Task01: getting to know database and SQL (Note 1)

This chapter mainly introduces the database , Consider ease of use and popularity , The course mainly uses MySql Conduct Introduce .
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1、 Database definition ：
Save a lot of data , A collection of data processed by a computer that can be accessed efficiently
The computer system used to manage the database is called the database management system （DBNS）
2、DBMS species ：
Classify according to data storage format ： Hierarchical database （HDB）
relational database （RDB）—— Also known as RDBMS
Object-oriented database （OODB）
XML database （XMLDB）
Key value storage system （KVS）,eg：MongoDB
Representative of RDBMS ：
* Oracle Database： Oracle's RDBMS
* SQL Server： Microsoft's RDBMS
* DB2：IBM The company's RDBMS
* PostgreSQL： Open source RDBMS
* MySQL： Open source RDBMS
3、RDBMS Common system structure
client / Server type （C/S type ）

I used the local MySQL The environment construction method is as follows ：
Save space , I will give you a detailed introduction and write pdf In the , You can click the link to view ：
http://tianchi-media.oss-cn-beijing.aliyuncs.com/dragonball/SQL/other/ Local MySQL Introduction to environment construction method .pdf
advantage ： free , Enhance hands-on ability .
shortcoming ： install 、 Configuration trouble , Data import 、 Exporting takes a long time .
4、SQL Concept ： The language in which the database library is operated

The table structure stored in the database is similar to excel Rows and columns in , As shown in the figure above ：
In the database , Lines are called records , Equivalent to a record
Columns are called fields , Represents the data items stored in the table
Where rows and columns meet is called a cell , Only one record can be entered in a cell
According to RDBMS Different categories of instructions given ：
（1） Data definition language （DDL） Used to create or delete Database and surface . The instructions are ：

• CREATE ： Create objects such as databases and tables

• DROP ： Delete objects such as databases and tables

• ALTER ： Modify the structure of objects such as databases and tables （2） Data manipulation language （DML） Used to query or change The records in the table . The instructions are ：

• SELECT ： Query data in table

• INSERT ： Insert new data into the table

• UPDATE ： Update the data in the table

• DELETE ： Delete data from table （3） Data control language （DCL） Used to confirm or cancel changes to data in a database . It can be done to RDBMS Whether or not the user has permission to operate the objects in the database （ Database table, etc ） To set . The instructions are ：

