# Query every book · Your first Rell app on Chromia · Learn Chromia > Queries let you read data from the blockchain. This lesson walks you through writing a query and then testing it. Source: https://chromia.com/learn/book-review/book-entity/write-queries/ Your first Rell app on ChromiaLesson 7 of 26 1. [01Your first Rell app on Chromia](https://chromia.com/learn/book-review/introduction/) 2. [02Set up your project](https://chromia.com/learn/book-review/setup/) 3. [03Rell project structure](https://chromia.com/learn/book-review/rell-structure/) Part 1: Define your first entity 1. [04Overview](https://chromia.com/learn/book-review/book-entity/) 2. [05Create your first entity](https://chromia.com/learn/book-review/book-entity/tables/) 3. [06Add your first operation](https://chromia.com/learn/book-review/book-entity/basic-operations/) 4. [07Query every book](https://chromia.com/learn/book-review/book-entity/write-queries/) Part 2: Add a related entity 1. [08Overview](https://chromia.com/learn/book-review/book-review-entity/) 2. [09Define the book review entity](https://chromia.com/learn/book-review/book-review-entity/tables/) 3. [10Add an operation for creating book reviews](https://chromia.com/learn/book-review/book-review-entity/basic-operations/) 4. [11Query all reviews for a book](https://chromia.com/learn/book-review/book-review-entity/write-queries/) Part 3: Checking and validating inputs 1. [12Overview](https://chromia.com/learn/book-review/input-verification/) 2. [13Verify inputs](https://chromia.com/learn/book-review/input-verification/input-verification/) 3. [14Return structured results from queries](https://chromia.com/learn/book-review/input-verification/structure/) Part 4: Signing transactions and filtering queries 1. [15Overview](https://chromia.com/learn/book-review/sign-transaction/) 2. [16Sign a transaction](https://chromia.com/learn/book-review/sign-transaction/sign-transaction/) 3. [17Filter and sort query results](https://chromia.com/learn/book-review/sign-transaction/structure/) Part 5: Blockchain state and transactions explained 1. [18Overview](https://chromia.com/learn/book-review/blockchain-transactions/) 2. [19How blockchain state and transactions work](https://chromia.com/learn/book-review/blockchain-transactions/query-transaction/) 3. [20Walking through an example](https://chromia.com/learn/book-review/blockchain-transactions/example/) Part 6: Create the client 1. [21Overview](https://chromia.com/learn/book-review/build-client/) 2. [22Prerequisites](https://chromia.com/learn/book-review/build-client/prerequisites/) 3. [23Establishing a connection to Chromia](https://chromia.com/learn/book-review/build-client/sign-transaction/) 4. [24Query the blockchain using postchain-client](https://chromia.com/learn/book-review/build-client/query-blockchain/) 5. [25Complete the example](https://chromia.com/learn/book-review/build-client/complete-example/) 1. [26What’s next?](https://chromia.com/learn/book-review/what-next/) [Learn](https://chromia.com/learn/)[Your first Rell app on Chromia](https://chromia.com/learn/book-review/) 07 / 26Part 1: Define your first entityBeginner # Query every book Queries let you read data from the blockchain. This lesson walks you through writing a query and then testing it. ## Define the query Add the following Rell code to `src/main/queries.rell`: src/main/queries.rell ``` query get_all_books() { return book @* { } ( .isbn, .title, .author ); } ``` ### Breakdown of the query * **`query` keyword:** Declares a new query. It plays a role comparable to a `SELECT` statement in SQL and returns data from the blockchain. * **`@*` operand:** Writing `book @*` tells Rell the query can return any number of `book` objects, including none. * **Filter criteria:** You place query conditions inside the curly braces `{}`. They are empty here, so every entry is returned. * **Attributes:** The part after the curly braces lists the attributes to return: `.isbn`, `.title`, and `.author`. The result is a collection of every book, where each item carries its `isbn`, `title`, and `author`. ## Update the unit tests Next, extend the tests so they verify the query's behavior. ### Step 1: Update imports In `src/test/book_review_test.rell`, change the import statement to the following: src/test/book\_review\_test.rell ```nolines import main.{ book, create_book, get_all_books }; ``` ### Step 2: Add the new test function Add this test function to the same file: src/test/book\_review\_test.rell ``` function test_get_books() { rell.test.tx() .op(create_book("123", "Book1", "Author1")) .op(create_book("124", "Book2", "Author2")) .run(); val books = get_all_books(); assert_equals(books.size(), 2); } ``` ### Breakdown of the test * **Transaction creation:** `rell.test.tx()` builds a transaction for testing. * **Adding operations:** The transaction receives two `create_book` operations. * **Execution:** Calling `run()` executes the transaction. * **Query and assertions:** `get_all_books()` fetches the books, and the test asserts that exactly 2 were returned. From your project folder, run the tests with this terminal command: shell ```nolines chr test ``` ## Test on a local Chromia node To check the query against a local Chromia node: 1. **Update or start the local Chromia node**: * **If your node is running**, update it with: shell ```nolines chr node update ``` * **If your node is not running**, start it with: shell ```nolines chr node start ``` note The effect of `chr node update` is not always immediate. The query might only become available once the next block has been processed. 2. **Run the query**: Open another terminal window or tab and run: shell ```nolines chr query get_all_books ``` The output lists the books stored so far, such as any you added earlier with the `create_book` operation. If you completed the previous steps, the result should resemble this: json ``` [ { "author": "George Orwell", "isbn": "ISBN1234", "title": "1984" } ] ``` This output shows that the query returns the stored book from the blockchain as expected. You have now stored entities on your test node's blockchain and written a query that returns a collection of all books. Written for this site from Chromia’s public course. Code samples are the course’s own. [← Previous**Add your first operation**](https://chromia.com/learn/book-review/book-entity/basic-operations/)[Next →**Part 2: Add a related entity**](https://chromia.com/learn/book-review/book-review-entity/) On this page * [Define the query](#define-the-query) * [Update the unit tests](#update-the-unit-tests) * [Test on a local Chromia node](#test-on-a-local-chromia-node)