Mastering TypeORM Query Builder: How to Write Insert Query Using CTE from Another Table

Are you tired of struggling with complex database queries? Do you want to take your TypeORM skills to the next level? Look no further! In this comprehensive guide, we’ll show you how to write an insert query using a Common Table Expression (CTE) from another table using TypeORM Query Builder.

What is a Common Table Expression (CTE)?

A Common Table Expression (CTE) is a temporary result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement. CTEs are particularly useful when you need to perform complex calculations or recursive operations on a dataset. In TypeORM, CTEs are supported through the Query Builder, which allows you to write expressive and efficient database queries.

Why Use a CTE with TypeORM Query Builder?

There are several reasons why you might want to use a CTE with TypeORM Query Builder:

• Improved performance**: CTEs can reduce the complexity of your queries and improve performance by reducing the number of joins and subqueries.
• Easier maintenance**: CTEs make it easier to maintain and modify your queries, as you can reuse the same expression multiple times.
• Increased readability**: CTEs can make your queries more readable by breaking down complex logic into smaller, more manageable pieces.

Setting Up TypeORM Query Builder

Before we dive into the example, make sure you have TypeORM installed in your project. If you’re new to TypeORM, start by creating a new project and installing the required dependencies:

npm install typeorm
npm install @types/typeorm

Create a new file called `typeorm.ts` and add the following code to configure TypeORM:

import { createConnection } from 'typeorm';

createConnection({
  type: 'postgres',
  url: 'localhost',
  username: 'username',
  password: 'password',
  database: 'database',
  entities: ['src/entities/*.js'],
  synchronize: true,
});

The Example Database

For this example, let’s assume we have two tables: `orders` and `order_items`.

Table	Columns
orders	id (primary key), customer_name, order_date
order_items	id (primary key), order_id (foreign key), product_name, quantity

We want to insert new orders with corresponding order items from a CTE that derives from the `order_items` table.

Creating the CTE

In TypeORM, you can create a CTE using the `createCommonTableExpression` method. In this example, we’ll create a CTE that selects all order items with a quantity greater than 10:

import { createCommonTableExpression } from 'typeorm';

const orderItemsCte = createCommonTableExpression(
  'order_items',
  'oi',
  `SELECT * FROM order_items WHERE quantity > 10`
);

Writing the Insert Query

Now that we have our CTE, we can use it to insert new orders with corresponding order items:

import { getRepository } from 'typeorm';
import { Order } from './entities/Order';

const orderRepository = getRepository(Order);

const query = orderRepository
  .createQueryBuilder('o')
  .insert()
  .into('orders')
  .values([
    {
      customer_name: 'John Doe',
      order_date: new Date(),
    },
  ])
  .output('inserted.*')
  .from(orderItemsCte, 'oi')
  .innerJoinAndSelect('o.orderItems', 'oi')
  .returning('o.*');

const result = await query.execute();

In this example, we:

1. Create a Query Builder instance for the `orders` table.
2. Specify the columns to insert using the `values` method.
3. Use the `from` method to reference the CTE.
4. Perform an inner join with the `order_items` table using the `innerJoinAndSelect` method.
5. Specify the columns to return using the `returning` method.
6. Execute the query using the `execute` method.

Explain the Magic

Let’s break down what’s happening behind the scenes:

• The CTE is executed first, selecting all order items with a quantity greater than 10.
• The Query Builder uses the CTE as a temporary result set to insert new orders with corresponding order items.
• The `innerJoinAndSelect` method performs an inner join with the `order_items` table, linking each order to its corresponding order items.
• The `returning` method specifies the columns to return from the inserted rows.

Conclusion

In this article, we’ve demonstrated how to write an insert query using a CTE from another table using TypeORM Query Builder. By leveraging CTEs, you can simplify complex database queries, improve performance, and increase readability.

Remember to practice and experiment with different scenarios to master TypeORM Query Builder. Happy coding!

Frequently Asked Question

Get ready to master the art of writing insert queries using CTE from another table in TypeORM Query Builder!