Icon of primary key in ER diagram 2. 唯一键(unique key) Unique key can be specified in create table and alter table statement. CREATETABLESCOTT.DEPT(DEPTNONUMBER(2,0),DNAMEVARCHAR2(14),LOCVARCHAR2(13),CONSTRAINTconstraint_nameUNIQUE(DEPTNO));ALTERTABLESCOTT.DEPTADDCONSTRAINTconstraint_nameUNIQUE...
一般只有一个 Foreign keys(连接两个table) 来自另一个table的primary key 可以改名字 可以有多个 矩形--代表一个实体(entity),也就是一个类别,在数据库中是一个table 椭圆--代表entity中的属性,如果是唯一(unique),则在属性下面画一条横线 FK(foreign key)-- 代表 ,用来连接两个table,也就是两个entity编辑...
An entity can be place, person, object, event or a concept, which stores data in the database. The characteristics of entities are must have an attribute, and a unique key. Every entity is made up of some 'attributes' which represent that entity. a.1 entity set 实体集是具有相同属性的...
实体是ER图中最基本的元素，表示现实世界中的一个独立的、有实际意义的对象。在ER图中，实体通常用矩形框表示，框内写上实体的名称。每个实体都具有一个唯一的标识符，称为实体的主键（Primary Key）。实体之间通过关系连接起来，形成一个网络结构。属性是实体或关系的特征或描述，用于描述实体或关系的特性。在ER...
唯一索引(Unique Key):唯一索引也是一种约束。唯一索引的属性列不能出现重复的数据,但是允许数据为 NULL,一张表允许创建多个唯一索引。建立唯一索引的目的大部分时候都是为了该属性列的数据的唯一性,而不是为了查询效率。 普通索引(Index):普通索引的唯一作用就是为了快速查询数据,一张表允许创建多个普通索引,并允许...
Attributesare characteristics of an entity (i.e., many-to-many or one-to-one). The primary key is crucial in establishing unique identification and relational schema. Relationshipsare associations between entities. Weak entitiesdepend on another entity. ...
Attributesare characteristics of an entity (i.e., many-to-many or one-to-one). The primary key is crucial in establishing unique identification and relational schema. Relationshipsare associations between entities. Weak entitiesdepend on another entity. ...
UNIQUE("AGE"), constraintPK_CS1 PRIMARY KEY("ID") )STORAGE(ON "MAIN", CLUSTERBTR) ; COMMENTON COLUMN "TABLE_1"."AGE" IS 'CS_AA'; COMMENTON COLUMN "TABLE_1"."COLUMN_11" IS 'CS_11'; COMMENTON COLUMN "TABLE_1"."C_2222" IS 'CS_'; ...
在弹出的对话框中,填写Name、Type、Length,并在最下方选择数据的类型Include in Primary Key、Nullable、Unique、Index;单击OK或者Apply,如下图所示。 单击左侧One-to-Many Relationship图标,在右侧的两个实体间进行连线,在弹出的界面单击OK,如下图,即可使得两个实体具有一对多的关系。
An article that contains everything on ER diagrams, entity symbols, attribute symbols, relationship symbols, cardinality, and several other ER diagram notations used worldwide.