Oracle Correlated Subquery

Summary: in this tutorial, you will learn about the Oracle correlated subquery which is a subquery whose some clauses refer to the column expressions in the outer query.

Introduction to the Oracle correlated subquery #

See the following products table in the sample database:

The following query returns the cheapest products from the products table using a subquery in the WHERE clause.

SELECT
  product_id,
  product_name,
  list_price
FROM
  products
WHERE
  list_price = (
    SELECT
      MIN(list_price)
    FROM
      products
  );Code language: SQL (Structured Query Language) (sql)

Try it

Let’s examine this query in more detail.

First, you can execute the subquery independently.

SELECT
  MIN(list_price)
FROM
  products;Code language: SQL (Structured Query Language) (sql)

Try it

Second, Oracle evaluates the subquery only once.

Third, after the subquery returns a result set, the outer query makes use of them. In other words, the outer query depends on the subquery. However, the subquery is isolated and not dependent on the values of the outer query.

Unlike the above subquery, a correlated subquery is a subquery that uses values from the outer query. In addition, a correlated subquery may be evaluated once for each row selected by the outer query. Because of this, a query that uses a correlated subquery could be slow.

A correlated subquery is also known as a repeating subquery or a synchronized subquery.

Oracle correlated subquery examples #

Let’s take some examples of the correlated subqueries to better understand how they work.

Oracle correlated subquery in the WHERE clause example #

The following query finds all products whose list price is above average for their category.

SELECT
  product_id,
  product_name,
  list_price
FROM
  products p
WHERE
  list_price > (
    SELECT
      AVG(list_price)
    FROM
      products
    WHERE
      category_id = p.category_id
  );Code language: SQL (Structured Query Language) (sql)

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In the above query, the outer query is:

SELECT
    product_id,
    product_name,
    list_price
FROM
    products p
WHERE
    list_price >
Code language: SQL (Structured Query Language) (sql)

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The correlated subquery is:

SELECT
  AVG(list_price)
FROM
  products
WHERE
  category_id = p.category_idCode language: SQL (Structured Query Language) (sql)

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For each product from the products table, Oracle has to execute the correlated subquery to calculate the average price by category.

Oracle correlated subquery in the SELECT clause example #

The following query returns all products and the average standard cost based on the product category:

SELECT
  product_id,
  product_name,
  standard_cost,
  ROUND(
    (
      SELECT
        AVG(standard_cost)
      FROM
        products
      WHERE
        category_id = p.category_id
    ),
    2
  ) avg_standard_cost
FROM
  products p
ORDER BY
  product_name;Code language: SQL (Structured Query Language) (sql)

Try it

For each product from the products table, Oracle executed the correlated subquery to calculate the average standard of cost for the product category.

Note that the above query used the ROUND() function to round the average standard cost to two decimals.

Oracle correlated subquery with the EXISTS operator example #

We usually use a correlated subquery with the EXISTS operator. For example, the following statement returns all customers who have no orders:

SELECT
  customer_id,
  name
FROM
  customers
WHERE
  NOT EXISTS (
    SELECT
      *
    FROM
      orders
    WHERE
      orders.customer_id = customers.customer_id
  )
ORDER BY
  name;Code language: SQL (Structured Query Language) (sql)

Try it

In this tutorial, you have learned about the Oracle correlated subquery. Now, you should understand how the correlated subqueries work and know how to apply them effectively to query data.