From 504e0bdb9d9554f461e210662e09670b2e215d89 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Wed, 21 May 2025 18:29:15 +0530
Subject: [PATCH 01/30] Update README.md

---
 README.md | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/README.md b/README.md
index 608c120..d24d06b 100644
--- a/README.md
+++ b/README.md
@@ -60,10 +60,20 @@ ON s.product_id = p.product_id
 
 [1581 - Customer Who Visited but Did Not Make Any Transactions](https://leetcode.com/problems/customer-who-visited-but-did-not-make-any-transactions/)
 ```sql
-SELECT customer_id, COUNT(*) as count_no_trans
-FROM Visits 
-WHERE visit_id NOT IN (SELECT DISTINCT visit_id FROM Transactions)
-GROUP BY customer_id
+select v.customer_id, count(v.visit_id) as count_no_trans
+from Visits v
+left join Transactions t
+On v.visit_id = t.visit_id
+Where t.transaction_id is Null
+group by v.customer_id;
+
+--2nd approach: using Subquery (This approach is not ideal as this problem can be solved using joins)
+SELECT customer_id, COUNT(visit_id) as count_no_trans 
+FROM Visits
+WHERE visit_id NOT IN (
+	SELECT visit_id FROM Transactions
+	)
+GROUP BY customer_id;
 ```
 
 [197 - Rising Temperature](https://leetcode.com/problems/rising-temperature/) 

From 86704faf9fec7faecf7ab53e0d157bba2c2a74f9 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Wed, 21 May 2025 18:40:16 +0530
Subject: [PATCH 02/30] Update README.md

---
 README.md | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/README.md b/README.md
index d24d06b..e9debd3 100644
--- a/README.md
+++ b/README.md
@@ -78,17 +78,18 @@ GROUP BY customer_id;
 
 [197 - Rising Temperature](https://leetcode.com/problems/rising-temperature/) 
 ```sql
-SELECT w1.id 
-FROM Weather w1, Weather w2
-WHERE DATEDIFF(w1.recordDate, w2.recordDate) = 1
-AND w1.temperature > w2.temperature
+SELECT 
+    w1.id
+FROM 
+    Weather w1
+JOIN 
+    Weather w2
+ON 
+    DATEDIFF(w1.recordDate, w2.recordDate) = 1
+WHERE 
+    w1.temperature > w2.temperature;
+
 
--- OR
-SELECT w1.id
-FROM Weather w1, Weather w2
-WHERE w1.temperature > w2.temperature
-AND SUBDATE(w1.recordDate, 1) = w2.recordDate
-```
 
 [1661 - Average Time of Process per Machine](https://leetcode.com/problems/average-time-of-process-per-machine/)
 ```sql

From cd81e88ff93254cf7af4c4d40f56a79b1d65b874 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Wed, 21 May 2025 18:41:25 +0530
Subject: [PATCH 03/30] Update README.md

---
 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/README.md b/README.md
index e9debd3..0fd00e4 100644
--- a/README.md
+++ b/README.md
@@ -88,7 +88,7 @@ ON
     DATEDIFF(w1.recordDate, w2.recordDate) = 1
 WHERE 
     w1.temperature > w2.temperature;
-
+```
 
 
 [1661 - Average Time of Process per Machine](https://leetcode.com/problems/average-time-of-process-per-machine/)

From 7f39c35074f8095bc93d98222292694b090f7426 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Wed, 21 May 2025 18:48:58 +0530
Subject: [PATCH 04/30] Update README.md

---
 README.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/README.md b/README.md
index 0fd00e4..6a9d982 100644
--- a/README.md
+++ b/README.md
@@ -88,6 +88,8 @@ ON
     DATEDIFF(w1.recordDate, w2.recordDate) = 1
 WHERE 
     w1.temperature > w2.temperature;
+
+-- OR We can use lag() window function to get previous_temperature and record_date columns beside our table and filter it
 ```
 
 

From 45b2974cb065f39ea0e8109b8f2a7fdbf6468c40 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Thu, 22 May 2025 18:00:44 +0530
Subject: [PATCH 05/30] Update README.md

---
 README.md | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/README.md b/README.md
index 6a9d982..102488a 100644
--- a/README.md
+++ b/README.md
@@ -103,6 +103,16 @@ FROM
  FROM Activity 
   GROUP BY machine_id, process_id) AS subq
 GROUP BY machine_id
+
+-- OR this is another approach but this put math expession in one go 
+SELECT 
+    machine_id,
+    round(SUM(CASE WHEN activity_type='start' THEN timestamp*-1 ELSE timestamp END)
+    / (SELECT COUNT(DISTINCT process_id)),3) AS processing_time
+FROM 
+    Activity
+GROUP BY machine_id;
+
 ```
 