• COMMIT ： Confirm changes to the data in the database

• ROLLBACK ： Cancel changes to the data in the database

• GRANT ： Give the user permission to operate

• REVOKE ： Cancel the user's operation permission 5、 Basic writing rules
  （1） Use a semicolon （;） ending
  （2） Case insensitive keywords , But the data inserted into the table is case sensitive
  （3）Windows The system does not distinguish the case of table name and field name by default (linux/mac Is different )
  （4） Constant is written in a fixed way eg：'abc', 1234, '26 Jan 2010', '10/01/26', '2010-01-26'…
  （5） Words need to be separated by half width spaces or line breaks , And you can't use full width spaces as word separators , Otherwise, there will be mistakes , Unexpected results .
  6、 Database creation （CREATE DATABASE sentence ）
  grammar ：
  create database< Database name >;
  7、 The creation of a table (CREATE TABLE sentence )
  grammar ：
  create table < Table name >
  (< Name 1>< data type >< Constraints required >
  < Name 2>< data type >< Constraints required >
  < Name n>< data type >< Constraints required >
  )
  8、 Naming rules
  （1） Only half width English letters can be used 、 Numbers 、 Underline （_） As a database 、 Table and column names
  （2） The name must start with half letter
  9、 Data type designation
  The tables created by the database , All columns must have a data type specified , Each column cannot store data that does not match the data type of the column .
  data type ：
  （1）INTEGER type Storage Integers The data type of the column of （ Digital ）, Can't store decimals .
  （2）CHAR type Store fixed length string , When the length of the string stored in the column does not reach the maximum length , Use half space to complement , Because it wastes storage space , So I don't use .
  （3）VARCHAR type Storage （ variable ） Length string , Fixed length string will be supplemented with half width spaces when the number of characters does not reach the maximum length , But variable length strings are different , Even if the number of characters does not reach the maximum length , I don't use half space to make up .
  （4）DATE type Storage date （ Specific date ） The data type of the column of （ Date type ）.
  10、 Setting of constraints
  Constraints are in addition to data types , The data stored in the column is Limit perhaps Additional Conditional function .
  NOT NULL yes Non empty constraint , That is, data must be entered in the column .
  PRIMARY KEY yes Primary key constraint , Represents that the column is unique , You can get the data of a specific row through this column .
  11、 Table deletion and update
  （1） Delete table Syntax ：
  Drop table < Table name >
  （2） Add columns ALTER TABLE sentence
  ALTER TABLE < Table name > ADD COLUMN < Definition of columns >;
  （3） Delete column ALTER TABLE sentence
  ALTER TABLE < Table name > DROP COLUMN < Name >;
  Be careful ：ALTER TABLE Statement and DROP TABLE The sentence is the same , Cannot recover after execution . Mistakenly added columns can be through ALTER TABLE Statement delete , Or delete all the tables and create them again .
  （4） Clear table contents
  TRUNCATE TABLE TABLE_NAME;
  advantage ： comparison drop``/``delete , truncate When used to clear data , The fastest .
  （5） Update of data
  UPDATE < Table name >
  SET < Name > = < expression > [, < Name 2>=< expression 2>...];
  WHERE < Conditions >; -- Optional , It's very important .
  ORDER BY Clause ; -- Optional
  LIMIT Clause ; -- Optional
  Be careful ： Use update Pay attention to adding where Conditions , Otherwise, all rows will be modified according to the statement
  Use UPDATE You can also update the column to NULL（ This update is commonly known as NULL Empty ）. At this time, you only need to write the value on the right of the assignment expression directly as NULL that will do
  UPDATE Statement and INSERT The sentence is the same , Can be NULL Use as a value .
  however , Only unset NOT NULL Columns with constraints and primary key constraints can be cleared to NULL. If the column with the above constraints is updated to NULL, Will go wrong , With this INSERT Same statement .
  （6） Multi column update
  UPDATE Of the statement SET Clause supports multiple columns as update objects at the same time .（ You can simplify the code by merging ）（7） towards product Insert data into the table
  INSERT INTO < Table name > ( Column 1, Column 2, Column 3, ……) VALUES ( value 1, value 2, value 3, ……);
  Be careful ：1、 Make a full column of the table INSERT when , You can omit the column list after the table name . VALUES The value of clause will be assigned to each column from left to right by default .
  2、 Do it once INSERT Statement inserts a row of data
  3、 When inserting multiple lines , It is usually necessary to cycle through the corresponding number of times INSERT sentence . In fact, a lot of RDBMS Both support inserting multiple rows of data at one time
  4、INSERT The statement wants to assign... To a column NULL When the value of , Can be directly in VALUES Clause in the list of values NULL. Want to insert NULL Columns must not be set NOT NULL constraint .
  5、 You can also insert default values into the table （ Initial value ）. You can create a table by CREATE TABLE Set... In the statement DEFAULT Constraints to set default values .
  6、 Use INSERT … SELECT Statement to copy data from other tables .
  The answer to the exercise ：
  1、create database Addressbook;
  create table Addressbook(
  regist_no integer not null,
  name varchar(128) not null,
  address varchar(256) not null,
  tel_no char(10),
  mail_address char(20),
  primary key(regist_no)
  );
  2、ALTER TABLE Addressbook ADD COLUMN postal_code char(8) not null;
  3、DROP TABLE Addressbook;
  4、# Deleted tables cannot be recovered , It can only be re inserted , Please be careful when deleting

author ：wxiaoyiwenrong

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