Data Engineering Weekly@data_weekly
Nov 8
Daily report says 1 order on a day nobody ordered.

calendar LEFT JOIN orders, then COUNT(*). The empty day still has its calendar row, so COUNT(*) = 1.

COUNT(o.order_id) gives 0.

leetdata.ai/problems/calen… #SQL
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Data Engineering Weekly@data_weekly
Nov 5
9 SQL topics, one problem each:

casting, % of total, 2nd highest salary, semi join, top 2 per group, crosstab, 15-min buckets, sentinel NULLs, new vs returning users.

The one that stalls you is this week's topic.

leetdata.ai/problems

#SQL
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300
Data Engineering Weekly@data_weekly
Nov 4
Same room, bookings a and b. Which WHERE catches every overlap?

A) a_in BETWEEN b_in AND b_out
B) a_in < b_out AND b_in < a_out
C) a_in = b_in
D) a_out > b_in

Reply with a letter.

leetdata.ai/problems/date-… #SQL
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Data Engineering Weekly@data_weekly
Nov 2
Data Engineering Weekly is on WhatsApp.

You get the weekly issue, SQL interview problems in order, and system design questions from real company write-ups.

A channel, not a group. Your number stays hidden.

whatsapp.com/channel/0029Va…
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416
Data Engineering Weekly@data_weekly
Oct 29
Two jobs append to one Iceberg table from snapshot 41. A commits first. B:

A) overwrites A's files
B) retries on the new snapshot, both land
C) breaks it till compaction
D) forks into 2 snapshots

leetdata.ai/paths/lakehouse
#dataengineering
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406
Data Engineering Weekly@data_weekly
Oct 28
8 join problems, in order:

inner, LEFT + IS NULL, anti-join, NOT IN + NULL trap, full outer, multi-condition, date range, many-to-many.

Number 4 returns zero rows, no error.

Free, in the browser:
leetdata.ai/problems

#SQL #dataengineering
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492
Data Engineering Weekly@data_weekly
Oct 27
Switching into data engineering? The interview won't stop at one SQL question.

Start each one free on leetdata.ai:
1. SQL & PySpark in your browser
2. Data modeling canvas
3. 45-min AI mock interview
4. Kafka & Spark cohorts

leetdata.ai

#dataengineering
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Data Engineering Weekly@data_weekly
Oct 24
Claude now builds live dashboards from raw data.

DEW #292's take: dashboards stay. They're how a company agrees on its numbers. BI tools feel it.

Who owns a dashboard nobody wrote SQL for?

dataengineeringweekly.com/p/data-enginee…

#bi
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Data Engineering Weekly@data_weekly
Oct 24
5 rows: 'a', '', ' ', NULL, 'b'

COUNT(*) = ?
COUNT(col) = ?
COUNT(NULLIF(TRIM(col), '')) = ?

Answer before you run it.

leetdata.ai/problems/blank…

#sql
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683
Data Engineering Weekly@data_weekly
Oct 23
Parquet is how columnar data sits on disk.
Arrow is how it sits in memory.

Engines that share Arrow pass data with zero copies, no serialization step.

From Hoyt Emerson's overview in DEW #292. Practice:
leetdata.ai/paths/lakehouse

#dataengineering
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Data Engineering Weekly@data_weekly
Oct 22
eBay monitors 2.6B listings without slowing the fee path.

Write a tiny event ID to a local ring buffer. Add 250 columns of context later in ClickHouse.

Keep the hot path small.

dataengineeringweekly.com/p/data-enginee…

#systemdesign
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Data Engineering Weekly@data_weekly
Oct 22
Data modeling round, one decision per scenario:

SCD2: insert, never update
Org chart: manager_id points at employees
Data vault: hubs, links, satellites
Schema evolution: additive only

Practise on a canvas:
leetdata.ai/modeling

#dataengineering
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Data Engineering Weekly@data_weekly
Oct 21
Zalando ranks schema-mapping matches by:

ORDER BY approval_count DESC, similarity DESC

A reader's catch: a 0.81 match approved often beats a 0.97 approved once.

Flip it, or blend the two?

dataengineeringweekly.com/p/data-enginee…

#sql
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Data Engineering Weekly@data_weekly
Oct 20
LinkedIn moved Spark shuffle onto Apache Celeborn.

Fetch failures: ~750/day to ~20
Shuffle cost per task: down ~90%
Runtime: down ~14%

The senior Spark question: where does your shuffle data live?

leetdata.ai/paths/spark

#apachespark
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Data Engineering Weekly@data_weekly
Oct 20
8 window-function problems, in order: running sum, LAG/LEAD, MoM growth, moving average, DENSE_RANK ties, QUALIFY, RANGE frames, sessions.

The trap: ROWS BETWEEN 6 PRECEDING is 7 rows, not 7 days.

leetdata.ai/problems

#sql #dataengineering
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Data Engineering Weekly@data_weekly
Oct 18
Quick SQL quiz.

A user signs up, comes back on day 6, then on day 8.

Day-7 retained: yes or no?

Answer in the replies. Try it first, free:
leetdata.ai/problems/day-n…

#dataengineering #sql
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Data Engineering Weekly@data_weekly
Oct 19
Replying to @data_weekly
Answer: no. Exactly day 7 means exactly, so day 6 and day 8 count for nothing. Join on activity_day = signup_date + 7, not a range, and the equality enforces "exactly" for free.
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Data Engineering Weekly@data_weekly
Oct 15
For engineers in India switching into data engineering:

All 200 leetdata.ai problems are free. SQL, Python, PySpark, hints, editorials. Plus 1 AI mock interview in week 1.

Practice for the rounds DSA prep skips.

leetdata.ai/problems

#dataengineering #sql
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Data Engineering Weekly@data_weekly
Oct 14
Ratings: 4.9, 4.8, 4.7, 4.7, 4.5

ROW_NUMBER: 1 2 3 4 5
RANK: 1 2 3 3 5
DENSE_RANK: 1 2 3 3 4

Ask for a "top 3" with RANK and you get 4 rows.

Top N per Group, free on leetdata.ai:
leetdata.ai/problems/top-n…

#dataengineering #sql
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Data Engineering Weekly@data_weekly
Oct 13
Affirm processes billions in credit risk and checkout workflows every day.

How do they handle real-time ML inference without crashing checkout latency?

They built an isolated "Intelligence Layer." 🧵👇
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Data Engineering Weekly@data_weekly
Oct 13
Replying to @data_weekly
• Agentic & Multi-Model Scoring: Dynamically routing risk, fraud, and credit logic.
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Data Engineering Weekly@data_weekly
Oct 13
Replying to @data_weekly
Want daily data engineering breakdowns, system architectures, and paper teardowns sent directly to your phone?

Join our new Data Engineering Weekly WhatsApp Channel! 🚀

👇 Join here: [whatsapp.com/channel/0029Va…]
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