标签：number custom char arc alt 创建 增删改 lte ble
1. 非空约束
列的为空性决定表中的行是否可以包含空值。空置（NULL）不同于零（0）/空白或者长度为零的字符串（“”）。
（1）创建非空约束
create table orders ( docentry int constraint pk_id primary key not null, --红色部分非空约束 customer_number varchar(20), order_number varchar(20) , order_line int, order_price float )
（2）修改非空约束
alter table orders alter column customer_number varchar(20) not null alter table orders alter column order_number varchar(20) not null alter table orders alter column order_line int not null
标签：number custom char arc alt 创建 增删改 lte ble
原文地址：https://www.cnblogs.com/howie-we/p/12075456.html
