Lighthouse

Level 4 · Missing costs on top

From Brian Mitchell, Finance Analyst

“I'm going through product costs one by one. Put the products with no cost at the very top so I fix those first, then the rest from cheapest cost to most expensive.”

Your goal

From products, return product_id, name and unit_cost, sorted so rows with a NULL unit_cost come first, then by unit_cost ascending, then by product_id ascending.

Read from the products tablenot done yet
Show exactly product_id name unit_costnot done yet
120 rows, sorted as askednot done yet

Your query

SQL

Loading the editor…

Reference solution

ResultPreview of products · press Run to see yours

Preview of products
product_id	name	category	subcategory	unit_price
101	Vale Velvet Loveseat	Furniture	Sofas	1659.00
102	Alder Oak Coffee Table	Furniture	Tables	919.00
103	Orchard Rattan Dining Chair	Furniture	Chairs	789.00
104	Upland Oak Bookcase	Furniture	Storage	429.00

Mote checks your result

Press Run. Each row that matches the goal becomes a firefly in your lantern.

Tables

Harbor & Pine store data, ready to query.

products120 rows · 8 columns

product_idinteger
namevarchar
categoryvarchar
subcategoryvarchar
unit_pricedecimal
unit_costdecimal
launched_ondate
is_activeboolean

First 5 rows of products
product_id	name	category	subcategory	unit_price	unit_cost	launched_on	is_active
101	Vale Velvet Loveseat	Furniture	Sofas	1659.00	NULL	2023-09-11	true
102	Alder Oak Coffee Table	Furniture	Tables	919.00	503.50	2024-05-16	true
103	Orchard Rattan Dining Chair	Furniture	Chairs	789.00	429.87	2023-05-27	true
104	Upland Oak Bookcase	Furniture	Storage	429.00	214.70	2022-12-20	true
105	Avery Leather Bed Frame	Furniture	Beds	449.00	232.70	2021-08-31	true

customers2,018 rows · 9 columns

customer_idinteger
first_namevarchar
last_namevarchar
emailvarchar
cityvarchar
statevarchar
signup_datedate
segmentvarchar
referral_sourcevarchar

First 5 rows of customers
customer_id	first_name	last_name	email	city	state	signup_date	segment	referral_source
1	Owen	Hudson	owen.hudson@icloud.com	San Francisco	CA	2022-06-02	consumer	search
2	Douglas	Mendoza	dmendoza69@gmail.com	Atlanta	GA	2022-06-02	consumer	NULL
3	Henry	Brooks	hbrooks@lakesidecafe.com	Los Angeles	CA	2022-06-04	business	NULL
4	Brian	Graham	bgraham@bluecedarhomes.com	Seattle	WA	2022-06-06	business	pinterest
5	Kyle	Jimenez	kjimenez27@yahoo.com	Indianapolis	IN	2022-06-07	consumer	NULL

orders12,000 rows · 7 columns

order_idinteger
customer_idinteger
order_datetimestamp
statusvarchar
channelvarchar
shipping_feedecimal
coupon_codevarchar

First 5 rows of orders
order_id	customer_id	order_date	status	channel	shipping_fee	coupon_code
1001	125	2023-01-01 20:51:19	completed	web	0.00	NULL
1002	156	2023-01-02 07:58:46	completed	web	0.00	NULL
1003	22	2023-01-02 10:25:27	completed	marketplace	4.99	WELCOME10
1004	48	2023-01-02 11:21:51	completed	marketplace	4.99	WELCOME10
1005	82	2023-01-02 11:43:36	completed	web	0.00	WELCOME10

order_items30,338 rows · 6 columns

order_item_idinteger
order_idinteger
product_idinteger
quantityinteger
unit_pricedecimal
discount_pctdecimal

First 5 rows of order_items
order_item_id	order_id	product_id	quantity	unit_price
1	1001	121	9	31.99
2	1001	154	3	11.99
3	1002	154	4	11.99
4	1002	194	1	149.00
5	1002	193	2	19.99

returns1,673 rows · 5 columns

return_idinteger
order_item_idinteger
returned_ondate
reasonvarchar
refund_amountdecimal

