add placeholder for relational.lookup block

core/src/datayoga_core/blocks/relational/lookup/block.schema.json

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+{
+  "title": "relational.write",
+  "description": "Lookup in a relational table",
+  "type": "object",
+  "additionalProperties": false,
+  "examples": [
+    {
+      "id": "load_snowflake",
+      "type": "relational.write",
+      "properties": {
+        "connection": "eu_datalake",
+        "table": "employees",
+        "schema": "dbo",
+        "load_strategy": "APPEND"
+      }
+    }
+  ],
+  "properties": {
+    "condition": {
+      "type": "string",
+      "title": "The lookup condition",
+      "description": "Use any valid SQL syntax. Use the alias `lookup` for the lookup table and `incoming` for the base table",
+      "examples": ["lookup.account_number = incoming.i_acct_no"]
+    },
+    "query": {
+      "type": "string",
+      "title": "Query string to use as an override to the built in query",
+      "description": "Use any valid SQL syntax. Use the alias `lookup` for the lookup table and `incoming` for the base table",
+      "examples": [
+        "select country_code,country_name from countries where is_active=1"
+      ]
+    },
+    "schema": {
+      "type": "string",
+      "title": "The table schema of the lookup table",
+      "description": "If not specified, no specific schema will be used when connecting to the database.",
+      "examples": ["dbo"]
+    },
+    "table": {
+      "type": "string",
+      "title": "The lookup table name",
+      "description": "Lookup table name",
+      "examples": ["countries"]
+    },
+    "order_by": {
+      "type": "array",
+      "title": "List of keys to use for ordering. Applicable for multiple matches",
+      "items": {
+        "type": ["string"],
+        "title": "name of column"
+      },
+      "examples": [["country_name"]]
+    },
+    "fields": {
+      "type": "array",
+      "title": "Columns to add to the output from the lookup table",
+      "items": {
+        "type": ["string", "object"],
+        "title": "name of column"
+      },
+      "examples": [["fname", { "lname": "last_name" }, "address", "gender"]]
+    },
+    "multiple_match_policy": {
+      "type": "string",
+      "enum": ["first", "last", "all"],
+      "description": "How to handle multiple matches in the lookup table",
+      "default": ""
+    }
+  },
+  "required": []
+}

docs/reference/blocks/relational_lookup.md

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+---
+parent: Blocks
+grand_parent: Reference
+---
+
+# relational\.write
+
+Lookup in a relational table
+
+
+**Properties**
+
+|Name|Type|Description|Required|
+|----|----|-----------|--------|
+|**condition**<br/>(The lookup condition)|`string`|Use any valid SQL syntax. Use the alias `lookup` for the lookup table and `incoming` for the base table<br/>|no|
+|**query**<br/>(Query string to use as an override to the built in query)|`string`|Use any valid SQL syntax. Use the alias `lookup` for the lookup table and `incoming` for the base table<br/>|no|
+|**schema**<br/>(The table schema of the lookup table)|`string`|If not specified, no specific schema will be used when connecting to the database.<br/>|no|
+|**table**<br/>(The lookup table name)|`string`|Lookup table name<br/>|no|
+|[**order\_by**](#order_by)<br/>(List of keys to use for ordering\. Applicable for multiple matches)|`array`||no|
+|[**fields**](#fields)<br/>(Columns to add to the output from the lookup table)|`array`||no|
+|**multiple\_match\_policy**|`string`|How to handle multiple matches in the lookup table<br/>Default: `""`<br/>Enum: `"first"`, `"last"`, `"all"`<br/>|no|
+
+**Additional Properties:** not allowed  
+**Example**
+
+```yaml
+id: load_snowflake
+type: relational.write
+properties:
+  connection: eu_datalake
+  table: employees
+  schema: dbo
+  load_strategy: APPEND
+
+```
+
+<a name="order_by"></a>
+## order\_by\[\]: List of keys to use for ordering\. Applicable for multiple matches
+
+**Items: name of column**
+
+**Item Type:** `string`  
+**Example**
+
+```yaml
+- country_name
+
+```
+
+<a name="fields"></a>
+## fields\[\]: Columns to add to the output from the lookup table
+
+**Items: name of column**
+
+**No properties.**
+
+**Example**
+
+```yaml
+- fname
+- lname: last_name
+- address
+- gender
+
+```
+
+