我们需要为主键指定“非空”吗？甲骨文/SQL
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Do we need to specify "not null" for primary key? Oracle/SQL
提问by Mr.Y
CREATE TABLE Person(
PersonId NUM(20),
...
)
ALTER TABLE Person
ADD(CONSTRAINT personpk PRIMARY KEY(PersonId))
As title, do I need to specify "not null" for PersonId? Or if I set it to primary key, it is automatically not null by default?
作为标题，我是否需要为 PersonId 指定“非空”？或者如果我把它设置为主键，默认情况下它自动不为空？
e.g:
CREATE TABLE Person(
PersonId NUM(20) NOT NULL,
...
回答by eaolson
create table mytable (
col1 number primary key,
col2 number,
col3 number not null
);
table MYTABLE created.
select table_name, column_name, nullable
from user_tab_cols where table_name = 'MYTABLE';
TABLE_NAME COLUMN_NAME NULLABLE
------------------------------ ------------------------------ --------
MYTABLE COL1 N
MYTABLE COL2 Y
MYTABLE COL3 N
So, no, you do not need to specify primary key columns as NOT NULL.
因此，不，您不需要将主键列指定为 NOT NULL。
回答by wolφi
Yes, as @eaolson said, you don't need to specify NOT NULL for primary key columns, they are set automatically to NOT NULL.
是的，正如@eaolson 所说，您不需要为主键列指定 NOT NULL，它们会自动设置为 NOT NULL。
However, Oracle keeps track that you didn't specify NOT NULL explicitly in case the primary key is disabled or dropped later on:
但是，Oracle 会跟踪您没有明确指定 NOT NULL，以防主键稍后被禁用或删除：
create table mytable (
col1 number,
col2 number not null
);
select table_name, column_name, nullable
from user_tab_columns where table_name = 'MYTABLE';
TABLE_NAME COLUMN_NAME NULLABLE
------------ ------------ ---------
MYTABLE COL1 Y
MYTABLE COL2 N
As expected, col1 is nullable and col2 NOT NULL. A primary key changes both columns to NOT NULL:
正如预期的那样，col1 可以为空，而 col2 不是 NULL。主键将两列都更改为 NOT NULL：
alter table mytable add primary key (col1, col2);
select table_name, column_name, nullable
from user_tab_columns where table_name = 'MYTABLE';
TABLE_NAME COLUMN_NAME NULLABLE
------------ ------------ ---------
MYTABLE COL1 N
MYTABLE COL2 N
If you disable or drop the primary key, both columns revert to the original state, co1 becomes nullable again:
如果禁用或删除主键，两列都恢复到原始状态，co1 再次变为可空：
alter table mytable disable primary key;
select table_name, column_name, nullable
from user_tab_columns where table_name = 'MYTABLE';
TABLE_NAME COLUMN_NAME NULLABLE
------------ ------------ ---------
MYTABLE COL1 Y
MYTABLE COL2 N
回答by Mudassir Hasan
Primary key by definition can never be Null. Primary key purpose is to uniquely identify records. A primary key is a combination of columns which uniquely specify a row.
根据定义，主键永远不能为 Null。主键的目的是唯一标识记录。主键是唯一指定行的列的组合。
A Null value represents lack of value. Even if two records have NULL in same column , the column values are not considered equal.
Null 值表示缺少值。即使两个记录在同一列中为 NULL，列值也不被视为相等。
回答by Ross
In most DBMS, since its a primary key (and the definition of is that it must be unique within the table) then it most certainly cannot be null.
在大多数 DBMS 中，由于它是一个主键（并且定义是它在表中必须是唯一的），那么它肯定不能为空。
