minor updates

Author: chaithrakc

README.md

@@ -251,7 +251,7 @@ where lat_n < 137.2345;
 ```
 https://www.hackerrank.com/challenges/weather-observation-station-13/problem
 
-***Note:*** Whenever problem asks for max/min, go with sorting (or) `max()/min()` functions. Whenever problem statement has "each" keyword then group by can be used
+**Note:** Whenever problem asks for max/min, go with sorting (or) `max()/min()` functions. Whenever problem statement has "each" keyword then group by can be used.
 
 4. `power()` function to square the numbers and sqrt() to square root
 https://www.w3schools.com/sql/func_mysql_power.asp
@@ -261,7 +261,9 @@ https://www.w3schools.com/sql/func_mysql_power.asp
 select lat_n, row_number() over (order by lat_n desc) as rnum2 from station;
 ```
 https://www.hackerrank.com/challenges/weather-observation-station-20/problem
+
 https://leetcode.com/problems/second-highest-salary/
+
 https://leetcode.com/problems/nth-highest-salary/
 
 ---------------------------------------------------------------------------------------------
@@ -275,10 +277,9 @@ https://www.hackerrank.com/challenges/the-report/problem
 
 2. if condition within the select clause
 ```
-select if(grade<8, NULL, name), grade, marks
-from students join grades
-where marks between min_mark and max_mark
-order by grade desc, name asc;
+select if(grade<8, null, name), grade, marks
+from students inner join grades on marks between min_mark and max_mark
+order by grade desc, name asc, marks asc;
 ```
 https://www.hackerrank.com/challenges/the-report/problem
 
@@ -346,9 +347,12 @@ https://leetcode.com/problems/consecutive-numbers/
 
 Joining condition is different
 ``` datediff(w1.recordDate,w2.recordDate) = 1```
+
 https://leetcode.com/problems/rising-temperature/
 
-**order of execution** <br>
+-------
+
+### order of execution
 (1) from <br>
 (2) where <br>
 (3) group by <br>
@@ -373,3 +377,41 @@ Note: "query execution order" is called logical query processing, as opposed to
 Do not mistake logical query execution for physical query execution, they are two different things. 
 
 The easiest way to find what is executed first, just look at the **execution plan** - in Microsoft SQL Server you even have the option of looking at the data transfer between operators live.
+
+---------
+### Date Handling in MySQL
+
+SQL Server MONTH() Function
+https://www.w3schools.com/sql/func_sqlserver_month.asp
+
+```
+select month('2017/08/25') as mm;
+```
+
+SQL Server DATEDIFF() Function
+https://www.w3schools.com/sql/func_sqlserver_datediff.asp
+```
+SELECT DATEDIFF(year, '2017/08/25', '2011/08/25') AS DateDiff;
+```
+```
++--------------+-------------------------------------------------------------------+
+|  Parameter   |                            Description                            |
++--------------+-------------------------------------------------------------------+
+| interval     | Required. The part to return. Can be one of the following values: |
+|              | year, yyyy, yy = Year                                             |
+|              | quarter, qq, q = Quarter                                          |
+|              | month, mm, m = month                                              |
+|              | dayofyear = Day of the year                                       |
+|              | day, dy, y = Day                                                  |
+|              | week, ww, wk = Week                                               |
+|              | weekday, dw, w = Weekday                                          |
+|              | hour, hh = hour                                                   |
+|              | minute, mi, n = Minute                                            |
+|              | second, ss, s = Second                                            |
+|              | millisecond, ms = Millisecond                                     |
+| date1, date2 | Required. The two dates to calculate the difference between       |
++--------------+-------------------------------------------------------------------+
+```
+
+
+

hackerrank/04basic_join/p04the_report.sql

@@ -19,95 +19,6 @@ descending order. If there is more than one student with the same grade (1-7) as
 order those particular students by their marks in ascending order.
 */
 
---step1: join without common field
-select * from Students join Grades;
-
-select * from Students inner join Grades;
-
-/*
-Sample Output
-19 Samantha 87 10 90 100
-19 Samantha 87 9 80 89
-19 Samantha 87 8 70 79
-19 Samantha 87 7 60 69
-19 Samantha 87 6 50 59
-19 Samantha 87 5 40 49
-19 Samantha 87 4 30 39
-19 Samantha 87 3 20 29
-19 Samantha 87 2 10 19
-19 Samantha 87 1 0 9
-21 Julia 96 10 90 100
-21 Julia 96 9 80 89
-21 Julia 96 8 70 79
-21 Julia 96 7 60 69
-21 Julia 96 6 50 59
-21 Julia 96 5 40 49
-21 Julia 96 4 30 39
-21 Julia 96 3 20 29
-21 Julia 96 2 10 19
-21 Julia 96 1 0 9
-11 Britney 95 10 90 100
-11 Britney 95 9 80 89
-11 Britney 95 8 70 79
-11 Britney 95 7 60 69
-11 Britney 95 6 50 59
-11 Britney 95 5 40 49
-11 Britney 95 4 30 39
-11 Britney 95 3 20 29
-11 Britney 95 2 10 19
-11 Britney 95 1 0 9
-32 Kristeen 100 10 90 100
-32 Kristeen 100 9 80 89
-32 Kristeen 100 8 70 79
-32 Kristeen 100 7 60 69
-32 Kristeen 100 6 50 59
-32 Kristeen 100 5 40 49
-32 Kristeen 100 4 30 39
-32 Kristeen 100 3 20 29
-32 Kristeen 100 2 10 19
-32 Kristeen 100 1 0 9
-*/
-
--- step 2: do the comparison for deciding the grade
-select name, grade, marks
-from students join grades
-where marks between min_mark and max_mark;
-
-select name, grade, marks
-from students inner join grades
-where marks between min_mark and max_mark;
-
-/*
-Samantha 9 87
-Julia 10 96
-Britney 10 95
-Kristeen 10 100
-Dyana 6 55
-Jenny 7 66
-Christene 9 88
-Meera 3 24
-Priya 8 76
-Priyanka 8 77
-Paige 8 74
-Jane 7 64
-Belvet 8 78
-Scarlet 9 80
-Salma 9 81
-Amanda 4 34
-Heraldo 10 94
-Stuart 10 99
-Aamina 8 77
-Amina 9 89
-Vivek 9 84
-*/
-
---step 3: rest of the conditions
-select if(grade<8, NULL, name), grade, marks
-from students join grades
-where marks between min_mark and max_mark
-order by grade desc, name asc;
-
-
 select if(grade<8, NULL, name), grade, marks
 from students inner join grades
 where marks between min_mark and max_mark
@@ -135,4 +46,14 @@ NULL 7 66
 NULL 6 55
 NULL 4 34
 NULL 3 24
+*/
+
+select if(grade<8, null, name), grade, marks
+from students inner join grades on marks between min_mark and max_mark
+order by grade desc, name asc, marks asc;
+
+/*
+As null, particularly not a legit value, is a pointer pointing to some invalid address, 
+woud not get compared which is why even using DESC after MARKS will get you same result i.e 
+in ascending order.
 */

leetcode/notes.md

@@ -0,0 +1,16 @@
+Easy
+
+
+Medium
+
+
+comparing list to another list using `in` operator
+```
+select dept.name as "Department", emp.name as "Employee", emp.salary as "Salary"
+from employee emp inner join department dept on emp.departmentId = dept.id
+where (emp.departmentId, emp.salary) in (select departmentId, max(salary)
+                                        from employee
+                                        group by departmentId);
+```
+https://leetcode.com/problems/department-highest-salary/
+