Sector file // sql

SQLDECODED.

Joins, indexes & queries

82 Questions56.3k Views3.4k Upvotes

15 easy51 med16 hard

0 of 82 solvedUp next: What is the difference between a primary key and a foreign key?

Easy

3 questions

What is the logical order of execution of a SQL query?

DSA · 1.1k views · order-of-execution

1.1k70

How do you find duplicate rows in a table?

DSA · 694 views · duplicates, group-by

6946

What is a CHECK constraint?

DSA · 122 views · check-constraint

12216

Medium

13 questions

Top-N per group with RANK() OVER (PARTITION BY) — how do you get top 2 per department without collapsing rows?

DSA · 1.2k views · window-functions, rank, partition

1.2k74

What is a subquery and what is a correlated subquery?

DSA · 1.2k views · subquery

1.2k4

LATERAL JOIN — how do you fetch the top 3 orders per customer efficiently?

DSA · 1.1k views · lateral-join, top-n, postgres

1.1k39

How does GROUP BY work with aggregate functions?

DSA · 1.1k views · group-by, aggregate

1.1k27

How do you aggregate values into a single string?

DSA · 1.1k views · string-agg

1.1k22

What do the LAG and LEAD window functions do?

DSA · 1.0k views · lag, lead

1.0k49

How do you calculate year-over-year or month-over-month growth in SQL?

DSA · 1.0k views · window-functions, analytics

1.0k43

What is the difference between ROW_NUMBER, RANK, and DENSE_RANK?

DSA · 740 views · window, ranking

7407

Hard

3 questions

RANK() vs DENSE_RANK() vs ROW_NUMBER() — when does the gap matter and how do you choose?

DSA · 1.1k views · window-functions, ranking, dense-rank

1.1k48

How do you solve gaps-and-islands problems in SQL?

DSA · 511 views · window-functions, gaps-and-islands

5115

LATERAL vs Window Function vs Correlated Subquery for top-N per group — which scales when?

System Design · 221 views · lateral-join, query-planning, performance

22179

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