 [577 - Employee Bonus](https://leetcode.com/problems/employee-bonus/solutions/)

From 74db512e5e20522bec5520ea1a96da3e349231e8 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Thu, 22 May 2025 18:03:21 +0530
Subject: [PATCH 06/30] Update README.md

---
 README.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/README.md b/README.md
index 102488a..18b177c 100644
--- a/README.md
+++ b/README.md
@@ -95,6 +95,8 @@ WHERE
 
 [1661 - Average Time of Process per Machine](https://leetcode.com/problems/average-time-of-process-per-machine/)
 ```sql
+
+-- 1ST approach is by writing subquery to make temporary table first and then we apply math function
 SELECT machine_id, ROUND(AVG(end - start), 3) AS processing_time
 FROM 
 (SELECT machine_id, process_id, 

From 07c03a4ed5dff4fb63ef5aaea26ef5c89f6c9891 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Thu, 22 May 2025 18:37:29 +0530
Subject: [PATCH 07/30] Update README.md

---
 README.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index 18b177c..789f881 100644
--- a/README.md
+++ b/README.md
@@ -119,12 +119,12 @@ GROUP BY machine_id;
 
 [577 - Employee Bonus](https://leetcode.com/problems/employee-bonus/solutions/)
 ```sql
+-- Note that e.name b.bonus can also be written in select statement
 SELECT name, bonus
 FROM Employee e
 LEFT JOIN Bonus b
 ON e.empId = b.empId
-WHERE bonus < 1000
-OR bonus IS NULL
+WHERE bonus < 1000 OR bonus IS NULL
 ```
 
 [1280 - Students and Examinations](https://leetcode.com/problems/students-and-examinations/)

From c1395d217ebc94be9d9b07d691bb9fe8a12e899d Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Thu, 22 May 2025 22:17:50 +0530
Subject: [PATCH 08/30] Update README.md

---
 README.md | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/README.md b/README.md
index 789f881..2bc59ed 100644
--- a/README.md
+++ b/README.md
@@ -129,14 +129,17 @@ WHERE bonus < 1000 OR bonus IS NULL
 
 [1280 - Students and Examinations](https://leetcode.com/problems/students-and-examinations/)
 ```sql
-SELECT a.student_id, a.student_name, b.subject_name, COUNT(c.subject_name) AS attended_exams
-FROM Students a
-JOIN Subjects b
-LEFT JOIN Examinations c
-ON a.student_id = c.student_id
-AND b.subject_name = c.subject_name
-GROUP BY 1, 3
-ORDER BY 1, 3 
+-- Beautiful and brain storming one cross join is must
+
+select st.student_id, st.student_name, s.subject_name, 
+count(e.student_id) as attended_exams
+
+from Students st
+cross join Subjects s
+left join Examinations e
+On st.student_id = e.student_id and s.subject_name = e.subject_name
+group by 1,2,3
+order by 1,2,3 
 ```
 [570. Managers with at Least 5 Direct Reports](https://leetcode.com/problems/managers-with-at-least-5-direct-reports)
 ```sql

From 881b25f311db1d6a805268c353a8e85d79905cc9 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Fri, 23 May 2025 11:45:50 +0530
Subject: [PATCH 09/30] Update README.md

---
 README.md | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/README.md b/README.md
index 2bc59ed..e07ef7f 100644
--- a/README.md
+++ b/README.md
@@ -152,13 +152,6 @@ WHERE id IN
    HAVING COUNT(*) >= 5
   )
 
