Learning immediately: https://edu.csdn.net/course/play/4364/77143?utm_source=blogtoedu
# 创建表时指定非空列
create table fk
(
sid int not null,
sname varchar(20) not null,
cardid varchar(20)
)** SQL Server after the specified non-null columns, add nulls not allowed to add record success;
Although mySQL specify the non-empty column, add error record is not a null value, such as the column is numeric 0 is inserted, is inserted into the character '' (not empty but no content Null).
insert into fk (sid, sname) values (1, null) --报错，不允许插入空值
# 如何查询没有内容的记录？
select * from fk where sname=''
# 如何查询为空的记录？
select * from fk where cardid is null
# 增加非空约束
alter table fk modify column sid integer(11) NOT NULL
# 取消非空约束，增加默认值
alter table fk modify column sid integer(11) default NULL
