mysql - 我们是否必须明确提及主键的非空约束？

Question

创建表时是否必须明确指出主键不应该为空？即使我们不提及主键的“not null”约束，SQL 也不允许我们将 NULL 值插入到主键字段中。因此，以下哪一项是好的做法？

1)    create table registration(
        id int primary key,
        name varchar(10)

    )

2)     create table registration(
        id int primary key not null,
        name varchar(10)

    )

Answer 1

No你不知道。

A PRIMARY KEY is a unique index where all key columns must be defined as NOT NULL. If they are not explicitly declared as NOT NULL, MySQL declares them so implicitly (and silently). A table can have only one PRIMARY KEY. The name of a PRIMARY KEY is always PRIMARY, which thus cannot be used as the name for any other kind of index.

不过，我认为明确说明是一种很好的做法 - 因为这会让事情变得更加清晰， 而且您不必每次有疑问时都查找这条规则。