Rebase da1660d: 'missing value per field pattern'

patterns/README.md

@@ -965,3 +965,62 @@ re-packaged using these formats.
 To keep the implementation and testing testing: only one recursive level is
 possible. A `resource` can list `resources` inside (like in the example). But
 the inner resources cannot contain resources again.
+
+## Missing values per field
+
+### Overview
+
+Characters representing missing values in a table can be defined for all fields in a [Tabular Data Resource](http://frictionlessdata.io/specs/tabular-data-resource/) using the [`missingValues`](http://frictionlessdata.io/specs/table-schema/#missing-values) property in a Table Schema. Values that match the `missingValues` are treated as `null`.
+
+The Missing values per field pattern allows different missing values to be specified for each field in a Table Schema. If not specified, each field inherits from values assigned to `missingValues` at the Tabular Data Resource level.
+
+For example, this data...
+
+item | description | price
+---- | ----------- | -----
+1    | Apple       | 0.99
+tba  | Banana      | -1
+3    | n/a         | 1.20
+
+...using this Table Schema...
+
+```javascript
+"schema":{
+  "fields": [
+    {
+      "name": "item",
+      "title": "An inventory item number",
+      "type": "integer"
+    },
+    {
+      "name": "description",
+      "title": "item description",
+      "type": "string",
+      "missingValues": [ "n/a"]
+    },
+    {
+      "name": "price",
+      "title": "cost price",
+      "type": "number",
+      "missingValues": [ "-1"]
+    }
+  ],
+  "missingValues": [ "tba", "" ]
+}
+```
+
+...would be interpreted as...
+
+item   | description | price
+------ | ----------- | ------
+1      | Apple       | 0.99
+`null` | Banana      | `null`
+3      | `null`      | 1.20
+
+### Specification
+
+A field MAY have a `missingValues` property that MUST be an `array` where each entry is a `string`. If not specified, each field inherits from the values assigned to [`missingValues`](http://frictionlessdata.io/specs/table-schema/#missing-values) at the Tabular Data Resource level.
+
+### Implementations
+
+None known.