Merge pull request #229 from Latsan/main

using multiple subqueries in one table

Author: milos-simic

sql-subqueries/how-to-use-SELECT-with-multiple-subqueries-to-same-table-in-SQL/sample-problem-illustration.sql

@@ -0,0 +1,14 @@
+SELECT s.name,
+       (SELECT e.exam_date
+        FROM Exam e
+        WHERE e.student_id = s.id 
+        AND e.exam_date >= ALL (SELECT exam_date FROM Exam WHERE student_id = s.id)) AS last_exam_date,
+       (SELECT c.name
+        FROM Exam e
+        JOIN Course c ON e.course_id = c.id
+        WHERE e.student_id = s.id 
+        AND e.exam_date >= ALL (SELECT exam_date FROM Exam WHERE student_id = s.id)) AS last_exam_course,
+       (SELECT COUNT(*)
+        FROM Exam e
+        WHERE e.student_id = s.id) AS total_exams
+FROM Student s;

sql-subqueries/how-to-use-SELECT-with-multiple-subqueries-to-same-table-in-SQL/solving-redundancy-inefficient-query-with-CTE.sql

@@ -0,0 +1,17 @@
+WITH RankedExams AS (
+    SELECT 
+        e.student_id,
+        c.name AS course_name,
+        e.exam_date,
+        ROW_NUMBER() OVER (PARTITION BY e.student_id ORDER BY e.exam_date DESC) AS rn,
+        COUNT(*) OVER (PARTITION BY e.student_id) AS total_exams
+    FROM Exam e
+    JOIN Course c ON e.course_id = c.id
+)
+SELECT 
+    s.name,
+    re.exam_date AS last_exam_date,
+    re.course_name AS last_exam_course,
+    re.total_exams
+FROM Student s
+LEFT JOIN RankedExams re ON s.id = re.student_id AND re.rn = 1;

sql-subqueries/how-to-use-SELECT-with-multiple-subqueries-to-same-table-in-SQL/solving-redundancy-inefficient-query-with-JOINs.sql

@@ -0,0 +1,13 @@
+SELECT s.name,
+       e.exam_date AS last_exam_date,
+       c.name AS last_exam_course,
+       (SELECT COUNT(*) FROM Exam WHERE student_id = s.id) AS total_exams
+FROM Student s
+LEFT JOIN Exam e ON s.id = e.student_id
+LEFT JOIN Course c ON e.course_id = c.id
+WHERE e.exam_date = (
+    SELECT exam_date
+    FROM Exam
+    WHERE student_id = s.id
+    AND exam_date >= ALL (SELECT exam_date FROM Exam WHERE student_id = s.id)
+);

sql-subqueries/how-to-use-SELECT-with-multiple-subqueries-to-same-table-in-SQL/subqueries-in-FROM-clause.sql

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+SELECT s.name, exam_counts.course_name, exam_counts.exam_count
+FROM Student s
+JOIN (
+    SELECT e.student_id, c.name AS course_name, COUNT(*) AS exam_count
+    FROM Exam e
+    JOIN Course c ON e.course_id = c.id
+    GROUP BY e.student_id, c.name
+    HAVING COUNT(*) = (
+        SELECT MAX(exam_count)
+        FROM (
+            SELECT student_id, course_id, COUNT(*) AS exam_count
+            FROM Exam
+            GROUP BY student_id, course_id
+        ) max_counts
+        WHERE max_counts.student_id = e.student_id
+    )
+) exam_counts ON s.id = exam_counts.student_id;

sql-subqueries/how-to-use-SELECT-with-multiple-subqueries-to-same-table-in-SQL/subqueries-in-SELECT-clause.sql

@@ -0,0 +1,5 @@
+SELECT s.name,
+       (SELECT COUNT(*)
+        FROM Exam e
+        WHERE e.student_id = s.id) AS exam_count
+FROM Student s;

sql-subqueries/how-to-use-SELECT-with-multiple-subqueries-to-same-table-in-SQL/subqueries-in-WHERE-clause.sql

@@ -0,0 +1,7 @@
+SELECT s.name
+FROM Student s
+WHERE 5 < (
+    SELECT COUNT(*)
+    FROM Exam e
+    WHERE e.student_id = s.id
+);