HardSQL challenge #366WINDOW FUNCTIONLAGANALYTICS

Largest Order Amount Jump

Find largest jump between consecutive order totals.

The challenge runs on the SQLabHub practice database (SQLite) and uses the orders table. You can solve it in the browser, with nothing to install.

Solve This SQL Challenge

The task

Find largest jump between consecutive order totals.

A correct answer returns 1 row with one column: MAX(diff).

Tables used in this challenge

orders 200 rows

id INTEGER (PK) · user_id INTEGER · total REAL · discount REAL · item_count INTEGER · status TEXT · order_date TEXT · shipped_date TEXT · shipping_city TEXT · shipping_country TEXT

First 3 rows of the orders table
iduser_idtotaldiscountitem_countstatusorder_dateshipped_dateshipping_cityshipping_country
1432638.110.253cancelled2024-04-24NULLLos AngelesUSA
2829470.195delivered2024-03-152023-04-09ChicagoUSA
3136683.240.082delivered2023-02-152023-07-10Los AngelesUSA

The first rows of each table are shown. See how all the tables relate on the database schema page.

Expected output

Expected output of the challenge
MAX(diff)
4692.719999999999

The complete result: 1 row. This is the output of the model solution on the practice database.

What you practise

  • Summarising many rows into one value with COUNT, SUM, AVG, MIN or MAX
  • Querying the result of another query with a subquery in FROM
  • Reading the previous row's value with LAG()
  • Deriving a new column from an expression and naming it with an alias

Hint and approach

Try the challenge first. Open these only when you need them.

Show the hint

LAG(total)

Show the step-by-step approach

How the model solution is built, in the order the database evaluates it:

  1. Build an inner query first (a derived table):
    1. Read from the orders table.
    2. Fetch total from the previous row with LAG(), ordered by order_date. Where there is no such row the value is NULL.
    3. Combine those values into diff: total - LAG(total) OVER (ORDER BY order_date).
  2. Aggregate over all the rows that are left: compute MAX(diff). With no GROUP BY, the result is a single row.

Model solution

Checking your session…

What to watch for

  • LAG(). LAG() returns NULL for the first row of each partition, and any arithmetic with that NULL is NULL too. Supply a default as the third argument if you need a number.
  • Derived table. Columns computed in the inner query are only visible outside under their alias, so every calculated column there needs a name.
  • Calculated columns. Dividing one integer by another truncates the result in SQLite. Multiply one side by 1.0 to get decimals.
Solve This SQL Challenge

Keep learning