Oracle 中实现自增ID

## 在 Oracle 11g 及之前参考如下文章
https://www.jianshu.com/p/e8e29df265a5

## 在 Oracle 12c 及之后
### 1. 语法
```
GENERATED (ALWAYS | (BY DEFAULT [ON NULL])) AS IDENTITY
   [sequence_options,...]
```

| 身份列声明 | 描述 |
| --- | --- |
| GENERATED ALWAYS AS IDENTITY	 | 序列生成器始终提供一个IDENTITY值。您不能为列指定值。 | 
| GENERATED BY DEFAULT AS IDENTITY | 每当您不提供列值时，序列生成器都会提供IDENTITY值 |
| GENERATED BY DEFAULT ON NULL AS IDENTITY | 如果您指定NULL columnn值，则序列生成器将提供下一个IDENTITY值|

### 2. 建表时声明
```
CREATE TABLE IF NOT EXISTS tname1 (
id INTEGER GENERATED ALWAYS AS IDENTITY
);
```
### 3. 在现有表中插入
```
ALTER TABLE SNSOFT.XD_INTEREST_ADJUSTMENT 
ADD "ID" NUMBER(10,0) GENERATED BY DEFAULT ON NULL AS IDENTITY
```
### 4. 修改现有id列
```
ALTER TABLE SNSOFT.XD_INTEREST_ADJUSTMENT 
MODIFY "ID" NUMBER(10,0) GENERATED BY DEFAULT ON NULL AS IDENTITY
```
### 5. 删除自增列
```
ALTER TABLE SNSOFT.XD_INTEREST_ADJUSTMENT DROP COLUMN ID;
```

### 6. 别忘了还原主键
```
alter table SNSOFT.XD_INTEREST_ADJUSTMENT add constraint XD_INTEREST_ADJUSTMENT_PK primary key ("ID");
```