--- OR
-SELECT a.name
-FROM Employee a
-JOIN Employee b
-WHERE a.id = b.managerId
-GROUP BY b.managerId
-HAVING COUNT(*) >= 5
 ```
 
 [1934. Confirmation Rate](https://leetcode.com/problems/confirmation-rate/)

From f142aaa883a79f0ef2035d29482af265a474fb05 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Tue, 27 May 2025 12:19:46 +0530
Subject: [PATCH 10/30] Update README.md

---
 README.md | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/README.md b/README.md
index e07ef7f..5662568 100644
--- a/README.md
+++ b/README.md
@@ -168,6 +168,25 @@ FROM Signups s
 LEFT JOIN Confirmations c 
 ON s.user_id = c.user_id 
 GROUP BY s.user_id;  
+
+-- solution written by my own and it is correct
+
+With cte as (
+    select sp.user_id, count(action) as requests_made,
+        round(sum(case when action = "confirmed" then 1 else 0 end),2) as 
+                    confirmed_action 
+                    from  Confirmations cf
+                    right join Signups sp
+                    on sp.user_id = cf.user_id
+                    group by cf.user_id                    
+
+ )
+select user_id, (case when 
+                requests_made = 0 then 0 else 
+               confirmed_action/requests_made end) as confirmation_rate
+from cte
+order by confirmation_rate
+
 ```
 
 [620. Not Boring Movies](https://leetcode.com/problems/not-boring-movies)

From c5f0e34947c620804cd86fed9838d3956b50455c Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Tue, 27 May 2025 13:09:29 +0530
Subject: [PATCH 11/30] Update README.md

---
 README.md | 17 -----------------
 1 file changed, 17 deletions(-)

diff --git a/README.md b/README.md
index 5662568..197f096 100644
--- a/README.md
+++ b/README.md
@@ -169,23 +169,6 @@ LEFT JOIN Confirmations c
 ON s.user_id = c.user_id 
 GROUP BY s.user_id;  
 
--- solution written by my own and it is correct
-
-With cte as (
-    select sp.user_id, count(action) as requests_made,
-        round(sum(case when action = "confirmed" then 1 else 0 end),2) as 
-                    confirmed_action 
-                    from  Confirmations cf
-                    right join Signups sp
-                    on sp.user_id = cf.user_id
-                    group by cf.user_id                    
-
- )
-select user_id, (case when 
-                requests_made = 0 then 0 else 
-               confirmed_action/requests_made end) as confirmation_rate
-from cte
-order by confirmation_rate
 
 ```
 

From 0565400ac9730f5166158f3c52cea9a560195711 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Tue, 27 May 2025 14:40:53 +0530
Subject: [PATCH 12/30] Update README.md

---
 README.md | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/README.md b/README.md
index 197f096..87ff74c 100644
--- a/README.md
+++ b/README.md
@@ -180,6 +180,12 @@ FROM Cinema
 WHERE id % 2 <> 0 
 AND description <> "boring"
 ORDER BY rating DESC
+
+-- To reduce run time, best way 
+select id, movie, description, rating
+from Cinema
+where id%2 !=0 and description != "boring"
+order by rating desc
 ```
 
 [1251. Average Selling Price](https://leetcode.com/problems/average-selling-price/)

From d725a50c74ce8eb104c1fbeec96926e045ea3abf Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Tue, 27 May 2025 15:14:24 +0530
Subject: [PATCH 13/30] Update README.md

---
 README.md | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/README.md b/README.md
index 87ff74c..9fd72f9 100644
--- a/README.md
+++ b/README.md
@@ -190,14 +190,17 @@ order by rating desc
 
 [1251. Average Selling Price](https://leetcode.com/problems/average-selling-price/)
 ```sql
--- avg(selling), round 2
-SELECT p.product_id, 
-  ROUND(SUM(price * units) / SUM(units), 2) AS average_price
-FROM Prices p
-LEFT JOIN UnitsSold s
-ON p.product_id = s.product_id
-AND purchase_date BETWEEN start_date AND end_date
-GROUP BY p.product_id
+-- avg(selling), round 2 
+-- I solved this problem by creating cte first with price*units and continued writing but that takes 3370 ms. 
+-- This code took only 835 ms. note that try to avoid CTEs if possible it takes more time for code to run
+select p.product_id, 
+        round(coalesce(sum(units * price)/sum(units),0),2) as average_price
+
+        from Prices p
+        left join UnitsSold u
+        On p.product_id = u.product_id and 
+        purchase_date between start_date and end_date
+        group by p.product_id
 ```
 
 [1075. Project Employees I](https://leetcode.com/problems/project-employees-i)

