🚀 LeetCode Database Solutions

Welcome! 👋

This repository contains my MySQL solutions to Database problems on LeetCode.

Each solution includes:

• 📖 Problem
• 📝 Example
• 💡 Idea
• 💻 Solution
• 🛠️ Code Explanation
• 🔄 Alternative Approaches
• ⏱️ Complexity Analysis

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📂 Problems

No.	#	Problem	Difficulty
1	175	Combine Two Tables	🟢 Easy
2	176	Second Highest Salary	🟡 Medium
3	177	Nth Highest Salary	🟡 Medium
4	178	Rank Scores	🟡 Medium
5	180	Consecutive Numbers	🟡 Medium
6	181	Employees Earning More Than Their Managers	🟢 Easy
7	182	Duplicate Emails	🟢 Easy
8	183	Customers Who Never Order	🟢 Easy
9	184	Department Highest Salary	🟡 Medium
10	185	Department Top Three Salaries	🔴 Hard
11	196	Delete Duplicate Emails	🟢 Easy
12	197	Rising Temperature	🟢 Easy
13	262	Trips and Users	🔴 Hard
14	511	Game Play Analysis I	🟢 Easy
15	550	Game Play Analysis IV	🟡 Medium
16	570	Managers with at Least 5 Direct Reports	🟡 Medium
17	577	Employee Bonus	🟢 Easy
18	584	Find Customer Referee	🟢 Easy
19	585	Investments in 2016	🟡 Medium
20	586	Customer Placing the Largest Number of Orders	🟢 Easy
21	595	Big Countries	🟢 Easy
22	596	Classes With at Least 5 Students	🟢 Easy
23	601	Human Traffic of Stadium	🔴 Hard
24	602	Friend Requests II: Who Has the Most Friends	🟡 Medium
25	607	Sales Person	🟢 Easy
26	608	Tree Node	🟡 Medium
27	610	Triangle Judgement	🟢 Easy
28	619	Biggest Single Number	🟢 Easy
29	620	Not Boring Movies	🟢 Easy
30	626	Exchange Seats	🟡 Medium
31	627	Swap Sex of Employees	🟢 Easy
32	1045	Customers Who Bought All Products	🟡 Medium
33	1050	Actors and Directors Who Cooperated At Least Three Times	🟢 Easy
34	1068	Product Sales Analysis I	🟢 Easy
35	1070	Product Sales Analysis III	🟡 Medium
36	1075	Project Employees I	🟢 Easy
37	1084	Sales Analysis III	🟢 Easy
38	1141	User Activity for the Past 30 Days I	🟢 Easy
39	1148	Article Views I	🟢 Easy
40	1158	Market Analysis I	🟡 Medium
41	1164	Product Price at a Given Date	🟡 Medium
42	1174	Immediate Food Delivery II	🟡 Medium
43	1179	Reformat Department Table	🟢 Easy
44	1193	Monthly Transactions I	🟡 Medium
45	1204	Last Person to Fit in the Bus	🟡 Medium
46	1211	Queries Quality and Percentage	🟢 Easy
47	1251	Average Selling Price	🟢 Easy
48	1280	Students and Examinations	🟢 Easy
49	1321	Restaurant Growth	🟡 Medium
50	1327	List the Products Ordered in a Period	🟢 Easy
51	1341	Movie Rating	🟡 Medium
52	1378	Replace Employee ID With The Unique Identifier	🟢 Easy
53	1393	Capital Gain/Loss	🟡 Medium
54	1407	Top Travellers	🟢 Easy
55	1484	Group Sold Products By The Date	🟢 Easy
56	1517	Find Users With Valid E-Mails	🟢 Easy
57	1527	Patients With a Condition	🟢 Easy
58	1581	Customer Who Visited but Did Not Make Any Transactions	🟢 Easy
59	1587	Bank Account Summary II	🟢 Easy
60	1633	Percentage of Users Attended a Contest	🟢 Easy
61	1661	Average Time of Process per Machine	🟢 Easy
62	1667	Fix Names in a Table1683. Invalid Tweets	🟢 Easy
63	1683	Invalid Tweets	🟢 Easy
64	1693	Daily Leads and Partners	🟢 Easy
65	1729	Find Followers Count	🟢 Easy
66	1731	The Number of Employees Which Report to Each Employee	🟢 Easy
67	1741	Find Total Time Spent by Each Employee	🟢 Easy
68	1757	Recyclable and Low Fat Products	🟢 Easy
69	1789	Primary Department for Each Employee	🟢 Easy
70	1795	Rearrange Products Table	🟢 Easy
71	1873	Calculate Special Bonus	🟢 Easy
72	1890	The Latest Login in 2020	🟢 Easy
73	1907	Count Salary Categories	🟡 Medium
74	1934	Confirmation Rate	🟡 Medium
75	1965	Employees With Missing Information	🟢 Easy
76	1978	Employees Whose Manager Left the Company	🟢 Easy
77	2356	Number of Unique Subjects Taught by Each Teacher	🟢 Easy
78	3220	Odd and Even Transactions	🟡 Medium
79	3374	First Letter Capitalization II	🔴 Hard
80	3421	Find Students Who Improved	🟡 Medium
81	3436	Find Valid Emails	🟢 Easy
82	3451	Find Invalid IP Addresses	🔴 Hard
83	3465	Find Products with Valid Serial Numbers	🟢 Easy
84	3475	DNA Pattern Recognition	🟡 Medium
85	3482	Analyze Organization Hierarchy	🔴 Hard
86	3497	Analyze Subscription Conversion	🟡 Medium
87	3521	Find Product Recommendation Pairs	🟡 Medium
88	3554	Find Category Recommendation Pairs	🔴 Hard
89	3564	Seasonal Sales Analysis	🟡 Medium
90	3570	Find Books with No Available Copies	🟢 Easy
91	3580	Find Consistently Improving Employees	🟡 Medium
92	3586	Find COVID Recovery Patients	🟡 Medium
93	3601	Find Drivers with Improved Fuel Efficiency	🟡 Medium
94	3611	Find Overbooked Employees	🟡 Medium
95	3617	Find Students with Study Spiral Pattern	🔴 Hard
96	3626	Find Stores with Inventory Imbalance	🟡 Medium
97	3642	Find Books with Polarized Opinions	🟡 Medium
98	3657	Find Loyal Customers	🟡 Medium
99	3673	Find Zombie Sessions	🔴 Hard
100	3705	Find Golden Hour Customers	🟡 Medium
101	3716	Find Churn Risk Customers	🟡 Medium
102	3764	Most Common Course Pairs	🔴 Hard
103	3793	Find Users with High Token Usage	🟢 Easy
104	3808	Find Emotionally Consistent Users	🟡 Medium
105	3832	Find Users with Persistent Behavior Patterns	🔴 Hard

⭐ New solutions are added regularly.