HashTable允许key和vaule为null。

HashTable允许key和vaule为null。


参考答案和解析
C

相关考题:

HashMap和Hashtable正确的说法有() A.都属于Map接口的类B.Hashtable类不是同步的,HashMap类是同步的C.Hashtable类不允许null键和null值D.HashMap类允许null键和null值

若用如下的SQL语句创建表S: CREATE TABLE S(S# CHAR(6)PRIMARY KEY, SNAME CHAR(8)NOT NULL,SEX CHAR(2),AGE INTEGER) 并且S表中已经有两行: (‘022345’,‘李明芳’,‘女’,23)和(‘021876’,‘张为’,‘男’,32) 今欲向S表插入新元组:Ⅰ.(‘022345’,‘彭明芳’,‘女’,23)Ⅱ.(‘020746’,‘杨云’,NULL,NULL)Ⅲ. (‘020721’,‘张为’,‘女’,21)哪一个(些)元组可以被成功插入?A.Ⅰ和ⅡB.Ⅰ和ⅢC.Ⅱ和ⅢD.全部

PRIMARY KEY约束和NULL约束不能同时定义在相同的列上。()

若用如下的SQL语句创建表S: CREATE TABLES (S# CHAR(6)PRIMARY KEY, SNAME CHAR(8)NOT NULL, SEX CHAR(2),AGE INTEGER) 并且S表中已经有两行: ('022345',‘李明芳’,‘女’,23)和('021876',‘张为’,‘男’,32) 今欲向S表插入新元组:( )。Ⅰ.('022345’,‘彭鸣明’,‘男’,25)Ⅱ.('020746',‘杨云’,NULL,NULL)Ⅲ.('020721',‘张为’,‘女’,21)A.全部B.Ⅰ和ⅡC.Ⅰ和ⅢD.Ⅱ和Ⅲ

若某企业的数据库系统中有如下所示的员工关系和仓库关系,每个仓库可有多名员工,但只有一名负责人。员工关系(cmployee):仓库关系(warehouse):则创建仓库表结构的SQL语句为(58)。A.CREATE TABLE(employeeID CHAR(2)NOT NULL UNIQUE, name CHAR(30)NOT NULL, address CHAR(40), pricipallD CHAR(3));B.CREATE warehouse(warehouseD CHAR(2)PRIMARY KEY, name CHAR(30), address CHAR(40), pficipalID CHAR(3));C.CREATE TABLE warehouse(warehouselD CHAR(2)PRIMARY KEY, name CHAR(30)NOT NULL, address CHAR(40), pricipalID CHAR(3), FOREIGN KEY(priciPalID)REFERENCES employee(employeeID));D.CREATE TABLE warehouse(warehouselD CHAR(2), name CHAR(30)NOT NULL, address CHAR(40), pricipalID CHAR(3), PRIMARY KEY(warehouselD), FOREIGN KEY(employeeID)REFERENCES emplo

Given the following requirements:Create a table to contain employee data, with a unique numeric identifier automatically assigned when a row is added, has an EDLEVEL column that permits only the values ‘C‘, ‘H‘ and ‘N‘, and permits inserts only when a corresponding value for the employee‘s department exists in the DEPARTMENT table.Which of the following CREATE statements will successfully create this table?()A.CREATE TABLE emp ( empno SMALLINT NEXTVAL GENERATED ALWAYS AS IDENTITY, firstname VARCHAR(30) NOT NULL, lastname VARCHAR(30) NOT NULL, workdept CHAR(3) NOT NULL, edlevel CHAR(1), PRIMARY KEY emp_pk (empno), FOREIGN KEY emp_workdept_fk ON (workdept) REFERENCES department (deptno), CHECK edlevel_ck VALUES (edlevel IN (‘C‘,‘H‘,‘N‘)), );B.CREATE TABLE emp ( empno SMALLINT NOT NULL GENERATED BY DEFAULT AS IDENTITY, firstname VARCHAR(30) NOT NULL, lastname VARCHAR(30) NOT NULL, workdept CHAR(3), edlevel CHAR(1), CONSTRAINT emp_pk PRIMARY KEY (empno), CONSTRAINT emp_workdept_fk FOREIGN KEY (workdept) REFERENCES department (deptno), CONSTRAINT edlevel_ck CHECK edlevel VALUES (‘C‘,‘H‘,‘N‘) );C.CREATE TABLE emp ( empno SMALLINT NEXTVAL GENERATED BY DEFAULT AS IDENTITY, firstname VARCHAR(30) NOT NULL, lastname VARCHAR(30) NOT NULL, workdept CHAR(3) NOT NULL, edlevel CHAR(1) CHECK IN (‘C‘,‘H‘,‘N‘)), CONSTRAINT emp_pk PRIMARY KEY (empno), CONSTRAINT emp_workdept_fk FOREIGN KEY department (deptno) REFERENCES (workdept) );D.CREATE TABLE emp ( empno SMALLINT NOT NULL GENERATED BY DEFAULT AS IDENTITY, firstname VARCHAR(30) NOT NULL, lastname VARCHAR(30) NOT NULL, workdept CHAR(3), edlevel CHAR(1), CONSTRAINT emp_pk PRIMARY KEY (empno), CONSTRAINT emp_workdept_fk FOREIGN KEY (workdept) REFERENCES department (deptno), CONSTRAINT edlevel_ck CHECK (edlevel IN (‘C‘,‘H‘,‘N‘)) );

