It is not supported to look beyond the current row in a CHECK constraint. http://www.postgresql.org/docs/9.1/interactive/sql-createtable.html says: A check constraint specified as a column constraint should reference that column’s value only, while an expression appearing in a table constraint can reference multiple columns. Currently, CHECK expressions cannot contain subqueries nor refer to variables other than columns … Read more
Since Table2 has a composite primary key (FileID, FileType), then any reference to it must also include both columns. ALTER TABLE dbo.Table1 ADD CONSTRAINT FK_Table1_Table2 FOREIGN KEY(FileID, FileType) REFERENCES Table2(FileID, FileType) Unless you have a unique constraint/index on the Table2.FileID field (but if so: why isn’t this the PK??), you cannot create a FK relationship … Read more
This is a bad design. Just make a 2-column primary key, and 2-column foreign keys to it. This is a fundamental anti-pattern called “encoding information in keys” which (thereby) are called “smart”, “intelligent” or “concatenated” keys. A good key is a “dumb” key. Eg:: Despite it now being easy to implement a Smart Key, it … Read more
To achieve what you want you need to provide some aditional configuration.Code First convention can identify bidirectional relationships, but not when there are multiple bidirectional relationships between two entities.You can add configuration (using Data Annotations or the Fluent API) to present this information to the model builder. With Data Annotations, you’ll use an annotation called … Read more
You must make your foreign key nullable: public class User { public int Id { get; set; } public int? CountryId { get; set; } public virtual Country Country { get; set; } }
None of that is necessary, especially not the doubling up the tables. Introduction Since the Standard for Modelling Relational Databases (IDEF1X) has been in common use for over 25 years (at least in the high quality, high performance end of the market), I use that terminology. Date & Darwen, despite1 consistent with the great work … Read more
Old question but adding answer so that one can get help Its two step process: Suppose, a table1 has a foreign key with column name fk_table2_id, with constraint name fk_name and table2 is referred table with key t2 (something like below in my diagram). table1 [ fk_table2_id ] –> table2 [t2] First step, DROP old … Read more
A foreign key must reference only one parent table. This is fundamental to both SQL syntax, and relational theory. A Polymorphic Association is when a given column may reference either of two or more parent tables. There’s no way you can declare that constraint in SQL. The Polymorphic Associations design breaks rules of relational database … Read more
In one-to-one relation one end must be principal and second end must be dependent. Principal end is the one which will be inserted first and which can exist without the dependent one. Dependent end is the one which must be inserted after the principal because it has foreign key to the principal. In case of … Read more