Create a one to many relationship using SQL Server

Question

How do you create a one to many relationship using SQL Server

User · Answer

This is a simple example of a classic Order example. Each Customer can have multiple Orders, and each Order can consist of multiple OrderLines.

You create a relation by adding a foreign key column. Each Order record has a CustomerID in it, that points to the ID of the Customer. Similarly, each OrderLine has an OrderID value. This is how the database diagram looks:

In this diagram, there are actual foreign key constraints. They are optional, but they ensure integrity of your data. Also, they make the structure of your database clearer to anyone using it.

I assume you know how to create the tables themselves. Then you just need to define the relationships between them. You can of course define constraints in T-SQL (as posted by several people), but they're also easily added using the designer. Using SQL Management Studio, you can right-click the Order table, click Design (I think it may be called Edit under 2005). Then anywhere in the window that opens right-click and select Relationships.

You will get another dialog, on the right there should be a grid view. One of the first lines reads "Tables and Columns Specification". Click that line, then click again on the little [...] button that appears on the right. You will get this dialog:

The Order table should already be selected on the right. Select the Customer table on the left dropdown. Then in the left grid, select the ID column. In the right grid, select the CustomerID column. Close the dialog, and the next. Press Ctrl+S to save.

Having this constraint will ensure that no Order records can exist without an accompanying Customer record.

To effectively query a database like this, you might want to read up on JOINs.

User · Answer

Define two tables  example A and B   with their own primary key Define a column in Table A as having a Foreign key relationship based on the primary key of Table B   This means that Table A can have one or more records relating to a single record in Table B   If you already have the tables in place  use the ALTER TABLE statement to create the foreign key constraint   ALTER TABLE A ADD CONSTRAINT fk b FOREIGN KEY  b id  references b id      fk b  Name of the foreign key constraint  must be unique to the database b id  Name of column in Table A you are creating the foreign key relationship on b  Name of table  in this case b id  Name of column in Table B

User · Answer

If you are not using SSMS then here is the syntax   ALTER TABLE  lt table name gt  ADD  lt constraint name gt  FOREIGN KEY    lt column name1 gt     lt column name2 gt    REFERENCES  lt table name gt    lt column name1 gt     lt column name2 gt     http   infogoal com sql sql-add-foreignkey htm

User · Answer

If you are talking about two kinds of enitities  say teachers and students  you would create two tables for each and a third one to store the relationship  This third table can have two columns  say teacherID and StudentId  If this is not what you are looking for  please elaborate your question

User · Answer

This is how I usually do it  sql server    Create Table Master   MasterID int identity 1 1  primary key  Stuff varchar 10    GO Create Table Detail   DetailID int identity 1 1  primary key  MasterID int references Master  --use  references  Stuff varchar 10   GO Insert into Master values  value   -- 1 row s  affected  GO Insert into Detail values  1   Value1   -- Works -- 1 row s  affected  insert into Detail values  2   Value2   -- Fails --Msg 547  Level 16  State 0  Line 2 --The INSERT statement conflicted with the FOREIGN KEY constraint  FK  Detail  MasterID  0C70CFB4    --The conflict occurred in database  Play   table  dbo Master   column  MasterID   --The statement has been terminated    As you can see the second insert into the detail fails because of the foreign key  Here s a good weblink that shows various syntax for defining FK during table creation or after   http   www 1keydata com sql sql-foreign-key html

[sql] Create a one to many relationship using SQL Server

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