From 2036d03a01ba8f9f6a53811e6d9e4963a82e5e2d Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Tue, 27 May 2025 15:17:51 +0530
Subject: [PATCH 14/30] Update README.md

---
 README.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/README.md b/README.md
index 9fd72f9..4bfac4b 100644
--- a/README.md
+++ b/README.md
@@ -193,6 +193,7 @@ order by rating desc
 -- avg(selling), round 2 
 -- I solved this problem by creating cte first with price*units and continued writing but that takes 3370 ms. 
 -- This code took only 835 ms. note that try to avoid CTEs if possible it takes more time for code to run
+
 select p.product_id, 
         round(coalesce(sum(units * price)/sum(units),0),2) as average_price
 
@@ -201,6 +202,7 @@ select p.product_id,
         On p.product_id = u.product_id and 
         purchase_date between start_date and end_date
         group by p.product_id
+
 ```
 
 [1075. Project Employees I](https://leetcode.com/problems/project-employees-i)

From 1cde99ae23910283caf842a74d9d043173d83c43 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Tue, 27 May 2025 18:47:14 +0530
Subject: [PATCH 15/30] Update README.md

---
 README.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/README.md b/README.md
index 4bfac4b..31d9954 100644
--- a/README.md
+++ b/README.md
@@ -217,6 +217,7 @@ GROUP BY project_id
 
 [1633. Percentage of Users Attended a Contest](https://leetcode.com/problems/percentage-of-users-attended-a-contest)
 ```sql
+-- Common sense problem. don't use joins. as denominator is same for each contest_id group, use subquery very simple
 -- % desc, contest_id asc, round 2
 SELECT r.contest_id,
        ROUND(COUNT(DISTINCT r.user_id) * 100 / (SELECT COUNT(DISTINCT user_id) FROM Users), 2) AS percentage

From 3133d2a42f181b2c4df69b24b7d05195bd1dc0cc Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Tue, 27 May 2025 19:41:53 +0530
Subject: [PATCH 16/30] Update README.md

---
 README.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/README.md b/README.md
index 31d9954..4968a5f 100644
--- a/README.md
+++ b/README.md
@@ -237,6 +237,8 @@ FROM Queries
 GROUP BY query_name
 
 -- OR
+-- This is nice approach. poor rating is defined in question as less than 3 rating so we need to count how many got less than 1/total ratings and then get percentage
+
 SELECT query_name, 
     ROUND(AVG(rating/position), 2) AS quality, 
     ROUND(SUM(

From 9e8c815b01338f0e73e7bd39375706fa83391ff6 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Tue, 27 May 2025 19:42:52 +0530
Subject: [PATCH 17/30] Update README.md

---
 README.md | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 4968a5f..8c1a998 100644
--- a/README.md
+++ b/README.md
@@ -237,7 +237,8 @@ FROM Queries
 GROUP BY query_name
 
 -- OR
--- This is nice approach. poor rating is defined in question as less than 3 rating so we need to count how many got less than 1/total ratings and then get percentage
+-- This is nice approach. poor rating is defined in question as less than 3 rating so we need to count
+-- how many got less than 3/total ratings and then get percentage
 
 SELECT query_name, 
     ROUND(AVG(rating/position), 2) AS quality, 

From b72625fda191eb5ad9b33254bf9ab85e9465e171 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Wed, 28 May 2025 14:02:39 +0530
Subject: [PATCH 18/30] Update README.md

---
 README.md | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/README.md b/README.md
index 8c1a998..a115b2c 100644
--- a/README.md
+++ b/README.md
@@ -288,6 +288,22 @@ FROM (
     SELECT *, RANK() OVER(partition by customer_id ORDER BY order_date) od
     FROM Delivery) temp
 WHERE temp.od = 1
+
+-- cte approach 
+
+With first_orders as (select customer_id, order_date, customer_pref_delivery_date
+from Delivery 
+where (customer_id, order_date) IN (
+                                     select customer_id, min(order_date)
+                                     from Delivery
+                                     group by customer_id )
+
+)
+
+select round(SUM(order_date = customer_pref_delivery_date)*100/ COUNT(*),2)
+   AS immediate_percentage
+   from first_orders
+
 ```
 
 [550. Game Play Analysis IV](https://leetcode.com/problems/game-play-analysis-iv/)

From 98a02113aeb6a7ed016e776e3559eea391d80929 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Wed, 28 May 2025 14:12:27 +0530
Subject: [PATCH 19/30] Update README.md

---
 README.md | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/README.md b/README.md
index a115b2c..c84fe15 100644
--- a/README.md
+++ b/README.md
@@ -272,6 +272,9 @@ GROUP BY 1, 2
 
 [1174. Immediate Food Delivery II](https://leetcode.com/problems/immediate-food-delivery-ii/)
 ```sql
+-- subquery in the WHERE clause, which gets the minimum order date (MIN(order_date)) for each customer.
+-- This means considering first order placed by a customer.
+
 SELECT
     ROUND((COUNT(CASE WHEN d.order_date = d.customer_pref_delivery_date THEN 1 END) / COUNT(*)) * 100, 2)  immediate_percentage
 FROM Delivery d

