06 2021 档案

 
25、 char、varchar、nchar、nvarchar、nvarchar2的区别
摘要:CHAR vs VARCHAR, NCHAR vs NVARCHAR CHAR, NCHAR : 固定长度(最多8000个英文,4000个汉字) 如定义CHAR(5),无论使用字符多少都会占用5字节 VARCHAR, NVARCHAR : 可变长度(最多8000个英文,4000个汉字)如定义VARC 阅读全文
posted @ 2021-06-11 13:00 leaves0529 阅读(169) 评论(0) 推荐(0)
24、Index
摘要:create INDEX projects_creater_ix on projects(creater) Drop INDEX projects_creater_ix 阅读全文
posted @ 2021-06-11 12:43 leaves0529 阅读(31) 评论(0) 推荐(0)
23、Constraint and delete
摘要:NOT NULL UNIQUE:CONSTRAINT uc_PersonID UNIQUE (Id_P,LastName) PRIMARY KEY:CONSTRAINT pk_PersonID PRIMARY KEY (Id_P) FOREIGN KEY:FOREIGN KEY (Id_P) REF 阅读全文
posted @ 2021-06-11 11:14 leaves0529 阅读(56) 评论(0) 推荐(0)
22、design mode : Normalization
摘要:1nf : atomic 2nf : partial dependencies 3nf : transitive dependency 阅读全文
posted @ 2021-06-11 10:43 leaves0529 阅读(50) 评论(0) 推荐(0)