Learn how to use subquery with coalesce in MySQL. Our MySQL Support team is here to help you with your questions and concerns.

How to use Subquery with COALESCE MySQL 

Did you know that the COALESCE function is used to return the first non-null value from a list of expressions in MySQL?

In fact, it helps handle situations where we want to access a non-null value from a set of options.

A subquery can be described as a query embedded within another query. It allows us to get data from one or more tables and use that data as a part of a larger query.

When we combine a subquery with COALESCE, we can get data from a subquery and use the result as input for the COALESCE function. This helps us handle cases where the main query needs to fill in missing values with data gotten from a subquery.

Let’s take a look at an example of how to use a subquery with COALESCE in MySQL:

Now, suppose we have two tables, namely, orders and order_discounts. While the orders table contains order information, the order_discounts table contains discount information for each order.

If we want to get a list of orders with their respective discounts, and if a discount is not available, we have to use a default discount value.

Fortunately, the COALESCE function with a subquery helps us accomplish this:

SELECT
o.order_id,
o.order_date,
COALESCE(
(SELECT discount_amount FROM order_discounts od WHERE od.order_id = o.order_id),
0.10 -- Default discount value if no discount is available
) AS applied_discount
FROM orders o;

Here, the outer query gets data from the orders table and the subquery gets the discount amount from the other table for each order. In case no discount is available, the subquery returns NULL.

The COALESCE function helps select the first non-null value from the subquery result and the default discount value. In case a discount is available in the subquery, it is used. Otherwise, the default discount value is used.

In other words, this query combines the power of subqueries and the COALESCE function to handle such cases.

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Conclusion

In summary, our Support Techs demonstrated how to use a subquery with COALESCE in MySQL.