diff --git a/backend/src/test/kotlin/com/opendatamask/connector/AzureSQLConnectorTest.kt b/backend/src/test/kotlin/com/opendatamask/connector/AzureSQLConnectorTest.kt
new file mode 100644
index 0000000..7523a2d
--- /dev/null
+++ b/backend/src/test/kotlin/com/opendatamask/connector/AzureSQLConnectorTest.kt
@@ -0,0 +1,75 @@
+package com.opendatamask.connector
+
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.Assertions.*
+
+class AzureSQLConnectorTest {
+
+    // Use H2 in MSSQLServer compatibility mode with null credentials so
+    // AzureSQLConnector takes the simple DriverManager.getConnection(url) path.
+    private val h2Url =
+        "jdbc:h2:mem:aztest_${System.nanoTime()};DB_CLOSE_DELAY=-1;MODE=MSSQLServer;DATABASE_TO_UPPER=FALSE"
+    private lateinit var connector: AzureSQLConnector
+
+    @BeforeEach
+    fun setup() {
+        connector = AzureSQLConnector(h2Url, null, null)
+        java.sql.DriverManager.getConnection(h2Url).use { conn ->
+            conn.createStatement().execute(
+                "CREATE TABLE IF NOT EXISTS test_users (id INT, name VARCHAR(100), email VARCHAR(200))"
+            )
+            conn.createStatement().execute("DELETE FROM test_users")
+            conn.createStatement().execute("INSERT INTO test_users VALUES (1, 'Alice', 'alice@example.com')")
+            conn.createStatement().execute("INSERT INTO test_users VALUES (2, 'Bob', 'bob@example.com')")
+        }
+    }
+
+    @Test
+    fun `testConnection returns true for valid connection`() {
+        assertTrue(connector.testConnection())
+    }
+
+    @Test
+    fun `fetchData returns all rows without limit or filter`() {
+        val rows = connector.fetchData("test_users")
+        assertEquals(2, rows.size)
+    }
+
+    @Test
+    fun `fetchData respects row limit`() {
+        val rows = connector.fetchData("test_users", limit = 1)
+        assertEquals(1, rows.size)
+    }
+
+    @Test
+    fun `fetchData with whereClause filters rows`() {
+        val rows = connector.fetchData("test_users", whereClause = "name = 'Alice'")
+        assertEquals(1, rows.size)
+        val name = rows[0]["NAME"] ?: rows[0]["name"]
+        assertEquals("Alice", name)
+    }
+
+    @Test
+    fun `writeData inserts rows and returns count`() {
+        val rows = listOf(
+            mapOf("id" to 3, "name" to "Charlie", "email" to "charlie@example.com")
+        )
+        val count = connector.writeData("test_users", rows)
+        assertEquals(1, count)
+        val all = connector.fetchData("test_users")
+        assertEquals(3, all.size)
+    }
+
+    @Test
+    fun `writeData returns 0 for empty list`() {
+        assertEquals(0, connector.writeData("test_users", emptyList()))
+    }
+
+    @Test
+    fun `truncateTable removes all rows`() {
+        connector.truncateTable("test_users")
+        val rows = connector.fetchData("test_users")
+        assertTrue(rows.isEmpty())
+    }
+}
diff --git a/backend/src/test/kotlin/com/opendatamask/connector/MongoDBConnectorTest.kt b/backend/src/test/kotlin/com/opendatamask/connector/MongoDBConnectorTest.kt
index 622e20a..1f0921a 100644
--- a/backend/src/test/kotlin/com/opendatamask/connector/MongoDBConnectorTest.kt
+++ b/backend/src/test/kotlin/com/opendatamask/connector/MongoDBConnectorTest.kt
@@ -1,11 +1,13 @@
 package com.opendatamask.connector
 
+import com.mongodb.client.FindIterable
 import com.mongodb.client.MongoClient
 import com.mongodb.client.MongoCollection
 import com.mongodb.client.MongoDatabase
 import org.bson.Document
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.Assertions.*
+import org.mockito.Mockito
 import org.mockito.kotlin.*
 
 class MongoDBConnectorTest {
@@ -16,6 +18,70 @@ class MongoDBConnectorTest {
         }
     }
 
+    /**
+     * Returns a FindIterable mock that uses RETURNS_DEEP_STUBS so that the
+     * MongoIterable.map(...).toList() call chain in fetchData() works without NPE.
+     * fetchData() will return an empty list, but the filter/limit arguments
+     * passed to find()/limit() can still be verified.
+     */
+    private fun mockFindIterable(): FindIterable<Document> {
+        val iterable = mock<FindIterable<Document>>(defaultAnswer = Mockito.RETURNS_DEEP_STUBS)
+        whenever(iterable.limit(any<Int>())).thenReturn(iterable)
+        return iterable
+    }
+
+    @Test
+    fun `fetchData uses empty filter when no whereClause provided`() {
+        val mockCollection = mock<MongoCollection<Document>>()
+        val mockDb = mock<MongoDatabase>()
+        val mockClient = mock<MongoClient>()
+        val findIterable = mockFindIterable()
+
+        whenever(mockClient.getDatabase("testdb")).thenReturn(mockDb)
+        whenever(mockDb.getCollection("users")).thenReturn(mockCollection)
+        whenever(mockCollection.find(any<Document>())).thenReturn(findIterable)
+
+        val connector = createConnector(mockClient)
+        connector.fetchData("users")
+
+        verify(mockCollection).find(Document())
+    }
+
+    @Test
+    fun `fetchData applies JSON filter when whereClause provided`() {
+        val mockCollection = mock<MongoCollection<Document>>()
+        val mockDb = mock<MongoDatabase>()
+        val mockClient = mock<MongoClient>()
+        val findIterable = mockFindIterable()
+        val expectedFilter = Document.parse("""{"age": 18}""")
+
+        whenever(mockClient.getDatabase("testdb")).thenReturn(mockDb)
+        whenever(mockDb.getCollection("users")).thenReturn(mockCollection)
+        whenever(mockCollection.find(any<Document>())).thenReturn(findIterable)
+
+        val connector = createConnector(mockClient)
+        connector.fetchData("users", whereClause = """{"age": 18}""")
+
+        verify(mockCollection).find(expectedFilter)
+    }
+
+    @Test
+    fun `fetchData applies limit when specified`() {
+        val mockCollection = mock<MongoCollection<Document>>()
+        val mockDb = mock<MongoDatabase>()
+        val mockClient = mock<MongoClient>()
+        val findIterable = mockFindIterable()
+
+        whenever(mockClient.getDatabase("testdb")).thenReturn(mockDb)
+        whenever(mockDb.getCollection("users")).thenReturn(mockCollection)
+        whenever(mockCollection.find(any<Document>())).thenReturn(findIterable)
+
+        val connector = createConnector(mockClient)
+        connector.fetchData("users", limit = 5)
+
+        verify(findIterable).limit(5)
+    }
+
     @Test
     fun `writeData calls insertMany and returns row count`() {
         val mockCollection = mock<MongoCollection<Document>>()