The beginning of the database when the database is simply do not know what (the school which did not open, only learned C, Oh 。。。), So when the class is always in the tangled issue, and then went to see their own some information about the database.
In fact, the database file structure is stored in one series of information sheets, this file structure makes it possible to access these tables, select the table column to sort the table, and according to various criteria to select rows. Databases usually have multiple indexes and many of the columns in these tables is associated, so that we can quickly access these tables. As with the communications, construction database must follow certain rules (communication to follow certain protocols to achieve). In a relational database, this rule is the paradigm. Paradigm is consistent with the pattern of relationships of a set of levels. Relational database must meet a different paradigm, now has six relational database paradigm: the first paradigm (1NF), second normal form (2NF), third normal form (3NF), fourth normal form (4NF), Fifth Normal Form (5NF ), the sixth paradigm (6NF). The first normal form (1NF) is a relational database must be satisfied the minimum requirements for relational databases. In the first paradigm to further meet the additional requirements on the basis of the paradigm known as the second normal form (2NF), the rest of the paradigm and so on. Under normal circumstances, the database need only meet the third normal form (3NF) on it.
Here's a simple paradigm to talk about the first (1NF), second normal form (2NF), third normal form (3NF):
A first paradigm (1NF)
Paradigm of the so-called first (1NF) is the column for each database table the basic data items are indivisible, the same column can not have multiple values, that is, entities (objects with similar java classes) in the property can not have more than one value or can not have duplicate attributes. If repeated properties, you may need to define a new entity. In the first paradigm (1NF) tables contain only one instance of each line of information. The first paradigm (1NF) relational database can also be said of the atomic (each property values are no longer the smallest unit of data points.) In short, the first paradigm (1NF) is no duplication of columns.
In any relational database, the first paradigm (1NF) is the basic requirement of the relational model does not satisfy the first normal form (1NF) of the database is not relational databases.
Second, the second normal form (2NF)
Second normal form (2NF) is in first normal form (1NF) built on the basis that meet the second paradigm (2NF) must first meet the first paradigm (1NF). Second normal form (2NF) requires a database table or row for each instance must be the only distinction. Usually need to achieve distinction as a table with a column, a unique identifier for each instance storage (that is, we say that the primary key.)
Second normal form (2NF) requires an entity of the property is totally dependent on primary key (primary key). The so-called is not entirely dependent on the existence depends only on the primary key part of the property, if it exists, then this property and this part of the main keyword should be separated out to form a new entity, the new entity and the original is the one to many relationship between entities . In short, the second normal form (2NF) is not part of the non-primary properties depend on the primary key.
Third, the third normal form (3NF)
By the same token meet the third paradigm (3NF) must first meet the second paradigm (2NF). In short, the third normal form (3NF) requires a database table is not included in the table has been included in other non-primary key information. Third normal form (3NF), also known to eliminate indirect dependencies (which are not totally dependent on the primary key information, you can remove it.)
In short, the relationship between time to follow the database design database design paradigm. The first paradigm (1NF) is the most fundamental requirements, only if they meet the first paradigm (1NF), in order to achieve a variety of other paradigms.