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# 转换规则和操作
<a name="CHAP_Tasks.CustomizingTasks.TableMapping.SelectionTransformation.Transformations"></a>

可使用转换操作指定要应用于所选架构、表或视图的任何转换。转换规则是可选的。

## 限制
<a name="CHAP_Tasks.CustomizingTasks.TableMapping.SelectionTransformation.Transformations.Limitations"></a>
+ 不能对同一个对象（架构、表、列、表-表空间或索引-表空间）应用多个转换规则操作。只要每个转换操作应用于不同的对象，那么可以在任何级别应用多个转换规则操作。但是，当使用数据掩蔽转换规则时，此限制不适用。在这种规则中，您可以对同一列进行其他转换，例如 `ADD-COLUMN` 或 `CHANGE-DATA-TYPE`。
+ 转换规则中的表名称和列名称是区分大小写的。例如，Oracle 或 Db2 数据库的表名称和列名称必须以大写形式提供。
+ 不支持使用 Right-to-Left语言的列名进行转换。
+ 对于名称中包含特殊字符（如 \$1、\$1、/、-）的列，无法执行转换。
+ 对于映射到 BLOB/CLOB 数据类型的列，唯一支持的转换是将该列放在目标上。
+ AWS DMS 不支持将两个源表复制到单个目标表。 AWS DMS 根据复制任务的转换规则，将记录从一个表复制到另一个表以及从一个列复制到另一个列。对象名称必须唯一，以防止重叠。

  例如，源表包含名为 `ID` 的列，而对应的目标表预先存在名为 `id` 的列。如果规则使用`ADD-COLUMN`语句添加名为的新列`id`，并使用 SQLite 语句在列中填充自定义值，则会创建一个名为但不支持的重复的`id`、模棱两可的对象。
+ 创建转换规则时，建议仅在选择规则指定多列时（例如，当您将 `column-name` 设置为 `%` 时）才使用 `data-type` 参数。建议不要使用 `data-type` 选择单列。
+ AWS DMS 不支持源对象和目标对象（表）位于同一数据库/架构上的转换规则。在转换规则中同时使用同一个表作为源表和目标表可能会导致意外且可能有害的结果，包括但不限于表数据的意外更改、表结构的修改甚至表被删除。

## 值
<a name="CHAP_Tasks.CustomizingTasks.TableMapping.SelectionTransformation.Transformations.Values"></a>

对于使用转换规则类型的表映射规则，可应用以下值。

[\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/zh_cn/dms/latest/userguide/CHAP_Tasks.CustomizingTasks.TableMapping.SelectionTransformation.Transformations.html)

## 示例
<a name="CHAP_Tasks.CustomizingTasks.TableMapping.SelectionTransformation.Transformations.Examples"></a>

**Example 重命名架构**  
以下示例将架构从 `Test`（源中）重命名为 `Test1`（目标中）。  

```
{

    "rules": [
        {
            "rule-type": "selection",
            "rule-id": "1",
            "rule-name": "1",
            "object-locator": {
                "schema-name": "Test",
                "table-name": "%"
            },
            "rule-action": "include"
        },
        {
            "rule-type": "transformation",
            "rule-id": "2",
            "rule-name": "2",
            "rule-action": "rename",
            "rule-target": "schema",
            "object-locator": {
                "schema-name": "Test"
            },
            "value": "Test1"
        }
    ]
}
```

**Example 重命名表**  
以下示例将表从 `Actor`（源中）重命名为 `Actor1`（目标中）。  

```
{
    "rules": [
        {
            "rule-type": "selection",
            "rule-id": "1",
            "rule-name": "1",
            "object-locator": {
                "schema-name": "Test",
                "table-name": "%"
            },
            "rule-action": "include"
        },
        {
            "rule-type": "transformation",
            "rule-id": "2",
            "rule-name": "2",
            "rule-action": "rename",
            "rule-target": "table",
            "object-locator": {
                "schema-name": "Test",
                "table-name": "Actor"
            },
            "value": "Actor1"
        }
    ]
}
```

**Example 重命名列**  
以下示例将列从 `first_name` 中的表 `Actor`（源中）重命名为 `fname`（目标中）。  

```
{
    "rules": [
        {
            "rule-type": "selection",
            "rule-id": "1",
            "rule-name": "1",
            "object-locator": {
                "schema-name": "test",
                "table-name": "%"
            },
            "rule-action": "include"
        },
         {
            "rule-type": "transformation",
            "rule-id": "4",
            "rule-name": "4",
            "rule-action": "rename",
            "rule-target": "column",
            "object-locator": {
                "schema-name": "test",
                "table-name": "Actor",
                "column-name" : "first_name"
            },
            "value": "fname"
        }
    ]
}
```