From 3945b8c334c3145090c104f659cdf0bddcce8b64 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Wed, 28 May 2025 16:14:15 +0530
Subject: [PATCH 20/30] Update README.md

---
 README.md | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/README.md b/README.md
index c84fe15..bf108ef 100644
--- a/README.md
+++ b/README.md
@@ -311,6 +311,13 @@ select round(SUM(order_date = customer_pref_delivery_date)*100/ COUNT(*),2)
 
 [550. Game Play Analysis IV](https://leetcode.com/problems/game-play-analysis-iv/)
 ```sql
+
+-- Visual Flow of Logic:
+-- Login Date: Find the first login date of each player, Recent Login: Add 1 day to the first login to define the "next day."
+-- Count of Players Logging in on the Next Day: Count how many players logged in on that "next day."
+-- Calculate Fraction: Divide that count by the total number of distinct players and round the result.
+-- The absence of a FROM clause after SELECT ROUND(...) AS fraction is valid because the CTEs and subqueries are already providing the necessary data.
+
 WITH login_date AS (SELECT player_id, MIN(event_date) AS first_login
 FROM Activity
 GROUP BY player_id),
@@ -323,6 +330,28 @@ SELECT ROUND((SELECT COUNT(DISTINCT(player_id))
 FROM Activity
 WHERE (player_id, event_date) IN 
 (SELECT player_id, next_day FROM recent_login)) / (SELECT COUNT(DISTINCT player_id) FROM Activity), 2) AS fraction
+
+--or  second logic: we subtract 1 day from the player's event_date to see if it matches the player's first login date.
+
+SELECT
+  ROUND(
+    COUNT(A1.player_id)
+    / (SELECT COUNT(DISTINCT A3.player_id) FROM Activity A3)
+  , 2) AS fraction
+FROM
+  Activity A1
+WHERE
+  (A1.player_id, DATE_SUB(A1.event_date, INTERVAL 1 DAY)) IN (
+    SELECT
+      A2.player_id,
+      MIN(A2.event_date)
+    FROM
+      Activity A2
+    GROUP BY
+      A2.player_id
+  );
+
+
 ```
 [2356. Number of Unique Subjects Taught by Each Teacher](https://leetcode.com/problems/number-of-unique-subjects-taught-by-each-teacher)
 ```sql

From 212cb47b3d1f37aacf5d08fb66d5884f04a9cee0 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Thu, 29 May 2025 19:06:32 +0530
Subject: [PATCH 21/30] Update README.md

---
 README.md | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/README.md b/README.md
index bf108ef..2d99598 100644
--- a/README.md
+++ b/README.md
@@ -437,12 +437,16 @@ HAVING COUNT(DISTINCT product_key) = (
 ](https://leetcode.com/problems/the-number-of-employees-which-report-to-each-employee/)
 
