Three paradigms of MySQL

Last week , Feeling comfortable in life , The project functions are almost written , Suddenly the senior said he would interview us this week java Basics , All of a sudden, I feel refreshed . Yesterday, I had an interview and the results came out , The following is the senior's evaluation of me

I forgot a lot about the database , Although usually used in projects , But I still haven't remembered some basic knowledge .

Three paradigms of database ：

The concept of paradigm ：

Some specifications to follow when designing a database , At present, there are six paradigms of relational database ： First normal form （1NF）、 Second normal form （2NF）、 Third normal form （3NF）、 buss - The COD paradigm （BCNF）、 Fourth normal form (4NF） And the fifth paradigm （5NF, Also called perfect paradigm ）.

Meeting the first three paradigms can design a more standardized database .

Some specifications to follow when designing a database , At present, there are six paradigms of relational database ： First normal form （1NF）、 Second normal form （2NF）、 Third normal form （3NF）、 buss - The COD paradigm （BCNF）、 Fourth normal form (4NF） And the fifth paradigm （5NF, Also called perfect paradigm ）.

One 、 First normal form (1NF)： No more columns can be split

Upper figure , There are two sub columns under the major column , Obviously, it does not conform to the first paradigm .

This is in line with the first paradigm .

Two 、 Second normal form (2NF)： On the basis of the first paradigm , Non primary key columns are completely dependent on the primary key , Instead of relying on a part of the primary key .

1. Function dependency ： If you pass A attribute ( Attribute group ), It's certain that the only B The value of the property , that B Depend on A. For example, the name shown above , Completely dependent on the student id
2. Completely function dependent ： If A It's an attribute group , be B The determination of attribute values depends on A All attribute values in the attribute group . Attribute groups refer to multiple fields , So for example, if we want to know a score , You must rely on the two attributes of student number and course name to determine a score , Other attributes cannot determine a certain score
3. Part of the function depends on ： If A It's an attribute group , be B The determination of attribute value depends on A Some fields of the attribute group can , For example, the student number and course name are an attribute group , In fact, the student name can be determined only by the student number
4. The transfer function depends on ： If A attribute ( Attribute group ), The only one can be determined B The value of the property , Re pass B The value of the attribute can be uniquely determined C The value of the property , For example, a student number determines a department name , A department name corresponds to a dean
5. Primary key ： In a table , An attribute or group of attributes , Completely dependent on all other properties , This attribute is called the table of the code , For example, the attribute group composed of student ID and course name in the above figure

1. Function dependency ： If you pass A attribute ( Attribute group ), It's certain that the only B The value of the property , that B Depend on A. For example, the name shown above , Completely dependent on the student id
2. Completely function dependent ： If A It's an attribute group , be B The determination of attribute values depends on A All attribute values in the attribute group . Attribute groups refer to multiple fields , So for example, if we want to know a score , You must rely on the two attributes of student number and major name to determine a score , Other attributes cannot determine a certain score
3. Part of the function depends on ： If A It's an attribute group , be B The determination of attribute value depends on A Some fields of the attribute group can , For example, student ID and major name are an attribute group , In fact, the student name can be determined only by the student number
4. The transfer function depends on ： If A attribute ( Attribute group ), The only one can be determined B The value of the property , Re pass B The value of the attribute can be uniquely determined C The value of the property , For example, a student ID determines a major name , A major name corresponds to a major leader .
5. Primary key ： In a table , An attribute or group of attributes , Completely dependent on all other properties , This attribute is called the table of the code , For example, the attribute group composed of student ID and major name in the above figure

Simply speaking , The second normal form is to eliminate the partial dependence of non primary keys on primary keys on the basis of the first normal form .
So we can divide the watch into two


3、 ... and 、 Third normal form (3NF)： On the basis of the second paradigm , Non primary key columns only depend on the primary key , Not dependent on other non primary keys .