Using common table expressions (CTEs) in MySQL

It can be challenging to write complicated queries that require multiple joins or subqueries. But fear not! With the introduction of Common Table Expressions (CTEs), the process of composing complex queries becomes much more manageable. In this video, we will explore what CTEs are, how they work, and how you can use them to simplify your queries.

What are common table expressions (CTEs)?

At their core, a Common Table Expression is a SQL statement that can be referenced within the context of a larger query. CTEs are supported in MySQL 8. CTEs allow queries to be broken down into smaller parts that can be more easily comprehended by us mere humans. By doing so, it becomes simpler to reason about and compose complex queries.

How to use CTEs in MySQL

In MySQL, CTEs can be created using the WITH keyword. For example:

with cte_name as (
    select ...
)
select ...
from cte_name

The WITH keyword is followed by the name of the CTE and the query that generates it in parentheses. After defining the CTE, we can reference it in another query. Here's an example that uses CTEs to find customers who have spent more than the average on purchases at a particular store:

with spend_last_6 as (
  select
    customer_id,
    sum(amount) as total_spend
  from
    payment
    inner join customer on customer.id = payment.customer_id
  where
    store_id = 1
    and payment_date > CURRENT_DATE - INTERVAL 6 MONTH
  group by
    cusomter_id
)

select * from spend_last_6

This query uses CTEs to define a temporary table called spend_last_6 which encapsulates all the logic required to calculate the amount a customer has spent in the last six months at store number one. Then we can select from it as if it were a normal table!

Conclusion

Common Table Expressions offer a powerful tool for composing complex queries in MySQL. By breaking down queries into smaller, more reusable parts, you can make your code more readable and maintainable. Furthermore, by using CTEs, you can reduce the number of times that MySQL needs to execute a query, resulting in faster and more efficient queries. If you haven't already, give CTEs a try in your next project, and see how much simpler your queries can become!