Basic understanding of oracle databases

I. Concepts
1. Database)

What is a database?
A database is a set of data that is organized according to a certain data model and stored in the second-level memory.
This data set has the following features:
Do not repeat as much as possible, and provide multiple application services for a specific organization in the optimal way. Its data structure is independent of the application that uses it,
The unified software manages and controls the addition, deletion, modification, and retrieval of data.
From the development history, the database is an advanced stage of data management, which is developed by the file management system.

What is a database system?
A database system is a software system that provides data for the actual and operational storage, maintenance, and application systems. It is a collection of storage media, processing objects, and management systems.
It is usually composed of software, databases, and data administrators.
The software mainly includes the operating system, various host languages, utilities, and database management systems.
Databases are centrally managed by the database management system. Data insertion, modification, and retrieval must be performed through the database management system.
The Data Administrator is responsible for creating, monitoring, and maintaining the entire database so that the data can be effectively used by anyone with the right to use it.
The database administrator is generally a person with high business level and deep qualifications.

Let's make a metaphor:
A database is a data warehouse. of course, warehouses must be organized in an orderly manner, which requires a set of management methods and management organizations. The combination of management methods and management organizations has become an organic system for managing warehouses.

2. Data Table space (Tablespace)
Data storage always requires space. Oracle divides a database into several spaces by function to store data.
Of course, the data stored on the disk is ultimately in the form of files, so a disk and a data table space contain more than one physical file

3. Oracle users
Multiple users in a database create and manage their own data. each user has its own permissions and can share data with other users.

4. Data Tables
In the warehouse, we may have multiple houses, each with multiple shelves and multiple layers.
We store data in databases and ultimately store and manage data tables.

5. Data Files
These concepts are logical, while data files are physical.
That is to say, a data file is really "something you can see" and is displayed as a real file on the disk.

Ii. Create

1. After the database is installed, a database system is installed, with a default database. Of course, we can create a new database.
Create database...

2. Now we should create a data table space to store the goods. At least we should first build the house. It is the space for storing things. The tablespace stores the data.
Format: create tablespace table name datafile 'data filename 'size tablespace size;
Example:
Create tablespace data_test datafile 'e: \ oracle \ oradata \ test \ data_1.dbf' size 2000 M;
Create tablespace idx_test datafile 'e: \ oracle \ oradata \ test \ idx_1.dbf' size 2000 M;

(* The data file name contains the full path, and the tablespace size is 2000 MB, which is 2000 mb)

3. After tablespace is created, you can create a user.
Format: create user username identified by password default tablespace table;
Example:
Create user study identified by study default tablespace data_test;
(* Create a username named study and password named study. The tablespace named data_test is missing. This is created in step 2 .)
(* Saving the tablespace indicates that study's data in the future will be stored in data_test, that is, stored in the corresponding physical file e: \ oracle \ oradata \ test \ data_1.dbf)
4. authorize new users
Grant connect, resource to study;
-- Grant the connect and resource permissions to study users.
Grant dba to study;
-- Grant the dba permission to study

5. Create a data table
On the above page, we have built the User study. Now we enter this user
Sqlplusw study/study @ test
Then you can create a data table in study.
Format: name of the create table data table. For detailed parameters, search for "oracle" "create table" "Syntax" on the Internet. Too many parameters will not be included here.
The following is an example.
Create table test_user (
No number (5) not null, -- pk
Username varchar2 (30) not null, -- User Name
Passpord varchar2 (30) not null, -- Password
Constraint pk_connectdb primary key (no)
) Storage (initial 10 k next 10 k pctincrease 0 );

* The following describes the meaning of the above commands.
Create table test_user -- create a data table
No number (5) not null, -- pk
(Column name or field name) data type (Data Length) the data column cannot be blank, It is a delimiter between columns-the post content is a comment

Constraint pk_connectdb primary key (no)
(Constraint) constraint name (primary key) (column name) means that the no value of all rows in the test_user table cannot be the same (this is the meaning of the primary key)

Storage (initial 10 k next 10 k pctincrease 0); this is complicated. If a data table needs to store a large amount of data, set the values after initial and next to a larger value, otherwise, set a smaller value.

Since the table space is not specified in the table created above, the table is stored in the study Default table space data_test.