[sql-server] Why do table names in SQL Server start with "dbo"?

Microsoft introduced schema in version 2008. For those who didn’t know about schema, and those who didn’t care, objects were put into a default schema dbo.

dbo stands for DataBase Owner, but that’s not really important.

Think of a schema as you would a folder for files:

• You don’t need to refer to the schema if the object is in the same or default schema
• You can reference an object in a different schema by using the schema as a prefix, the way you can reference a file in a different folder.
• You can’t have two objects with the same name in a single schema, but you can in different schema
• Using schema can help you to organise a larger number of objects
• Schema can also be assigned to particular users and roles, so you can control access to who can do what.

You can always access any object from any schema.

Because dbo is the default, you normally don’t need to specify it within a single database:

SELECT * FROM customers;
SELECT * FROM dbo.customers;

mean the same thing.

I am inclined to disagree with the notion of always using the dbo. prefix, since the more you clutter your code with unnecessary detail, the harder it is to read and manage.

For the most part, you can ignore the schema. However, the schema will make itself apparent in the following situations:

1. If you view the tables in either the object navigator or in an external application, such as Microsoft Excel or Access, you will see the dbo. prefix. You can still ignore it.

2. If you reference a table in another database, you will need its full name in the form database.schema.table:

   SELECT * FROM bookshop.dbo.customers;
   

3. For historical reasons, if you write a user defined scalar function, you will need to call it with the schema prefix:

   CREATE FUNCTION tax(@amount DECIMAL(6,2) RETURNS DECIMAL(6,2) AS
   BEGIN
       RETURN @amount * 0.1;
   END;
   GO
   SELECT total, dbo.tax(total) FROM pricelist;
   

   This does not apply to other objects, such as table functions, procedures and views.

You can use schema to overcome naming conflicts. For example, if every user has a personal schema, they can create additional objects without having to fight with other users over the name.