DB
- Function 作用
- Relational DB 关系型数据库
- NoSQL DB 非关系型数据库
- Concept 概念
  - View
  - Schema
  - Package
  - Trigger
  - Sequence
  - Synonym
  - Null
- Other Topic
  - Index
  - Sharding
  - Txn
  - Turning

DB

Function 作用

数据库，是用来存储数据的。

问：那为啥不直接用Excel或者txt文本文件呢？答：简单的场景如记自己的账，可以。但是，当数据量大，场景复杂，如银行记账，就不够用了。

一款数据库软件，要保证：

查询（读）方便
存储（写）稳定

Relational DB 关系型数据库

Oracle
[[mysql]]
[[h2]]

NoSQL DB 非关系型数据库

Redis
HBase
MongoDb
Neo4J

![[db_nosql_list.png]]

Concept 概念

以下是关系型数据库的一些概念，但是大部分对于非关系型数据库也适用。

View

In database theory, a view is the result set of a stored query on the data, which the database users can query just as they would in a persistent database collection object.

视图是一种虚拟的表，具有和物理表相同的功能。可以对视图进行增，改，查，操作，试图通常是有一个表或者多个表的行或列的子集。对视图的修改不影响基本表。它使得我们获取数据更容易，相比多表查询。

Schema

A database schema is a way to logically group objects such as tables, views, stored procedures etc. Think of a schema as a container of objects.

Package

A package is a schema object that groups logically related PL/SQL types, variables, constants, subprograms, cursors, and exceptions. A package is compiled and stored in the database, where many applications can share its contents.

Trigger

A trigger is a special kind of stored procedure that automatically executes when an event occurs in the database server. DML triggers execute when a user tries to modify data through a data manipulation language (DML) event. DML events are INSERT, UPDATE, or DELETE statements on a table or view.

Sequence

Sequence will allow you to populate primary key with a unique, serialized number.

Synonym

A synonym is an alias or alternate name for a table, view, sequence, or other schema object.

Null

Null表示的是未知，不确定
Null的比较需要用IS/IS NOT，不能用=/!=
- select * from users where deleted_at = null; -- no results
- select * from users where deleted_at is null; -- some results
将Null值转化为其它值
- NVL(value,0)

DB