Relationship between schema and instance
It is similar to types and variables in programming languages.
Schema is analogous to type information of a variable in a program.（变量类型） Instance is analogous to the value of a variable.（变量取值）
Schema（模式）Schema is the overall design of the database.（数据库的整体设计）
Several schemas of DBS- Physical schema（物理模式/内模式/存储模式）: database design at the physical level.
- Logical schema（逻辑模式/模式/概念模式）: database design at the logical level.
- Sub schema（子模式/外模式）: describe different views at the view level
