如何在与外键和主键没有相同字段的两个表之间创建关系

Question

I received an excel sheet from a client. It has three lines which I cannot understand.

In test table's row 4 (line 6) "RelationTo" column's testCategoryList.CategoryId. Does it mean that test table's foreign key testCategory is the Primary key of testCategoryList table?

But, test table's foreign key testCategory is not the primary key of testCategoryList table. Other two rows (line 7 and line 8) have the same structure.

testCategoryList table:

testSubjectList table:

testLevelList table:

Am I missing something? Those three lines don't make any sense.

Answer 1

I think your confusion might be over naming conventions.

Are you assuming that a foreign key column name must be identical to a primary key column name in order to match them? This is absolutely not true. There was a time in pre-relational database days where some storage technologies did have this requirement, but it is not part of the SQL standard at all.

Nevertheless, there is a naming convention which is popular with many people in which foreign key names match primary key names exactly. This enables something called natural joins .

However, not everyone agrees that this naming convention is a good idea. A couple of problems with it are that (a) if you have two FK's from one table to another, you have to let go of the convention to avoid duplicate column names, and (b) you end up having to put the table name in front of every single column to avoid unintended natural joins, such as person.name against company.name.

What your spreadsheet is clearly showing is that the FKs on the test table point to the PKs of the other three tables. For example the CREATE TABLE script for the test table would include:

...
CONSTRAINT FK_TEST__TESTCATEGORYLIST FOREIGN KEY
    IX_TEST__TESTCATEGORY (testCategory)
    REFERENCES testCategoryList (categoryId)
...