 ```sql
-SELECT e1.employee_id, e1.name, COUNT(e2.employee_id) reports_count, ROUND(AVG(e2.age)) average_age 
-FROM Employees e1, Employees e2
-WHERE e1.employee_id = e2.reports_to
-GROUP BY e1.employee_id
-HAVING reports_count > 0
-ORDER BY e1.employee_id
+
+select e1.employee_id, e1.name, count(e2.employee_id) as reports_count,
+       round(avg(e2.age)) as average_age
+
+from Employees e1
+left join Employees e2
+On e1.employee_id = e2.reports_to
+group by e1.employee_id, e1.name
+having count(e2.employee_id) >=1
+order by 1
 ```
 [1789. Primary Department for Each Employee
 ](https://leetcode.com/problems/primary-department-for-each-employee/?envType=study-plan-v2&envId=top-sql-50)

From 531e9074c64097c80f66429a58ef14ba50ea4073 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Sat, 31 May 2025 13:17:58 +0530
Subject: [PATCH 22/30] Update README.md

---
 README.md | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/README.md b/README.md
index 2d99598..5f22a62 100644
--- a/README.md
+++ b/README.md
@@ -521,14 +521,13 @@ WHEN product_id NOT IN
 
 [1978. Employees Whose Manager Left the Company](https://leetcode.com/problems/employees-whose-manager-left-the-company)
 ```sql
-SELECT employee_id
-FROM Employees
-WHERE manager_id NOT IN (
-    SELECT employee_id 
-    FROM Employees
-)
-AND salary < 30000
-ORDER BY employee_id
+select employee_id 
+
+from Employees
+where salary < 30000 and manager_id not in (
+                                       select employee_id from Employees
+                                        )
+order by employee_id
 ```
 
 [185. Department Top Three Salaries](https://leetcode.com/problems/department-top-three-salaries)

From 3ca73bfa4da44832e2d33c6f667b984763234d7e Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Sat, 31 May 2025 14:16:08 +0530
Subject: [PATCH 23/30] Update README.md

---
 README.md | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/README.md b/README.md
index 5f22a62..c447cdc 100644
--- a/README.md
+++ b/README.md
@@ -554,6 +554,12 @@ WHERE r.salary_rank <=3;
 SELECT user_id, CONCAT(UPPER(LEFT(name, 1)), LOWER(RIGHT(name, LENGTH(name)-1))) AS name
 FROM Users
 ORDER BY user_id
+
+--OR  Using Concat and substr functions
+select user_id,
+   concat(Upper(substr(name,1,1)), lower(substr(name,2))) as name
+from Users
+order by user_id
 ```
 
 [1527. Patients With a Condition](https://leetcode.com/problems/patients-with-a-condition)

From 11857a03eedadd9f0b269f1d6014003556bc52af Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Sat, 31 May 2025 17:21:00 +0530
Subject: [PATCH 24/30] Update README.md

---
 README.md | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/README.md b/README.md
index c447cdc..2505b5e 100644
--- a/README.md
+++ b/README.md
@@ -421,6 +421,16 @@ SELECT COALESCE(
   ORDER BY num DESC
   LIMIT 1), null) 
   AS num
+
+-- OR CTE APPROACH
+
+With cte as (select num, count(num) as single_occurance
+            from MyNumbers 
+            group by num
+            having count(num) =1 
+               )
+select max(num) as num
+from cte
 ```
 
 [1045. Customers Who Bought All Products](https://leetcode.com/problems/customers-who-bought-all-products/)

From 3d87bd20aed28a3f29176448863a85fa0bc7c772 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Sat, 31 May 2025 18:40:39 +0530
Subject: [PATCH 25/30] Update README.md

---
 README.md | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/README.md b/README.md
index 2505b5e..73a0f39 100644
--- a/README.md
+++ b/README.md
@@ -586,6 +586,17 @@ DELETE p
 FROM Person p, Person q
 WHERE p.id > q.id
 AND q.Email = p.Email
+
+-- OR best approach scalable for large datasets
+DELETE FROM person
+WHERE id NOT IN (
+    SELECT * FROM (
+        SELECT MIN(id)
+        FROM person
+        GROUP BY email
+    ) AS valid_ids
+);
+
 ```
 
 [176. Second Highest Salary](https://leetcode.com/problems/second-highest-salary)

From 5cdb4caeb1743a300b12413dffca747aa7104a95 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Sat, 31 May 2025 19:03:51 +0530
Subject: [PATCH 26/30] Update README.md

