外键可以为NULL吗?

现有的答案集中在单列场景。如果我们考虑多列外键，我们使用SQL标准中定义的MATCH [SIMPLE | PARTIAL | FULL]子句有更多的选项:

PostgreSQL-CREATE TABLE A value inserted into the referencing column(s) is matched against the values of the referenced table and referenced columns using the given match type. There are three match types: MATCH FULL, MATCH PARTIAL, and MATCH SIMPLE (which is the default). MATCH FULL will not allow one column of a multicolumn foreign key to be null unless all foreign key columns are null; if they are all null, the row is not required to have a match in the referenced table. MATCH SIMPLE allows any of the foreign key columns to be null; if any of them are null, the row is not required to have a match in the referenced table. MATCH PARTIAL is not yet implemented. (Of course, NOT NULL constraints can be applied to the referencing column(s) to prevent these cases from arising.)

例子:

CREATE TABLE A(a VARCHAR(10), b VARCHAR(10), d DATE , UNIQUE(a,b));
INSERT INTO A(a, b, d) 
VALUES (NULL, NULL, NOW()),('a', NULL, NOW()),(NULL, 'b', NOW()),('c', 'b', NOW());

CREATE TABLE B(id INT PRIMARY KEY, ref_a VARCHAR(10), ref_b VARCHAR(10));

-- MATCH SIMPLE - default behaviour nulls are allowed
ALTER TABLE B ADD CONSTRAINT B_Fk FOREIGN KEY (ref_a, ref_b) 
REFERENCES A(a,b) MATCH SIMPLE;

INSERT INTO B(id, ref_a, ref_b) VALUES (1, NULL, 'b');  

-- (NULL/'x') 'x' value does not exists in A table, but insert is valid
INSERT INTO B(id, ref_a, ref_b) VALUES (2, NULL, 'x');  

ALTER TABLE B DROP CONSTRAINT IF EXISTS B_Fk; -- cleanup

-- MATCH PARTIAL - not implemented
ALTER TABLE B ADD CONSTRAINT B_Fk FOREIGN KEY (ref_a, ref_b) 
REFERENCES A(a,b) MATCH PARTIAL;
-- ERROR:  MATCH PARTIAL not yet implemented

DELETE FROM B; ALTER TABLE B DROP CONSTRAINT IF EXISTS B_Fk; -- cleanup

-- MATCH FULL nulls are not allowed
ALTER TABLE B ADD CONSTRAINT B_Fk FOREIGN KEY (ref_a, ref_b) 
REFERENCES A(a,b) MATCH FULL;

-- FK is defined, inserting NULL as part of FK
INSERT INTO B(id, ref_a, ref_b) VALUES (1, NULL, 'b');
-- ERROR:  MATCH FULL does not allow mixing of null and nonnull key values.

-- FK is defined, inserting all NULLs - valid
INSERT INTO B(id, ref_a, ref_b) VALUES (1, NULL, NULL);

db < > fiddle演示

外键可以为NULL吗?

现有的答案集中在单列场景。如果我们考虑多列外键，我们使用SQL标准中定义的MATCH [SIMPLE | PARTIAL | FULL]子句有更多的选项:

PostgreSQL-CREATE TABLE A value inserted into the referencing column(s) is matched against the values of the referenced table and referenced columns using the given match type. There are three match types: MATCH FULL, MATCH PARTIAL, and MATCH SIMPLE (which is the default). MATCH FULL will not allow one column of a multicolumn foreign key to be null unless all foreign key columns are null; if they are all null, the row is not required to have a match in the referenced table. MATCH SIMPLE allows any of the foreign key columns to be null; if any of them are null, the row is not required to have a match in the referenced table. MATCH PARTIAL is not yet implemented. (Of course, NOT NULL constraints can be applied to the referencing column(s) to prevent these cases from arising.)