First 5 rows of returns
return_id	order_item_id	returned_on	reason	refund_amount
1	14	2023-01-10	damaged	83.69
2	42	2023-01-10	wrong_item	19.99
3	105	2023-01-14	wrong_item	10.99
4	24	2023-01-16	changed_mind	26.99
5	126	2023-01-16	late	20.99

employees60 rows · 8 columns

employee_idinteger
first_namevarchar
last_namevarchar
departmentvarchar
titlevarchar
manager_idinteger
hire_datedate
salaryinteger

First 5 rows of employees
employee_id	first_name	last_name	department	title	manager_id	hire_date	salary
1	Catherine	Hale	Executive	Chief Executive Officer	NULL	2016-03-01	285000
2	Robert	Kim	Executive	Chief Operating Officer	1	2017-01-09	235000
3	Megan	Collins	Sales	Head of Sales	1	2018-04-16	172000
4	Chloe	Gutierrez	Sales	Account Executive	3	2020-01-21	72000
5	Frank	Bell	Sales	Account Executive	3	2023-05-21	73000

web_sessions40,000 rows · 7 columns

session_idinteger
customer_idinteger
started_attimestamp
devicevarchar
sourcevarchar
pages_viewedinteger
convertedboolean

First 5 rows of web_sessions
session_id	customer_id	started_at	device	source	pages_viewed	converted
1	NULL	2023-01-01 23:02:24	mobile	paid	1	false
2	18	2023-01-01 15:07:49	desktop	paid	1	false
3	NULL	2023-01-01 16:55:46	desktop	direct	7	false
4	NULL	2023-01-01 12:43:28	mobile	organic	3	false
5	NULL	2023-01-01 13:06:21	mobile	social	2	false

marketing_spend3,276 rows · 3 columns

spend_datedate
channelvarchar
amountdecimal

First 5 rows of marketing_spend
spend_date	channel	amount
2023-01-01	paid	492.99
2023-01-01	email	37.38
2023-01-01	social	200.53
2023-01-02	paid	709.13
2023-01-02	email	64.13

The lesson

NULL is not a number, so where does it go in a sort? The SQL standard leaves it to each database, and they disagree:

database	`ASC`	`DESC`
Postgres, Snowflake, Oracle	NULLs last	NULLs first
DuckDB (since 0.8)	NULLs last	NULLs last
MySQL, SQL Server	NULLs first	NULLs last
BigQuery	NULLs first	NULLs last

Postgres treats NULL as larger than any value, MySQL as smaller, and DuckDB puts NULLs last in both directions by default. The same query can show your missing values at the top in one tool and bury them at the bottom in another.

Say what you want

Postgres, DuckDB, BigQuery, Snowflake and Oracle let you choose explicitly:

ORDER BY unit_cost ASC NULLS FIRST, product_id
ORDER BY unit_cost DESC NULLS LAST, product_id

MySQL does not support NULLS FIRST/LAST. The common workaround sorts on a NULL check first:

-- MySQL: NULLs first, then ascending
ORDER BY unit_cost IS NULL DESC, unit_cost ASC

unit_cost IS NULL is 1 for missing rows and 0 otherwise, so sorting it descending puts the missing ones on top. This trick works in DuckDB and Postgres too.

Why it matters

Top-N with DESC. In Postgres, ORDER BY revenue DESC LIMIT 10 puts NULL revenues first, so your “top 10 products” could start with products that have no data at all. Add NULLS LAST whenever a sort column can be NULL and you are taking a top-N.
Work queues. Like Brian’s request, a list of things to fix should put the missing values first, deliberately.
Portability. Queries move between tools. A query that works by accident because of one database’s default breaks when someone runs it elsewhere.

Common mistakes

Filtering NULLs out when the request was to sort them. If the ticket says “on top”, keep them.
Writing NULLS FIRST once and expecting it to apply to every column. It applies only to the sort key it follows.
Forgetting the tie-breaker. All NULL rows tie with each other, so they need product_id to have a fixed order.

Level 4 · Missing costs on top

Your goal

Your query

Reference solution

Mote checks your result

What Brian gets, and why

Tables

The lesson

Say what you want

Why it matters

Common mistakes

Lantern lit!

Level 4 · Missing costs on top

The request

Your goal

Your query

Reference solution

Mote checks your result

What Brian gets, and why

Tables

The lesson

Say what you want

Why it matters

Common mistakes