**Example 重命名 Oracle 表表空间**  
以下示例将 Oracle 源中名为 `Actor` 的表的名为 `SetSpace` 的表空间重命名为 Oracle 目标端点中的 `SceneTblSpace`。  

```
{
    "rules": [
        {
            "rule-type": "selection",
            "rule-id": "1",
            "rule-name": "1",
            "object-locator": {
                "schema-name": "Play",
                "table-name": "%"
            },
            "rule-action": "include"
        },
        {
            "rule-type": "transformation",
            "rule-id": "2",
            "rule-name": "2",
            "rule-action": "rename",
            "rule-target": "table-tablespace",
            "object-locator": {
                "schema-name": "Play",
                "table-name": "Actor",
                "table-tablespace-name": "SetSpace"
            },
            "value": "SceneTblSpace"
        }
    ]
}
```

**Example 重命名 Oracle 索引表空间**  
以下示例将 Oracle 源中名为 `Actor` 的表的名为 `SetISpace` 的索引表空间重命名为 Oracle 目标端点中的 `SceneIdxSpace`。  

```
{
    "rules": [
        {
            "rule-type": "selection",
            "rule-id": "1",
            "rule-name": "1",
            "object-locator": {
                "schema-name": "Play",
                "table-name": "%"
            },
            "rule-action": "include"
        },
        {
            "rule-type": "transformation",
            "rule-id": "2",
            "rule-name": "2",
            "rule-action": "rename",
            "rule-target": "table-tablespace",
            "object-locator": {
                "schema-name": "Play",
                "table-name": "Actor",
                "table-tablespace-name": "SetISpace"
            },
            "value": "SceneIdxSpace"
        }
    ]
}
```

**Example 添加列**  
以下示例将 `datetime` 列添加到架构 `test` 的表 `Actor` 中。  

```
{
    "rules": [
        {
            "rule-type": "selection",
            "rule-id": "1",
            "rule-name": "1",
            "object-locator": {
                "schema-name": "test",
                "table-name": "%"
            },
            "rule-action": "include"
        },
        {
            "rule-type": "transformation",
            "rule-id": "2",
            "rule-name": "2",
            "rule-action": "add-column",
            "rule-target": "column",
            "object-locator": {
                "schema-name": "test",
                "table-name": "actor"
            },
            "value": "last_updated",
            "data-type": {
                "type": "datetime",
                "precision": 6
            }
        }
    ]
}
```

**Example 删除列**  
以下示例将转换源中名为 `Actor` 的表，以从目标中删除以字符 `col` 开头的所有列。  

```
{
 	"rules": [{
		"rule-type": "selection",
		"rule-id": "1",
		"rule-name": "1",
		"object-locator": {
			"schema-name": "test",
			"table-name": "%"
		},
		"rule-action": "include"
	}, {
		"rule-type": "transformation",
		"rule-id": "2",
		"rule-name": "2",
		"rule-action": "remove-column",
		"rule-target": "column",
		"object-locator": {
			"schema-name": "test",
			"table-name": "Actor",
			"column-name": "col%"
		}
	}]
 }
```

**Example 转换为小写形式**  
以下示例将表名从 `ACTOR`（源中）转换为 `actor`（目标中）。  

```
{
	"rules": [{
		"rule-type": "selection",
		"rule-id": "1",
		"rule-name": "1",
		"object-locator": {
			"schema-name": "test",
			"table-name": "%"
		},
		"rule-action": "include"
	}, {
		"rule-type": "transformation",
		"rule-id": "2",
		"rule-name": "2",
		"rule-action": "convert-lowercase",
		"rule-target": "table",
		"object-locator": {
			"schema-name": "test",
			"table-name": "ACTOR"
		}
	}]
}
```

**Example 转换为大写形式**  
以下示例将所有表和所有架构的所有列从小写形式（源中）转换为大写形式（目标中）。  

```
{
    "rules": [
        {
            "rule-type": "selection",
            "rule-id": "1",
            "rule-name": "1",
            "object-locator": {
                "schema-name": "test",
                "table-name": "%"
            },
            "rule-action": "include"
        },
        {
            "rule-type": "transformation",
            "rule-id": "2",
            "rule-name": "2",
            "rule-action": "convert-uppercase",
            "rule-target": "column",
            "object-locator": {
                "schema-name": "%",
                "table-name": "%",
                "column-name": "%"
            }
        }
    ]
}
```

**Example 添加前缀**  
以下示例转换源中的所有表，以便在目标中向这些表添加前缀 `DMS_`。  

```
{
 	"rules": [{
		"rule-type": "selection",
		"rule-id": "1",
		"rule-name": "1",
		"object-locator": {
			"schema-name": "test",
			"table-name": "%"
		},
		"rule-action": "include"
	}, {
		"rule-type": "transformation",
		"rule-id": "2",
		"rule-name": "2",
		"rule-action": "add-prefix",
		"rule-target": "table",
		"object-locator": {
			"schema-name": "test",
			"table-name": "%"
		},
		"value": "DMS_"
	}]
 
}
```

