SQL笔记（约束、外键、casewhen）

SQL CHECK 约束

CREATE TABLE Salaries
(name VARCHAR(32) PRIMARY KEY,
 sex CHAR(1) NOT NULL,
 salary INTEGER NOT NULL,
 CHECK (salary<200000));

CREATE TABLE Salaries
(name VARCHAR(32) PRIMARY KEY,
 sex CHAR(1) NOT NULL,
 salary INTEGER NOT NULL,
 CONSTRAINT check_salary CHECK ( sex = '2' AND salary <= 200000 ));

update:

 

 

添加外键：

alter table book add constraint book_category foreign key (cid) references category(cid);