rownum如何生成

MySQL

```sql

SET @rownum:=0;

SELECT @rownum:=@rownum+1 AS rownum, table_name.

FROM table_name

ORDER BY column_name;

```

PostgreSQL

```sql

SELECT row_number() OVER (ORDER BY column_name) AS rownum, table_name.

FROM table_name;

```

SQL Server

在SQL Server中，同样可以使用`ROW_NUMBER()`窗口函数。

```sql

SELECT ROW_NUMBER() OVER (ORDER BY column_name) AS rownum, table_name.

FROM table_name;

```

Oracle

在Oracle中，也使用`ROW_NUMBER()`窗口函数。

```sql

SELECT row_number() OVER (ORDER BY column_name) AS rownum, table_name.

FROM table_name;

```

SQLite

在SQLite中，可以使用`ROW_NUMBER()`窗口函数。

```sql

SELECT row_number() OVER (ORDER BY column_name) AS rownum, table_name.

FROM table_name;

```

在使用这些方法时，你需要根据你的具体需求选择一个合适的排序字段`column_name`来决定行号的排序方式。通常情况下，我们会选择主键或者业务上需要的某个排序字段来确保行号的连续性和可预测性。