As https://www.informit.com/articles/article.aspx?p=30669

所有对象的名称在一定范围内必须是唯一的。每一个 数据库必须有唯一的名称;模式名称不能重复 在单个数据库的范围内，表的名称必须为 在单个模式的作用域中是唯一的，列名必须是 在表中是唯一的。类中索引的名称必须唯一 数据库。

架构->平面图

数据库->房子

桌子->房间

来自PostgreSQL文档:

A database contains one or more named schemas, which in turn contain tables. Schemas also contain other kinds of named objects, including data types, functions, and operators. The same object name can be used in different schemas without conflict; for example, both schema1 and myschema can contain tables named mytable. Unlike databases, schemas are not rigidly separated: a user can access objects in any of the schemas in the database he is connected to, if he has privileges to do so. There are several reasons why one might want to use schemas: To allow many users to use one database without interfering with each other. To organize database objects into logical groups to make them more manageable. Third-party applications can be put into separate schemas so they do not collide with the names of other objects. Schemas are analogous to directories at the operating system level, except that schemas cannot be nested.

我试着根据我对以下类比的理解来回答:

A database is like the house In the house there are several types of rooms. Assuming that you're living in a really big house. You really don't want your living rooms, bedrooms, bathrooms, mezzanines, treehouses, etc. to look the same. They each need a blueprint to tell how to build/use them. In other words, they each need a schema to tell how to build/use a bathroom, for example. Of course, you may have several bedrooms, each looks slightly different. You and your wife/husband's bedroom is slightly different from your kids' bedroom. Each bedroom is analogous to a table in your database. A DBMS is like a butler in the house. He manages literally everything.

A schema is not a plan for the entire database. It is a plan/container for a subset of objects (ex.tables) inside a a database. This goes to say that you can have multiple objects(ex. tables) inside one database which don't neccessarily fall under the same functional category. So you can group them under various schemas and give them different user access permissions. That said, I am unsure whether you can have one table under multiple schemas. The Management Studio UI gives a dropdown to assign a schema to a table, and hence making it possible to choose only one schema. I guess if you do it with TSQL, it might create 2 (or multiple) different objects with different object Ids.

简而言之，模式是整个数据库的定义，因此它包括表、视图、存储过程、索引、主键和外键等。