Question

I need a plain English explanation of “schema”, as in Database schema & XML schema.

EDIT:
When someone says to create a database schema, does it mean creating constraints for the fields in the tables?

Answer 1

Definition

A schema defines the structure of the data, whether you store it in a database or an XML file or elsewhere.

Example
Suppose you use a relational database server to store your company’s data, and you have a “Customers” table, to track information for each customer. The schema for the Customer table might dictate that each customer entry should have a firstname, lastname, and phone number. When you create a customer entry you can store only those fields.

Constraints

Schema can specify whether a field is optional or required. In this example, you might require that ALL of those fields be present. If the schema were enforced by some mechanism, it would not be possible to enter a customer entry into the database without specifying a phone number. Or, your schema might specify a mobile phone number, as optional.

In this example, the schema doesn’t have a slot for “size” as an element within the customer structure, so you cannot insert that element into any customer entry. On the other hand, a T-shirt company might use a customer schema that includes a “size” field. Schemas vary by the thing they are describing, and by the people or parties that use the schema.

As Applied to XML

XML schema just defines the structure of a conformant XML document. There’s a W3C standard for formally describing the schema, XML Schema Definition, aka XSD. (See the primer for an informal description of it) There are also other ways to formally describe the structure, or schema of an XML doc, which are not dejure standard but serve the same purpose. Relax-NG is one.

Informal Schema

There are also informal – or at least, less formal – ways of describing schema, particularly around XML files or other formats. Providing am example XML file and a text description is one common, informal way of specifying the schema. I call this “informal” because a sample doc cannot possibly describe everything about the schema. But, a sample doc coupled with a few lines of english text describing what’s intended, is often a good enough, or even the best, way to describe an XML schema. This is basically the way the majority of the web works.

Consider the case of JSON. It is now being used to store and transmit information, in some cases as a alternative to XML. As far as I know, there is no formal mechanism for describing a schema for JSON. There’s no way to specify what an acceptable or “compliant” JSON document looks like. What people do instead is provide an example or template, and that’s good enough. There’s no enforcement. If you provide a JSON packet that conforms to the expected format, then it works as expected. If you don’t, then it won’t work.

Informal schema is also the way most old-school paper-based businesses work. They hand out a form and give some examples – “if you want to place an order, fill out the form in this way”.

Enforcement or Validation

Schema may or may not be “enforced” automatically. A relational database server typically provides a strict schema enforcement. You MUST define the schema in order to create a table, and any database clients MUST conform to the schema. Some XML-based systems perform document validation against an XSD to enforce schema. Many do not. As I said, most of the web is much less formal.

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In answer to the second part of your question,
creating a database schema for a Customer means defining those fields that belong to the customer entity (in an entity sense), or columns that can be stored in the customer table (in a database), or the attributes and child elements of a Customer element (in an xml document), as well as the constraints on same (is the field required, optional, etc)..

A constraint places a limit on the type or format of the data in a particular field. One example constraint is whether the field is required or optional. Another constraint might be: “a customer name cannot be solely comprised of numeric characters.” A more strict constraint might say: “a customer name may not have any numeric characters at all.” For the T-shirt company, a constraint might be: the “size” element must be one of {S,M,L,XL}. Formal schema languages allow you to specify these constraints explicitly, in a way that a computer program could verify compliance. In informal schema specifications, you would just write or say, “the size must be one of S,M,L,XL” and that covers it.