ABAP第三步：建立域、数据元素和搜索帮助
top-down: proceeding by breaking large general aspects (as of a problem) into smaller more detailed constituents : working from the general to the specific
bottom-up: progressing upward from the lowest levels (as of a stratified organization or system)
A Data Model have three basic components: Entities, Attributes, Entity Types.
Entity: An entity is a uniquely identifiable object for which information is to be collected.
Attribute: An attribute is a characteristic of an entity.
Entity Type: An entity type is a set of entities with common attributes.
Primary Key: A primary key is that attribute or combination of attributes that uniquely identifies an entity of a particular type. A customer ID number might serve as the primary key of the customers entity type.
Relationships between entities can be classified in several ways:
Cardinality: the cardinality of a relationship describes the number of entities of one type that relate to entities of other types.
Optionality: The optionality of a relationship describes whether or not entities of one type must refer to entities of another type.
Category: A relationship’s category provides information about whether one entity is identified by its relationship to another entity
重要的编辑键
key Function
F8 执行程序
F5 复制游标所在列的内容
F11 储存档案
CTRL+F11 删除游标所在列
域（Domain）：描述一个字段类型和长度等信息；
数据元素（Data Element）：描述一个字段的用途。
使用时，先定义域，再定义数据元素使用哪个域，最后定义一个表字段使用哪些数据元素。
Create new Domain: SE11
Input the Data Type, No.Characters, Output Length. Saved and active.
Create new Data Element: SE11
Maintain Data Element in Table: SE11
Input the appropriate Data Element
Create the Search Help: SE11
Double click “YCT_NAME” to establish Data Element
Input Parameters “YCT_ID”, save and active
SE16 open Data Browser, we can see YCT_ID have “Search Help”
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