The following statements:CREATE TABLE t1 (col1 INT NOT NULL, PRIMARY KEY(col1)); CREATE TABLE t2 (col1 INT NOT NULL, col2 CHAR(1) NOT NULL, PRIMARY KEY (col1, col2), FOREIGN KEY (col1) REFERENCES t1 (col1) ON DELETE CASCADE ON UPDATE RESTRICT); CREATE TABLE t3 (col1 INT NOT NULL, col2 INT NOT NULL, PRIMARY KEY (col1, col2),FOREIGN KEY (col1) REFERENCES t1 (col1) ON DELETE NO ACTION ON UPDATE RESTRICT);INSERT INTO t1 VALUES (1), (2); INSERT INTO t2 VALUES (1, ‘a‘), (1, ‘b‘), (2,‘c‘); INSERT INTO t3 VALUES (1, 100), (2, 200), (2,300);How many rows will be deleted by the following DELETE statement? DELETE FROM t1 WHERE col1= 1;()A.4B.3C.1D.0

_____A.PRIMARY KEY (部门号) NOT NULL UNIQUEB.PRIMARY KEY (部门名) UNIQUEC.FOREIGN KEY (负责人) REFERENCES 员工 (姓名)D.FOREIGN KEY (负责人) REFERENCES 员工 (员工号)A.B.C.D.

若用如下的SQL语句创建表S, CREATE TABLES (S# CHAR(6)PRIMARY KEY, SNAME CHAR(8)NOT NULL,SEX CHAR(2),AGE INTEGER)下列各元组可插入S表的是______。Ⅰ.('022345','彭鸣明','男','25')Ⅱ.('020746','杨云',NULL,NULL)Ⅲ.('020721','张为','女','21')A.全部B.Ⅰ和ⅡC.Ⅰ和ⅢD.Ⅱ和Ⅲ

Which values are NOT permitted to be part of the primary key?A.alpha-numeric charactersB.punctuationC.special charactersD.NULL

KV结构的集合,在处理null值的存储上有细微的区别,下列哪些说法是正确的: A.TreeMap的key不可以为nullB.TreeMap的key可以为nullC.ConcurrentHashMap的key可以为nullD.ConcurrentHashMap的value可以为null

Hashtable,HashMap,ConcurrentHashMap都是Map的实现类,它们在处理null值的存储上有细微的区别,下列哪些说法是正确的: A.Hashtable的KV都不可以为nullB.HashMap的KV都可以为nullC.HashMap的K不可以为null,V可以为nullD.ConcurrentHashMap的KV都不可以为null

下列关于完整性约束的叙述中,正确的是()。A、一个表可以有多个PRIMARY KEY约束B、在任何情况下PRIMARY KEY约束都可以被删除C、一个表可以有多个UNIQUE约束D、使用NULL/NOT NULL可以设定某列的值允许为空值或不允许为空值

If a unique constraint is defined on column COL1 of table TAB1, what are the characteristics of COL1?()A、COL1 will accept NULL values and can be referenced in another table's foreign key specification.B、COL1 will not accept NULL values and cannot be referenced in another tables foreign key specification.C、COL1 will not accept NULL values and can be referenced in another tables foreign key specification.D、COL1 will accept NULL values and cannot be referenced in another tables foreign key specification.

HashMap和Hashtable正确的说法有()    A、都属于Map接口的类B、Hashtable 类不是同步的,HashMap类是同步的C、Hashtable 类不允许 null 键和 null 值D、HashMap 类允许null 键和null 值

HashMap和Hashtable区别?

Given the following requirements: Create a table to contain employee data, with a unique numeric identifier automatically assigned when a row is added, has an EDLEVEL column that permits only the values 'C', 'H' and 'N', and permits inserts only when a corresponding value for the employee's department exists in the DEPARTMENT table. Which of the following CREATE statements will successfully create this table?()A、CREATE TABLE emp ( empno SMALLINT NEXTVAL GENERATED ALWAYS AS IDENTITY, firstname VARCHAR(30) NOT NULL, lastname VARCHAR(30) NOT NULL, workdept CHAR(3) NOT NULL, edlevel CHAR(1), PRIMARY KEY emp_pk (empno), FOREIGN KEY emp_workdept_fk ON (workdept) REFERENCES department (deptno), CHECK edlevel_ck VALUES (edlevel IN ('C','H','N')), );B、CREATE TABLE emp ( empno SMALLINT NOT NULL GENERATED BY DEFAULT AS IDENTITY, firstname VARCHAR(30) NOT NULL, lastname VARCHAR(30) NOT NULL, workdept CHAR(3), edlevel CHAR(1), CONSTRAINT emp_pk PRIMARY KEY (empno), CONSTRAINT emp_workdept_fk FOREIGN KEY (workdept) REFERENCES department (deptno), CONSTRAINT edlevel_ck CHECK edlevel VALUES ('C','H','N') );C、CREATE TABLE emp ( empno SMALLINT NEXTVAL GENERATED BY DEFAULT AS IDENTITY, firstname VARCHAR(30) NOT NULL, lastname VARCHAR(30) NOT NULL, workdept CHAR(3) NOT NULL, edlevel CHAR(1) CHECK IN ('C','H','N')), CONSTRAINT emp_pk PRIMARY KEY (empno), CONSTRAINT emp_workdept_fk FOREIGN KEY department (deptno) REFERENCES (workdept) );D、CREATE TABLE emp ( empno SMALLINT NOT NULL GENERATED BY DEFAULT AS IDENTITY, firstname VARCHAR(30) NOT NULL, lastname VARCHAR(30) NOT NULL, workdept CHAR(3), edlevel CHAR(1), CONSTRAINT emp_pk PRIMARY KEY (empno), CONSTRAINT emp_workdept_fk FOREIGN KEY (workdept) REFERENCES department (deptno), CONSTRAINT edlevel_ck CHECK (edlevel IN ('C','H','N')) );

