SQL_Basic_Join.md

Prepare SQL Basic Join

Average population of each continent

Given the CITY and COUNTRY tables, query the names of all the continents (COUNTRY.Continent) and their respective average city populations (CITY.Population) rounded down to the nearest integer.

Note: CITY.CountryCode and COUNTRY.Code are matching key columns.

Input Format

The CITY and COUNTRY tables are described as follows:

CITY

Field	Type
ID	NUMBER
NAME	VARCHAR2 (17)
COUNTRYCODE	VARCHAR2 (3)
DISTRICT	VARCHAR2 (20)
POPULATION	NUMBER

COINTRY

Field	Type
Code	char(3)
Name	char(52)
Continent	char(50)
Region	char(26)
SurfaceArea	float(10,2)
IndepYear	smallint(6)
Population	int(11)
LifeExpectancy	float(3,1)
GNP	float(10,2)
GNPOld	float(10,2)
LocalName	char(45)
GovernmentForm	char(45)
HeadOfState	char(60)
Capital	int(11)
Code2	char(2)

Solution

SELECT Country.Continent, FLOOR(AVG(City.Population))
FROM Country, City 
WHERE Country.Code = City.CountryCode 
GROUP BY Country.Continent ;

The Report

You are given two tables: Students and Grades. Students contains three columns ID, Name and Marks.

Column	Type
ID	Integer
Name	String
Marks	Integer

Grades contains the following data:

Grade	Min_Mark	Max_Mark
1	0	9
2	10	19
3	20	29
4	30	39
5	40	49
6	50	59
7	60	69
8	70	79
9	80	89
10	90	100

Ketty gives Eve a task to generate a report containing three columns: Name, Grade and Mark. Ketty doesn't want the NAMES of those students who received a grade lower than 8. The report must be in descending order by grade -- i.e. higher grades are entered first. If there is more than one student with the same grade (8-10) assigned to them, order those particular students by their name alphabetically. Finally, if the grade is lower than 8, use "NULL" as their name and list them by their grades in descending order. If there is more than one student with the same grade (1-7) assigned to them, order those particular students by their marks in ascending order.

Write a query to help Eve.

Sample Input

ID	Name	Marks
1	Julia	88
2	Samantha	68
3	Maria	99
4	Scarlet	78
5	Ashley	63
6	Jane	81

Sample Output

Maria 10 99
Jane 9 81
Julia 9 88 
Scarlet 8 78
NULL 7 63
NULL 7 68

Note

Print "NULL" as the name if the grade is less than 8.

Explanation Consider the following table with the grades assigned to the students:

ID	Name	Marks	Grade
1	Julia	88	9
2	Samantha	68	7
3	Maria	99	10
4	Scarlet	78	8
5	Ashley	63	7
6	Jane	81	9

So, the following students got 8, 9 or 10 grades:

Maria (grade 10) Jane (grade 9) Julia (grade 9) Scarlet (grade 8)

Solution

SELECT 
CASE WHEN grd.grade < 8 THEN NULL 
WHEN grd.grade >= 8 THEN std.name END,
grd.grade, std.marks FROM students std, grades grd
WHERE std.marks BETWEEN grd.min_mark AND grd.max_mark
ORDER BY grd.grade DESC, std.name ASC;