---
 README.md | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/README.md b/README.md
index 73a0f39..b2f6cfe 100644
--- a/README.md
+++ b/README.md
@@ -684,6 +684,14 @@ GROUP_CONCAT(DISTINCT product) AS 'products'
 FROM Activities
 GROUP BY sell_date
 ORDER BY sell_date
+
+-- little modification
+
+select sell_date, count(distinct product) as num_sold, 
+ group_concat(distinct product order by product asc) as products
+
+from Activities
+group by sell_date
 ```
 
 [1341. Movie Rating](https://leetcode.com/problems/movie-rating/)

From 12282cc343ad4b74a8050a47a13bc85bee611d43 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Tue, 10 Jun 2025 11:29:37 +0530
Subject: [PATCH 27/30] Update README.md

---
 README.md | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/README.md b/README.md
index b2f6cfe..1209ef2 100644
--- a/README.md
+++ b/README.md
@@ -674,6 +674,17 @@ ON p.product_id = o.product_id
 WHERE DATE_FORMAT(order_date, '%Y-%m') = '2020-02'
 GROUP BY p.product_name
 HAVING SUM(o.unit) >= 100
+
+-- OR we can do like this without using date_format 
+
+select product_name, sum(unit) as unit
+from Orders o
+left join Products p
+on o.product_id = p.product_id
+where order_date between '2020-02-01' and '2020-02-29'
+group by o.product_id, product_name
+having sum(unit) >= 100
+
 ```
 
 [1484. Group Sold Products By The Date](https://leetcode.com/problems/group-sold-products-by-the-date/)

From c47040ab2db0dc3063014a5147813a09c932e238 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Tue, 10 Jun 2025 12:23:24 +0530
Subject: [PATCH 28/30] Update README.md

---
 README.md | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/README.md b/README.md
index 1209ef2..24db68a 100644
--- a/README.md
+++ b/README.md
@@ -618,6 +618,12 @@ AS SecondHighestSalary
 SELECT *
 FROM Users
 WHERE mail REGEXP '^[A-Za-z][A-Za-z0-9_\.\-]*@leetcode\\.com$'
+
+-- OR slight regex variation don't forgot to use REGEXP keyword and single quotes around the expression
+
+select * 
+from Users
+where mail REGEXP '^[a-zA-Z][a-zA-Z0-9._-]*@leetcode\\.com$'
 ```
 
 [1204. Last Person to Fit in the Bus](https://leetcode.com/problems/last-person-to-fit-in-the-bus/)

From 26e0d748bdaa014f6404d81c50ebd22791b82a2b Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Tue, 10 Jun 2025 17:30:02 +0530
Subject: [PATCH 29/30] Update README.md

---
 README.md | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/README.md b/README.md
index 24db68a..7d90111 100644
--- a/README.md
+++ b/README.md
@@ -661,11 +661,22 @@ FROM Accounts
 -- swap every two consecutives
 -- num(students): odd? no swap for last one
 
-SELECT id, 
-CASE WHEN MOD(id,2)=0 THEN (LAG(student) OVER (ORDER BY id))
-ELSE (LEAD(student, 1, student) OVER (ORDER BY id))
-END AS 'Student'
-FROM Seat
+select id, 
+case when 
+     mod(id,2) = 0 then lag(student) over(order by id)
+     else lead(student,1,student) over(order by id) end 
+     as student
+from Seat
+
+-- Very easy way. 
+-- here id in order by is id column we created with case when. not original id column present in the table
+
+select case when id % 2 =1 and id+1 in (select id from Seat) then id+1
+            when id % 2 =0 then id-1
+            else id
+        end as id,student
+        from Seat
+        order by id;
 ```
 
 [1327. List the Products Ordered in a Period](https://leetcode.com/problems/list-the-products-ordered-in-a-period/)

From 5e75d93480663c89a0974014c1135dcfe671bc60 Mon Sep 17 00:00:00 2001
From: Learner457 <135804937+Learner457@users.noreply.github.com>
Date: Thu, 12 Jun 2025 18:21:58 +0530
Subject: [PATCH 30/30] Update README.md

---
 README.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index 7d90111..3d9e642 100644
--- a/README.md
+++ b/README.md
@@ -754,9 +754,9 @@ SELECT visited_on, amount, ROUND(amount/7, 2) AS average_amount
 FROM (
     SELECT DISTINCT visited_on,
     SUM(amount) OVER(ORDER BY visited_on RANGE BETWEEN INTERVAL 6 DAY PRECEDING AND CURRENT ROW) AS amount,
-    MIN(visited_on) OVER() day_1
+    MIN(visited_on) OVER() as day_1
     FROM Customer
-) t
+) t1
 WHERE visited_on >= day_1+6;
 ```