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Introduction
Basic CTE
9. Multiple CTEs
Summary

Instruction

Excellent. You can actually have as many CTEs in a single query as you need. Each of them should be separated with a comma, and the WITH keyword should only appear once, at the beginning.

WITH some_name1 AS ( your_CTE1 ),
some_name2 AS ( you_CTE2 ),
...
SELECT ...

Do remember that WITH appears only once, at the beginning. The other CTEs are separated with commas. DO NOT put a comma after the last CTE.

Introducing multiple CTEs usually makes sense when they refer to each other. We'll get to know such CTEs in the next part. For now, we may think of other usages: for instance, you can use set operations like UNION to show results from both CTEs. Suppose we want to show the top projects from two separate categories and some specific parameters, we could provide a query like this:

WITH succ_traveling AS (
  SELECT
    project_id,
    category,
    SUM(amount) AS sum_amount
  FROM project
  JOIN donation
  ON donation.project_id = project.id
  WHERE category = 'traveling'
  GROUP BY project_id, category, minimal_amount
  HAVING SUM(amount) >= 1.25 * minimal_amount),

succ_games AS (
  SELECT
    project_id,
    category,
    SUM(amount) AS sum_amount
  FROM project 
  JOIN donation 
  ON donation.project_id = project.id
  WHERE category = 'games'
  GROUP BY project_id, category, minimal_amount
  HAVING SUM(amount) >= 2 * minimal_amount)

SELECT 
  project_id, 
  category, 
  sum_amount
FROM succ_traveling
UNION
SELECT 
  project_id, 
  category, 
  sum_amount
FROM succ_games
ORDER BY sum_amount DESC;

In this example, we want to show projects with category traveling, which collected at least 25% more money than minimal_amount and projects with category games, which collected at least twice as much money as they initially needed. Once we defined both CTEs, we could show all results together using UNION in the outer query.

Exercise

In January 2016, supporters who donated at least 10% of the minimal_amount in one donation received a gift. In February 2016, they had to donate 20% of the minimal_amount to get the same gift. Show columns amount and donated for all such donations from January and February for which a gift was awarded.

Stuck? Here's a hint!

Create two CTEs: one for January, one for February. Join donation with project to get the desired result.

In order to check if donation occurred in January 2016 use: WHERE donated BETWEEN '2016-01-01' AND '2016-01-31'.

Remember that February 2016 had 29 days.