有没有可能在Postgressql中创建一个可空的外键约束？(Is it possible to create a nullable foreign key constraint in Postgre

有没有可能在Postgressql中创建一个可空的外键约束？(Is it possible to create a nullable foreign key constraint in PostgreSQL?)

我在表'XY'上有一个可空的int8列'z_id'。 'z_id'是'Z'表的主键。 如果在persistense期间提供'z_id'，我想要一个约束将'Z'实体映射到'XY'表。

I have a nullable int8 column 'z_id' on a table 'XY'. The 'z_id' is the primary key of the 'Z' table. I'd like to have a constraint that maps the 'Z' entity to the 'XY' table if the 'z_id' is provided during persistense.

最满意答案

如果我正确理解你，你会尝试：

t=# create table z(i int primary key); CREATE TABLE t=# insert into z values (1); INSERT 0 1 t=# create table xy(i int); CREATE TABLE t=# insert into xy values(1),(null); INSERT 0 2 t=# alter table xy add constraint fk foreign key (i) references z(i); ALTER TABLE t=# select * from xy; i --- 1 (2 rows)

你可能有同样的怀疑 ？

If I understand you correctly, you try to:

t=# create table z(i int primary key); CREATE TABLE t=# insert into z values (1); INSERT 0 1 t=# create table xy(i int); CREATE TABLE t=# insert into xy values(1),(null); INSERT 0 2 t=# alter table xy add constraint fk foreign key (i) references z(i); ALTER TABLE t=# select * from xy; i --- 1 (2 rows)

And you probably had same doubdts?..