How to delete relationship in sql

How to delete rows in tables that contain foreign keys to other tables | The Forums

how to delete relationship in sql

Microsoft Access Delete Query SQL Syntax with examples of deleting to remove records from tables that are in a one-to-many relationship with other tables. Unfortunately, I dont think you can get the constraint name out of the database unless you created it SQL as well: ALTER TABLE M_Employees ADD. The dependencies mentioned hint on DROP rather than DELETE. How do I delete a table in SQL Server? How do I compare data for all the tables in SQL developer and SQL server?.

Don't worry about exactly what this means for now, just think of it as another aspect of entity relationships. Adding this clause, and setting it to CASCADE basically means that if the row being referenced is deleted, the row referencing it is also deleted.

Determining what to do in situations where you delete a row that is referenced by another row is an important design decision, and is part of the concept of maintaining referential integrity.

SQL Drop Constraint Syntax

One-to-Many Okay, time to get back to our different table relationship types with a look at one-to-many. A one-to-many relationship exists between two entities if an entity instance in one of the tables can be associated with multiple records entity instances in the other table.

The opposite relationship does not exist; that is, each entity instance in the second table can only be associated with one entity instance in the first table. A book has many reviews.

SQL Server Foreign Key Update and Delete Rules

A review belongs to only one book. Let's set up the necessary data. There's a key difference worth pointing out in the statement for our reviews table however: In other words a book can have many reviews. Now we have created our books and reviews tables, let's add some data to them.

23- SQL Server Databases : Understand relation delete & update rules + finalizing module relations

Since a column in reviews references data in books we must first ensure that the data exists in the books table for us to reference. We set up the table in this way for our example because we wanted to focus on the one-to-many relationship type.

how to delete relationship in sql

If we had added such a Foreign Key to reviews we'd effectively be setting up a Many-to-Many relationship between books and users, which is what we'll look at next. Many-to-Many A many-to-many relationship exists between two entities if for one entity instance there may be multiple records in the other table, and vice versa.

A user can check out many books. A book can be checked out by many users over time. In order to implement this sort of relationship we need to introduce a third, cross-reference, table.

We already have our books and users tables, so we just need to create the cross-reference table: Each row of the checkouts table uses these two Foreign Keys to create an association between rows of users and books. We can see on the first row of checkouts, the user with an id of 1 is associated with the book with an id of 1. On the second row, the same user is also associated with the book with an id of 2.

On the third row a different user, with and id of 2, is associated with the same book from the previous row. On the fourth row, the user with an id of 5 is associated with the book with an id of 3.

how to delete relationship in sql

Don't worry if you don't completely understand this right away, we'll take a look shortly at what these associations look like in terms of the data in users and books. A foreign key value may be NULL and indicates a particular record has no parent record.

But if a value exists, then it is bound to have an associated value in a parent table.

how to delete relationship in sql

When applying update or delete operations on parent tables there may be different requirements about the effect on associated values in child tables. There are four available options in SQL Server and as follows: There may be different rules for each of the update and delete operations on a single FK constraint. Before proceeding with the demo, here is summary of the effects for update and delete operations: Specification Update operation on parent table Delete operation on parent table No Action Not allowed.

Error message would be generated. Cascade Associated values in child table would also be updated.

how to delete relationship in sql

Associated records in child table would also be deleted. Foreign key column should allow NULL values to specify this rule. Associated values in child table would be set to NULL. Set Default Associated values in child table would be set to default value specified in column definition.