docs: user guide for merge insert (#2083)

Closes #2062

nodejs/examples/merge_insert.test.ts

@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+import { expect, test } from "@jest/globals";
+import * as lancedb from "@lancedb/lancedb";
+
+test("basic upsert", async () => {
+  const db = await lancedb.connect("memory://");
+
+  // --8<-- [start:upsert_basic]
+  const table = await db.createTable("users", [
+    { id: 0, name: "Alice" },
+    { id: 1, name: "Bob" },
+  ]);
+
+  const newUsers = [
+    { id: 1, name: "Bobby" },
+    { id: 2, name: "Charlie" },
+  ];
+  await table
+    .mergeInsert("id")
+    .whenMatchedUpdateAll()
+    .whenNotMatchedInsertAll()
+    .execute(newUsers);
+
+  await table.countRows(); // 3
+  // --8<-- [end:upsert_basic]
+  expect(await table.countRows()).toBe(3);
+
+  // --8<-- [start:insert_if_not_exists]
+  const table2 = await db.createTable("domains", [
+    { domain: "google.com", name: "Google" },
+    { domain: "github.com", name: "GitHub" },
+  ]);
+
+  const newDomains = [
+    { domain: "google.com", name: "Google" },
+    { domain: "facebook.com", name: "Facebook" },
+  ];
+  await table2
+    .mergeInsert("domain")
+    .whenNotMatchedInsertAll()
+    .execute(newDomains);
+  await table2.countRows(); // 3
+  // --8<-- [end:insert_if_not_exists]
+  expect(await table2.countRows()).toBe(3);
+
+  // --8<-- [start:replace_range]
+  const table3 = await db.createTable("chunks", [
+    { doc_id: 0, chunk_id: 0, text: "Hello" },
+    { doc_id: 0, chunk_id: 1, text: "World" },
+    { doc_id: 1, chunk_id: 0, text: "Foo" },
+    { doc_id: 1, chunk_id: 1, text: "Bar" },
+  ]);
+
+  const newChunks = [{ doc_id: 1, chunk_id: 0, text: "Baz" }];
+
+  await table3
+    .mergeInsert(["doc_id", "chunk_id"])
+    .whenMatchedUpdateAll()
+    .whenNotMatchedInsertAll()
+    .whenNotMatchedBySourceDelete({ where: "doc_id = 1" })
+    .execute(newChunks);
+
+  await table3.countRows("doc_id = 1"); // 1
+  // --8<-- [end:replace_range]
+  expect(await table3.countRows("doc_id = 1")).toBe(1);
+});