问题
I'm attempting to setup a foreign key in table cell_lines that will reference the topographic_region column of the composite primary key in table topographic_regions.
Each time I run the last three lines of code trying to add the foreign key, I receive Error Code 1215: cannot add foreign key constraint.
Now, the foreign key column name (topographic_region) in cell_lines only matches one of the composite primary key column names in topographic_regions, the other composite primary key column name being topographic_region_id. Do I usually need to address both components of a composite primary key when creating a foreign key?
A follow up problem is that I've actually already tried addressing both components of a composite primary key using a composite foreign key constraint, and I was still presented with an Error Code 1215: cannot add foreign key constraint.
What can I do to solve this problem, and is there anymore information you would like me to provide in order to do so? I'm happy to respond.
Thanks for reading. I'm very new to mySQL.
create table topographic_regions(
topographic_regions_id int not null auto_increment,
topographic_region int(10),
karyotypes varchar(255),
constraint pk_topographicID primary key (topographic_regions_id, topographic_region)
);
create table cell_lines(
cell_lines_id int not null auto_increment,
cell_line varchar(50),
topographic_region int(10),
constraint pk_cellID primary key (cell_lines_id, cell_line)
);
alter table cell_lines
add foreign key (topographic_region)
references topographic_regions(topographic_region);
回答1:
This is the problem with composite PKs. In fact, your autonumber topographic_region_id will be unique and you should use that for the PK, and the FK. topographic_region sounds like it is also unique so you should add a unique index to it.
回答2:
A foreign key is some columns whose subrow values have to appear as subrow values in another table where they are a candidate key. If you really had a foreign key from cell_lines to topographic_regions then cell_lines would have a topographic_region_name column and you would need:
alter table cell_lines
add foreign key (topographic_regions_id, topographic_region)
references topographic_regions(topographic_regions_id, topographic_region);
I suspect that (topographic_regions_id, topographic_region) is not a candidate key of topographic_regions and that topographic_regions_id is enough to identify a region just because you decided cell_lines doesn't have a topographic_region column. Although it may be that a cell line doesn't identify a particular topographic region. But then topographic_regions_id is not a foreign key in cell_lines, since it isn't a key in topographic_regions. (There is no easy way in SQL to constrain that some columns' subrow values have to appear as subrow values in another table where they are not a superset of a candidate key.)
If topographic_regions_id is unique in topographic_regions then it is a candidate key and should be declared UNIQUE NOT NULL. If topographic_region_name is unique in topographic_regions then it is a candidate key and should be declared UNIQUE NOT NULL. If either is a candidate key then (topographic_regions_id, topographic_region) is not a candidate key. You can pick one candidate key to be declared PRIMARY KEY which just means UNIQUE NOT NULL. Ditto for cell_line. Since your _id columns are auto_increment, I suspect they are unique, in which case neither table has a composite candidate key.
(All this assuming none of your columns can be NULL.)
来源：https://stackoverflow.com/questions/26891456/how-do-i-reference-a-composite-primary-key-with-a-foreign-key-using-mysql