允许有空值,创建唯一约束的语法如下: CREATE TABLE 表名 ( 列名1 UNIQUE, 列名2 ..., ...
上述语句表示在table_name表的column1, column2, ...列上创建一个名为index_name的唯一索引。 当违反唯一性约束时,Oracle数据库会抛出以下错误之一: 1、ORA00001: 常见的错误,当插入或更新操作试图创建重复的键值时抛出。 “`sql ORA00001: unique constraint (schema_name.constraint_name) violated ...
不能删除unique/primary key上的索引。在这种情况下,我们只有先删除约束。 再次drop索引时,提示索引已经不存在,说明已经在删除约束的同时,把索引删掉了。 当约束列上没有索引时,在创建unique constraint 时,oracle会自动创建unique index,并且该索引不能删除,当删除unique constraint 时,unique index会自动删除。
触发器(Trigger)、表(Table)、索引(Index)、约束(Constraint)、视图(View)、序列(Sequence)、同义词(Synonym)、簇(Cluster)、过程(Procedure)、函数(Function)、包(Package)、触发器(Trigger)、对象类型(Object Type) 、数据库链(Database Link) 5、SQL语句和SQL*Plus命令的区别; ...
Life’s not quite so “simple” for a Non-Unique index. Note also and most importantly that life gets no easier for a Non-Unique index that polices a PK or Unique key constraint. Even though there’s a PK or Unique constraint on a column, to Oracle, it’s just another Non-U...
public void setIndexID(DBObjectIDid) Set the Index that implements this UniqueConstraint Parameters: id- the ID of the Index isUnique public static boolean isUnique(Columncol) Returns true if the given table is constrained by a UniqueConstraint (including PKConstraint) in its parent table. T...