计算机二级考试精讲（Computer secondary examination）.doc

计算机二级考试精讲（Computer secondary examination） This article is contributed by lo Doc documents may have a poor browsing experience at the WAP end. It is recommended that you choose TXT, or download the source file to the native view. Test: test 1: basic knowledge of database knowledge summary: summary of knowledge points In this chapter and the fourth chapter of public basic knowledge database design basic content basically the same (other knowledge may refer to the public based knowledge related knowledge) database system is the core of the database management system database management system The relationship between two entities is: one-to-one and many-to-many, one-to-many, etc., this subject according to the specific analysis. One-on-one, (often one-to-one many-to-many, one-to-many, etc., subject) data model, the hierarchical model, network model and relation model (most of the current is a relational model, such as Access) the hierarchical model, the hierarchical model mesh models, relational model () using two-dimensional table to show links between entities and entity data model is the relational model a relational model in relational database, every relationship is a two-dimensional table (Access database, the table is relationship) key words: key words: its values can uniquely identifies a tuple properties or combinations of properties Keywords: external key word: a primary key field is not in, but a primary key of another table, this field is called the close relationship between key operation (: a relationship operation (often the examination site) : a collection of traditional operation (and, worse, pay) 2 special relationship operation (selection, projection, join, natural join) from relationship to find out the operation of tuple satisfies the given condition known as choosing from relational schema specified in a number of properties of a new relationship called projection projection make two of the same attribute values in the tuple join together to form