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Introduction
CTEs with data modifying statements
13. Nested CTEs with data modifying
Summary

## Instruction

Fantastic! So far you have done exercises based on the CTEs with data modifying statements. You may also use INSERT, UPDATE, and DELETE statements with nested CTEs. Imagine you need to raise prices of all store products by 7 percent, if the average selling price of products in the store is the lowest from among all the stores. Look at the example below.

WITH avg_prices AS (
SELECT
grocery_store_id,
avg(price) AS avg_price
FROM product
GROUP BY grocery_store_id
),
min_avg_prices AS (
SELECT
MIN(avg_price) AS min_price
FROM avg_prices
)

UPDATE product
SET price = price + price * 0.07
WHERE grocery_store_id IN (
SELECT
grocery_store_id
FROM avg_prices ap
JOIN min_avg_prices map
ON ap.avg_price = map.min_price
);


We construct two nested CTEs. The first uses store ID to identify and average product prices for each store. The second CTE selects the minimum average price from those returned by the first.

In the UPDATE statement we calculate and apply a new product price, increased by 7 percent, to grocery stores whose IDs are identified with the lowest average product price.

To obtain IDs of grocery stores with the least average prices, we compare grocery_store_ids to the ids obtained from the subquery. The subquery compares minimal value from the second CTE and average value from first CTE, getting the grocery_store_id from the first CTE.

## Exercise

Code a storewide (i.e., all-product) price change for the store with the most customer satisfaction points.

In the first CTE, select the sums of customer satisfaction points for all orders for each grocery store. In the second CTE, select the ID of the grocery store(s) with the highest number of points, using the first CTE.

Update product prices, increasing by \$0.05 for the grocery stores with the highest number of points.

### Stuck? Here's a hint!

In first WITH use the SUM aggregate function to select the sum of all points gathered by each grocery store.

In the second WITH clause, select records with only those grocery stores which obtain the highest sum of points. Use the MAX aggregate function with the value obtained from the first CTE.

In the UPDATE statement, use the IN clause with subquery, selecting the IDs of grocery stores from the second CTE.