In database terms, a schema (pronounced “skee-muh” or “skee-mah”) is the organisation and structure of a database. Both schemas and schemata can be used as plural forms.
A database schema can be represented in a visual diagram, which shows the database objects and their relationship with each other.
Above is a simple example of a schema diagram. It shows three tables, along with their data types, relationships between the tables, as well as their primary keys and foreign keys.
Here is a more complex example of a database schema:
In this case, the schema diagram has been separated into four sections:
- Customer Data: Data related to the customers, such as their name, address, etc
- Business: Data required to run the business, such as staff, store locations, payment details, etc
- Inventory: Details on all products. In this case the products are movies, so it contains data such as movie title, its category, the actors, etc.
- Views: Special view on data used for appraisals.
So by looking at these schema diagrams, we could go ahead and create a database. In fact, MySQL Workbench allows you to generate a
CREATE TABLE script straight from the diagram. You can then use the script to create a database. You can even reverse engineer a database into a diagram.
Is a Schema and a Database the Same Thing?
There’s a lot of confusion about schemas when it comes to databases. The question often arises whether there’s a difference between schemas and databases and if so, what is the difference.
Depends on the Vendor
Part of the reason for the confusion is that database systems tend to approach schemas in their own way.
- The MySQL documentation states that physically,
a schema is synonymous with a database. Therefore, a schema and a database are the same thing.
- However, the Oracle Database documentation states that certain objects can be stored inside a database but not inside a schema. Therefore, a schema and a database are two different things.
- And according to this SQL Server technical article, a schema is a separate entity inside the database. So, they are two different things.
So, depending on the RDBMS you use, schemas and databases may or may not be the same thing.
What about the SQL Standard?
The ISO/IEC 9075-1 SQL standard defines a schema as
a persistent, named collection of descriptors.
If you were confused before, hope I haven’t just made it worse…
Another reason for the confusion is probably due to the fact that the term schema has such a broad meaning. It has different connotations within different contexts.
The word schema originates from the Greek word skhēma, which means form, figure, shape, or plan.
Schema is used in psychology to describe an organised pattern of thought or behaviour that organises categories of information and the relationships among them.
Before designing a database, we also need to look at the categories of information and the relationships among them. We need to create a conceptual schema before we even start with the physical schema within the DBMS.
In software development, when discussing schemas, one could be discussing conceptual schemas, physical schemas, internal schemas, external schemas, logical schemas, etc . Each of these has its own specific meaning.
Schema Definitions by DBMS
Here’s a quick definition of schema from the three leading database systems:
Conceptually, a schema is a set of interrelated database objects, such as tables, table columns, data types of the columns, indexes, foreign keys, and so on.
In MySQL, physically, a schema is synonymous with a database. You can substitute the keyword
DATABASEin MySQL SQL syntax, for example using
CREATE SCHEMAinstead of
Source: “MySQL Glossary”. MySQL 5.7 Reference Manual. MySQL. Retrieved 6 June 2016.
The names of tables, fields, data types, and primary and foreign keys of a database.
“Glossary”. SQL Server 2016 Technical Documentation. Microsoft Developer Network. Retrieved 6 June 2016.
Oracle Database‘s schema system is quite different to the other systems. Oracle’s schema is very much tied to the database user.
A schema is a collection of logical structures of data, or schema objects. A schema is owned by a database user and has the same name as that user. Each user owns a single schema.
Source: “Database Objects”. Oracle Database Online Documentation 12c Release 1 (12.1). Oracle Help Center. Retrieved 6 June 2016.
This article on schema definitions by DBMS provides more detail.
Despite their differences in defining schemas, each of the three aforementioned DBMSs support the
CREATE SCHEMA statement.
And that’s where the similarity ends.
CREATE SCHEMA creates a database.
This is because
CREATE SCHEMA is a synonym for
CREATE DATABASE. In other words, you can use
CREATE SCHEMA or
CREATE DATABASE to do the same thing.
In Oracle Database, the
CREATE SCHEMA statement doesn’t actually create a schema. This is because a schema is already created with each database user.
In Oracle, the
CREATE USER statement creates the schema.
CREATE SCHEMA statement lets you populate your schema with tables and views and grant privileges on those objects without having to issue multiple SQL statements in multiple transactions.
In SQL Server,
CREATE SCHEMA will create a schema by the name you give it.
Unlike MySQL, the
CREATE SCHEMA statement creates a schema that is defined separately to the database.
Unlike Oracle, the
CREATE SCHEMA statement actually creates the schema.
In SQL Server, once you create the schema, you can then add users and objects to it.
The term schema can be used within many different contexts. In the context of creating schemas within a specific database management system, you’ll need to work with however that DBMS defines schemas.
And when you switch to a new DBMS, be sure to look up how that system defines schemas.