例子:

CREATE TABLE A(a VARCHAR(10), b VARCHAR(10), d DATE , UNIQUE(a,b));
INSERT INTO A(a, b, d) 
VALUES (NULL, NULL, NOW()),('a', NULL, NOW()),(NULL, 'b', NOW()),('c', 'b', NOW());

CREATE TABLE B(id INT PRIMARY KEY, ref_a VARCHAR(10), ref_b VARCHAR(10));

-- MATCH SIMPLE - default behaviour nulls are allowed
ALTER TABLE B ADD CONSTRAINT B_Fk FOREIGN KEY (ref_a, ref_b) 
REFERENCES A(a,b) MATCH SIMPLE;

INSERT INTO B(id, ref_a, ref_b) VALUES (1, NULL, 'b');  

-- (NULL/'x') 'x' value does not exists in A table, but insert is valid
INSERT INTO B(id, ref_a, ref_b) VALUES (2, NULL, 'x');  

ALTER TABLE B DROP CONSTRAINT IF EXISTS B_Fk; -- cleanup

-- MATCH PARTIAL - not implemented
ALTER TABLE B ADD CONSTRAINT B_Fk FOREIGN KEY (ref_a, ref_b) 
REFERENCES A(a,b) MATCH PARTIAL;
-- ERROR:  MATCH PARTIAL not yet implemented

DELETE FROM B; ALTER TABLE B DROP CONSTRAINT IF EXISTS B_Fk; -- cleanup

-- MATCH FULL nulls are not allowed
ALTER TABLE B ADD CONSTRAINT B_Fk FOREIGN KEY (ref_a, ref_b) 
REFERENCES A(a,b) MATCH FULL;

-- FK is defined, inserting NULL as part of FK
INSERT INTO B(id, ref_a, ref_b) VALUES (1, NULL, 'b');
-- ERROR:  MATCH FULL does not allow mixing of null and nonnull key values.

-- FK is defined, inserting all NULLs - valid
INSERT INTO B(id, ref_a, ref_b) VALUES (1, NULL, NULL);

db < > fiddle演示

下面是一个使用Oracle语法的例子: 首先，让我们创建一个表COUNTRY

CREATE TABLE TBL_COUNTRY ( COUNTRY_ID VARCHAR2 (50) NOT NULL ) ;
ALTER TABLE TBL_COUNTRY ADD CONSTRAINT COUNTRY_PK PRIMARY KEY ( COUNTRY_ID ) ;

创建表PROVINCE

CREATE TABLE TBL_PROVINCE(
PROVINCE_ID VARCHAR2 (50) NOT NULL ,
COUNTRY_ID  VARCHAR2 (50)
);
ALTER TABLE TBL_PROVINCE ADD CONSTRAINT PROVINCE_PK PRIMARY KEY ( PROVINCE_ID ) ;
ALTER TABLE TBL_PROVINCE ADD CONSTRAINT PROVINCE_COUNTRY_FK FOREIGN KEY ( COUNTRY_ID ) REFERENCES TBL_COUNTRY ( COUNTRY_ID ) ;

这在Oracle中运行得非常好。注意第二个表中的COUNTRY_ID外键没有“NOT NULL”。

现在要向PROVINCE表中插入一行，只需指定PROVINCE_ID就足够了。但是，如果您选择指定一个COUNTRY_ID，那么它必须已经存在于COUNTRY表中。

我认为一个表的外键也是另一个表的主键。所以它不允许空值。所以外键没有空值的问题。

外键的思想是基于引用一个已经存在于主表中的值的概念。这就是为什么在另一个表中它被称为外键。这个概念被称为参考完整性。如果外键被声明为空字段，它将违反引用完整性的逻辑。它指的是什么?它只能引用主表中存在的内容。因此，我认为将外键字段声明为null是错误的。

1 -是的，因为至少SQL Server 2000。

2 -是的，只要它不是UNIQUE约束或链接到唯一索引。