Add uniqueKeys property to Table Schema (#30)

* Updated the spec

* Updated the profile

* Updated letter case

* Updated json formatting

* Updated the example

* Fixed profile

* Added `non-empty array`

* Improved wording

content/docs/specifications/table-schema.md

@@ -650,6 +650,47 @@ Here's an example with an array primary key:
       "primaryKey": ["a", "c"]
      }
 
+### Unique Keys
+
+A unique key is a field or a set of fields that are required to have unique logical values in each row in the table. It is directly modeled on the concept of unique constraint in SQL.
+
+The `uniqueKeys` property, if present, `MUST` be a non-empty array. Each entry in the array `MUST` be a `uniqueKey`. A `uniqueKey` `MUST` be an array of strings with each string corresponding to one of the field `name` values in the `fields` array, denoting that the unique key is made up of those fields. It is acceptable to have an array with a single value, indicating just one field in the unique key.
+
+An example of using the `uniqueKeys` property:
+
+```json
+"fields": [
+  {
+    "name": "a"
+  },
+  {
+    "name": "b"
+  },
+  {
+    "name": "c"
+  }
+],
+"uniqueKeys": [
+  ["a"],
+  ["a", "b"],
+  ["a", "c"]
+]
+```
+
+In the case of the definition above, the data in the table has to be considered valid only if:
+
+- each row has a unique logical value in the field `a`
+- each row has a unique set of logical values in the fields `a` and `b`
+- each row has a unique set of logical values in the fields `a` and `c`
+
+#### Handling `null` values
+
+All the field values that are on the logical level are considered to be `null` values `MUST` be excluded from the uniqueness check, as the `uniqueKeys` property is modeled on the concept of unique constraint in SQL.
+
+#### Relation to `constraints.unique`
+
+In contrast with `field.constraints.unique`, `uniqueKeys` allows to define uniqueness as a combination of fields. Both properties `SHOULD` be assessed separately.
+
 ### Foreign Keys
 
 A foreign key is a reference where values in a field (or fields) on the

profiles/dictionary/schema.yaml

@@ -42,6 +42,8 @@ tableSchema:
           }
     primaryKey:
       "$ref": "#/definitions/tableSchemaPrimaryKey"
+    uniqueKeys:
+      "$ref": "#/definitions/tableSchemaUniqueKeys"
     foreignKeys:
       type: array
       minItems: 1
@@ -141,6 +143,16 @@ tableSchemaPrimaryKey:
           "last_name"
         ]
       }
+tableSchemaUniqueKeys:
+  type: array
+  minItems: 1
+  uniqueItems: true
+  items:
+    type: array
+    minItems: 1
+    uniqueItems: true
+    items:
+      type: string
 tableSchemaForeignKey:
   title: Table Schema Foreign Key
   description: Table Schema Foreign Key