**Example 替换前缀**  
以下示例将转换源中包含前缀 `Pre_` 的所有列，以在目标中将前缀替换为 `NewPre_`。  

```
{
    "rules": [
        {
            "rule-type": "selection",
            "rule-id": "1",
            "rule-name": "1",
            "object-locator": {
                "schema-name": "test",
                "table-name": "%"
            },
            "rule-action": "include"
        },
        {
            "rule-type": "transformation",
            "rule-id": "2",
            "rule-name": "2",
            "rule-action": "replace-prefix",
            "rule-target": "column",
            "object-locator": {
                "schema-name": "%",
                "table-name": "%",
                "column-name": "%"
            },
            "value": "NewPre_",
            "old-value": "Pre_"
        }
    ]
}
```

**Example 删除后缀**  
以下示例转换源中的所有表，以从目标中删除这些表的后缀 `_DMS`。  

```
{
	"rules": [{
		"rule-type": "selection",
		"rule-id": "1",
		"rule-name": "1",
		"object-locator": {
			"schema-name": "test",
			"table-name": "%"
		},
		"rule-action": "include"
	}, {
		"rule-type": "transformation",
		"rule-id": "2",
		"rule-name": "2",
		"rule-action": "remove-suffix",
		"rule-target": "table",
		"object-locator": {
			"schema-name": "test",
			"table-name": "%"
		},
		"value": "_DMS"
	}]
}
```

**Example 定义主键**  
以下示例在迁移到目标端点的 `ITEM` 表的三个列上定义一个名为 `ITEM-primary-key` 的主键。  

```
{
	"rules": [{
		"rule-type": "selection",
		"rule-id": "1",
		"rule-name": "1",
		"object-locator": {
			"schema-name": "inventory",
			"table-name": "%"
		},
		"rule-action": "include"
	}, {
		"rule-type": "transformation",
		"rule-id": "2",
		"rule-name": "2",
		"rule-action": "define-primary-key",
		"rule-target": "table",
		"object-locator": {
			"schema-name": "inventory",
			"table-name": "ITEM"
		},
		"primary-key-def": {
			"name": "ITEM-primary-key",
			"columns": [
				"ITEM-NAME",
				"BOM-MODEL-NUM",
				"BOM-PART-NUM"
			]
              }
	}]
}
```

**Example 定义唯一索引**  
以下示例在迁移到目标端点的 `ITEM` 表的三个列上定义一个名为 `ITEM-unique-idx` 的唯一索引。  

```
{
	"rules": [{
		"rule-type": "selection",
		"rule-id": "1",
		"rule-name": "1",
		"object-locator": {
			"schema-name": "inventory",
			"table-name": "%"
		},
		"rule-action": "include"
	}, {
		"rule-type": "transformation",
		"rule-id": "2",
		"rule-name": "2",
		"rule-action": "define-primary-key",
		"rule-target": "table",
		"object-locator": {
			"schema-name": "inventory",
			"table-name": "ITEM"
		},
		"primary-key-def": {
			"name": "ITEM-unique-idx",
			"origin": "unique-index",
			"columns": [
				"ITEM-NAME",
				"BOM-MODEL-NUM",
				"BOM-PART-NUM"
			]
              }
	}]
}
```

**Example 更改目标列的数据类型**  
以下示例将名为 `SALE_AMOUNT` 的目标列的数据类型从现有数据类型更改为 `int8`。  

```
{
    "rule-type": "transformation",
    "rule-id": "1",
    "rule-name": "RuleName 1",
    "rule-action": "change-data-type",
    "rule-target": "column",
    "object-locator": {
        "schema-name": "dbo",
        "table-name": "dms",
        "column-name": "SALE_AMOUNT"
    },
    "data-type": {
        "type": "int8"
    }
}
```

**Example 添加之前映像列**  
对于名为 `emp_no` 的源列，下面示例中的转换规则会在目标中添加名为 `BI_emp_no` 的新列。  

```
{
	"rules": [{
			"rule-type": "selection",
			"rule-id": "1",
			"rule-name": "1",
			"object-locator": {
				"schema-name": "%",
				"table-name": "%"
			},
			"rule-action": "include"
		},
		{
			"rule-type": "transformation",
			"rule-id": "2",
			"rule-name": "2",
			"rule-target": "column",
			"object-locator": {
				"schema-name": "%",
				"table-name": "employees"
			},
			"rule-action": "add-before-image-columns",
			"before-image-def": {
				"column-prefix": "BI_",
				"column-suffix": "",
				"column-filter": "pk-only"
			}
		}
	]
}
```
在此，下面的语句用 1 填充相应行中的 `BI_emp_no` 列。  

```
UPDATE employees SET emp_no = 3 WHERE BI_emp_no = 1;
```
在向支持的 AWS DMS 目标写入 CDC 更新时，该`BI_emp_no`列可以判断`emp_no`列中哪些行有更新的值。