键与超键

SQL 关系型数据库入门

Timo Grossenbacher

Data Journalist

当前的数据库模型

带主键的数据库模型

什么是键？

能唯一标识记录的属性（集）
仍可删除属性时：称为超键
不能再删属性时：极小超键，即键

     license_no     | serial_no |    make    |  model  | year
 -------------------+-----------+------------+---------+------
 Texas ABC-739      | A69352    | Ford       | Mustang |    2
 Florida TVP-347    | B43696    | Oldsmobile | Cutlass |    5
 New York MPO-22    | X83554    | Oldsmobile | Delta   |    1
 California 432-TFY | C43742    | Mercedes   | 190-D   |   99
 California RSK-629 | Y82935    | Toyota     | Camry   |    4
 Texas RSK-629      | U028365   | Jaguar     | XJS     |    4

SK1 = {license_no, serial_no, make, model, year}

SK2 = {license_no, serial_no, make, model}

SK3 = {make, model, year}, SK4 = {license_no, serial_no}, SKi, ..., SKn

改编自 Elmasri, Navathe (2011)：Fundamentals of Database Systems, 6th Ed., Pearson

     license_no     | serial_no |    make    |  model  | year
 -------------------+-----------+------------+---------+------
 Texas ABC-739      | A69352    | Ford       | Mustang |    2
 Florida TVP-347    | B43696    | Oldsmobile | Cutlass |    5
 New York MPO-22    | X83554    | Oldsmobile | Delta   |    1
 California 432-TFY | C43742    | Mercedes   | 190-D   |   99
 California RSK-629 | Y82935    | Toyota     | Camry   |    4
 Texas RSK-629      | U028365   | Jaguar     | XJS     |    4

K1 = {license_no}; K2 = {serial_no}; K3 = {model}; K4 = {make, year}

K1 至 K3 仅含一个属性
从 K4 中删去"make"或"year"都会产生重复
只有一个候选键可被选为"选定"键

一起看看有哪些键！

SQL 关系型数据库入门