非空与唯一性约束

SQL 关系型数据库入门

Timo Grossenbacher

Data Journalist

非空约束

CREATE TABLE students (
 ssn integer not null,
 lastname varchar(64) not null,
 home_phone integer,
 office_phone integer
);

NULL != NULL

创建表时…

CREATE TABLE students (
 ssn integer not null,
 lastname varchar(64) not null,
 home_phone integer,
 office_phone integer
);

创建后…

ALTER TABLE students 
ALTER COLUMN home_phone 
SET NOT NULL;

ALTER TABLE students 
ALTER COLUMN ssn 
DROP NOT NULL;

CREATE TABLE table_name (
 column_name UNIQUE
);

ALTER TABLE table_name
ADD CONSTRAINT some_name UNIQUE(column_name);

SQL 关系型数据库入门