接上，现有两张表，department和employee

1、为department的manager添加唯一性约束，保证manager列中不出现重复值（unique_manager是随便设置的约束名称）

alter table department
add constraint unique_manager unique(manager);

关于唯一性约束的各种用法UNIQUE使用

2、将George换成Army

update department
set manager='Army'
where manager='George';
update employee
set name='Army'
where name='George';

3、为department的manager添加非空约束，保证不出现空值

alter table department
alter column manager set not null;

各种约束的添加方式不同，详见添加primary key，not null，unique，default约束

4、限制department的所有budget>100000

alter table department
add constraint check_budget check(budget>100000);

详细用法见添加not null，default，check