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

我认为最好考虑一下表中可能存在的基数。 我们可以让最小的基数为0。当它是可选的时，相关表的元组的最小参与可以是零，现在你面临着外键值被允许为空的必要性。

但答案是，这完全取决于业务。

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

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。

这取决于这个外键在关系中扮演什么角色。

如果这个外键也是你的关系中的键属性，那么它不能为NULL 如果这个外键是关系中的一个普通属性，那么它可以为NULL。

默认情况下，对外键没有约束，外键可以为null和重复。

在创建表/修改表时，如果你添加了唯一性或非空的约束，那么只有它不允许空值/重复值。