蛋塔賣你 (Data Mining)

資料庫常出現null值，但大致來說，其可能有下列三種可能性：

1. Unknown value - 例如某位員工的生日無從得知，所以就用null

2. Unavailable or withheld value - 例如某位員工只有家用電話號碼而無公司用電話號碼

3. Not applicable attribute - 例如某位學生沒有大學學歷，故沒有主修科目的資料

三種刪除資料的限制的處理方式

1. Restric ; 無法刪除

2.Cascade; 當刪除資料時，一併行除其相關連的資料

3.Set null or set default; 當刪除資料時，將期關連的資料設為null或設為預設值

常用的SQL指令如下：

SELECT <attribute and function list>

FROM <table list>

[WHERE <condition>]

[GROUP BY <grouping attribute>]

[HAVING <group condition>]

[ORDOR BY <attribute>]

Aggreagte Function in SQL

COUNT(*), COUNT (DISTINCT *), AVG(*), SUM(*), MAX(*), MIN(*)

EXISTS, NOT EXISTS Funciton in SQL

SELECT

FROM 

WHERE EXISTS (........有值=True)

SELECT

FROM

WHERE NOT EXISTS (.........無值=Ture)

常見的資料庫限制(Database Constraint)大致上可分為三類：

1.Inherent model-based constaints 或 Implicit constraints

2.Schema-based constaints 或Explicit constraints

3.Application-based 或Semantic constaints或Business rule

在資料庫系統的定義中：

Tuple代表資料中的行;

Attribute代表每個欄位的屬性

Table代表各欄位的關係

Atomic 指的是不能再切割的值

Domain指的是所允許格式的範圍

Relation schema就是meta-data指的是一組Attribute清單

Conceptual design: that can be represented and manipulated using some computerized tools so that it can be easily maintained, modified, and transformed into a data baesd implementation. Such as: ER model (Entity-Relationship model)

Logical design：that can be expressed in a data model implemented in a commerical DBMS. Such as: Relational Data Model

Physical design: during which further specifications are provided for storing and accessing the database

Source: Database Systems by Rmaes Elmasri

下表可以看出其在不同特性上的差異：