+---------------+---------+
|Column Name |Type|+---------------+---------+
| student_id | int || student_name | varchar |+---------------+---------+
student_id is the columnwithuniquevaluesfor this table.
Eachrowof this tablecontains the name and the id of a student in school A.
All student_name aredistinct.
Table: SchoolB
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+---------------+---------+
|Column Name |Type|+---------------+---------+
| student_id | int || student_name | varchar |+---------------+---------+
student_id is the columnwithuniquevaluesfor this table.
Eachrowof this tablecontains the name and the id of a student in school B.
All student_name aredistinct.
Table: SchoolC
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+---------------+---------+
|Column Name |Type|+---------------+---------+
| student_id | int || student_name | varchar |+---------------+---------+
student_id is the columnwithuniquevaluesfor this table.
Eachrowof this tablecontains the name and the id of a student in school C.
All student_name aredistinct.
There is a country with three schools, where each student is enrolled in exactly one school. The country is joining a competition and wants to select one student from each school to represent the country such that:
member_A is selected from SchoolA,
member_B is selected from SchoolB,
member_C is selected from SchoolC, and
The selected students’ names and IDs are pairwise distinct (i.e. no two students share the same name, and no two students share the same ID).
Write a solution to find all the possible triplets representing the country under the given constraints.
Input:
SchoolA table:
+------------+--------------+
| student_id | student_name |+------------+--------------+
|1| Alice ||2| Bob |+------------+--------------+
SchoolB table:
+------------+--------------+
| student_id | student_name |+------------+--------------+
|3| Tom |+------------+--------------+
SchoolC table:
+------------+--------------+
| student_id | student_name |+------------+--------------+
|3| Tom ||2| Jerry ||10| Alice |+------------+--------------+
Output:
+----------+----------+----------+
| member_A | member_B | member_C |+----------+----------+----------+
| Alice | Tom | Jerry || Bob | Tom | Alice |+----------+----------+----------+
Explanation:
Let us see all the possible triplets.
- (Alice, Tom, Tom) --> Rejected because member_B and member_C have the same name and the same ID.
- (Alice, Tom, Jerry) --> Valid triplet.
- (Alice, Tom, Alice) --> Rejected because member_A and member_C have the same name.
- (Bob, Tom, Tom) --> Rejected because member_B and member_C have the same name and the same ID.
- (Bob, Tom, Jerry) --> Rejected because member_A and member_C have the same ID.
- (Bob, Tom, Alice) --> Valid triplet.
The key idea is to generate all possible combinations of one student from each school, then filter out any triplet where two students share the same name or the same ID. Since all students are enrolled in only one school and names/IDs are unique within each school, we only need to check for pairwise distinctness across schools.
SELECT a.student_name AS member_A,
b.student_name AS member_B,
c.student_name AS member_C
FROM SchoolA a
CROSSJOIN SchoolB b
CROSSJOIN SchoolC cWHERE a.student_name <> b.student_name
AND a.student_name <>c.student_name
AND b.student_name <>c.student_name
AND a.student_id <> b.student_id
AND a.student_id <>c.student_id
AND b.student_id <>c.student_id;