有关hashMap跟hashTable的区别,说法正确的是()。A、HashMap和Hashtable都实现了Map接口B、HashMap是非synchronized,而Hashtable是synchronizedC、HashTable使用Enumeration,HashMap使用IteratorD、Hashtable直接使用对象的hashCode,HashMap重新计算hash值,而且用与代替求模

HashMap和HashTable的描述,错误的是()。A、他们都实现了Map接口B、HashMap非线程安全,在多个线程访问Hashtable时,不需要自己为它的方法实现同步,而HashMap就必须为之提供额外同步。C、HashMap允许将null作为一个entry的key或者value,而Hashtable不允许D、通过contains方法可以判断一个对象是否存在于HashMap或者Hashtable中

Which two statements are true about constraints? ()A、The UNIQUE constraint does not permit a null value for the column.B、A UNIQUE index gets created for columns with PRIMARY KEY and UNIQUE constraints.C、The PRIMARY KEY and FOREIGN KEY constraints create a UNIQUE index.D、The NOT NULL constraint ensures that null values are not permitted for the column.

You need to select a class that is optimized for key-based item retrieval from both small and large collections. Which class should you choose?()A、OrderedDictionary classB、HybridDictionary classC、ListDictionary classD、Hashtable class

单选题If a unique constraint is defined on column COL1 of table TAB1, what are the characteristics of COL1?()ACOL1 will accept NULL values and can be referenced in another table's foreign key specification.BCOL1 will not accept NULL values and cannot be referenced in another tables foreign key specification.CCOL1 will not accept NULL values and can be referenced in another tables foreign key specification.DCOL1 will accept NULL values and cannot be referenced in another tables foreign key specification.

多选题在设计数据库时,要充分考虑数据的完整性或准确性。下面关于primary key和unique的描述错误的是()A设为unique的列的值是不能重复的,用来唯一区别unique列的值Bprimary key列不可以有null值,而unique列是可以有null的Cprimary key列和unique列都不可以有null值Dprimary key用来在表中设置主键,主键列的值是可以重复的,用来唯一标识表中的每一条记录

多选题下列关于完整性约束的叙述中,正确的是()。A一个表可以有多个PRIMARY KEY约束B在任何情况下PRIMARY KEY约束都可以被删除C一个表可以有多个UNIQUE约束D使用NULL/NOT NULL可以设定某列的值允许为空值或不允许为空值

多选题Which two statements are true about constraints? ()AThe UNIQUE constraint does not permit a null value for the column.BA UNIQUE index gets created for columns with PRIMARY KEY and UNIQUE constraints.CThe PRIMARY KEY and FOREIGN KEY constraints create a UNIQUE index.DThe NOT NULL constraint ensures that null values are not permitted for the column.

单选题The following statements: CREATE TABLE t1 (col1 INT NOT NULL, PRIMARY KEY(col1)); CREATE TABLE t2 (col1 INT NOT NULL, col2 CHAR(1) NOT NULL, PRIMARY KEY (col1, col2), FOREIGN KEY (col1) REFERENCES t1 (col1) ON DELETE CASCADE ON UPDATE RESTRICT); CREATE TABLE t3 (col1 INT NOT NULL, col2 INT NOT NULL, PRIMARY KEY (col1, col2),FOREIGN KEY (col1) REFERENCES t1 (col1) ON DELETE NO ACTION ON UPDATE RESTRICT);INSERT INTO t1 VALUES (1), (2); INSERT INTO t2 VALUES (1, 'a'), (1, 'b'), (2,'c'); INSERT INTO t3 VALUES (1, 100), (2, 200), (2,300); How many rows will be deleted by the following DELETE statement? DELETE FROM t1 WHERE col1= 1;()A4B3C1D0

判断题PRIMARY KEY约束和NULL约束不能同时定义在相同的列上。A对B错

多选题HashMap和Hashtable正确的说法有()A都属于Map接口的类BHashtable 类不是同步的,HashMap类是同步的CHashtable 类不允许 null 键和 null 值DHashMap 类允许null 键和null 值