Finishes aggregate questions.

database/notes/06-sql-joins-aggregation-worksheet-answers.md

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+# Queries
+
+## Joins
+
+1. Get all batch names along with their instructor names.
+
+```sql
+SELECT 
+    b.name, i.first_name, i.last_name
+FROM
+    batches b
+        JOIN
+    instructors i ON b.instructor_id = i.id;
+```
+
+2. Get all students along with their batch names if present else `NULL`.
+
+```sql
+SELECT 
+    s.first_name, s.last_name, b.name
+FROM
+    students s
+        LEFT JOIN
+    batches b ON s.batch_id = b.id;
+```
+3. Get all students along with their batch names. Also, fetch all the batches which have no students.
+
+```sql
+SELECT 
+    s.first_name, s.last_name, b.name
+FROM
+    students s
+        RIGHT JOIN
+    batches b ON s.batch_id = b.id
+```
+4. Get all the combinations of batches and instructors.
+
+```sql
+SELECT 
+    b.name, i.first_name, i.last_name
+FROM
+    batches b, instructors i
+```
+5. Get all students with their instructors. If a student has no instructor, then show `NULL` for the instructor's name.
+
+```sql
+SELECT 
+    s.first_name, s.last_name, i.first_name, i.last_name
+FROM
+    students s
+        LEFT JOIN
+    batches b ON s.batch_id = b.id
+        LEFT JOIN
+    instructors i ON b.instructor_id = i.id;
+```
+
+## Aggregation
+1. Get the maximum IQ in all students (Try without aggregation first).
+
+```sql
+SELECT 
+    first_name, iq
+FROM
+    students
+ORDER BY iq DESC
+LIMIT 1;
+```
+
+2. Get the maximum IQ in all students (With aggregation).
+
+```sql
+SELECT 
+    MAX(IQ) AS 'IQ'
+FROM
+    students;
+```
+
+3. Get the oldest batch from the batches table.
+
+```sql
+SELECT 
+    MIN(start_date) AS 'start date'
+FROM
+    batches;
+```
+
+4. Fetch the number of batches that start with the word `Jedi`.
+
+```sql
+SELECT 
+    COUNT(id)
+FROM
+    batches
+WHERE
+    name LIKE 'Jedi%';
+```
+
+5. Get the average IQ of all students (Without using `AVG`)
+
+```sql
+SELECT 
+    SUM(iq) / COUNT(iq) as 'Average IQ'
+FROM
+    students;
+```
+
+6. Get the average IQ of students in all batches.
+
+```sql
+SELECT 
+    AVG(IQ) AS 'IQ'
+FROM
+    students
+WHERE
+    batch_id IS NOT NULL;
+```
+7. Find the average IQ of students in each batch.
+
+```sql
+SELECT 
+    batch_id, AVG(iq)
+FROM
+    students
+GROUP BY batch_id;
+```
+
+8. Find the total number of students in each batch.
+
+```sql
+SELECT 
+	b.name, COUNT(s.id)
+FROM
+    batches b
+        LEFT JOIN
+    students s ON b.id = s.batch_id
+GROUP BY b.id;
+```
+
+9. Get the total number of batches taught by each instructor.
+
+```sql
+SELECT 
+    i.first_name, COUNT(b.id)
+FROM
+    instructors i
+        LEFT JOIN
+    batches b ON i.id = b.instructor_id
+GROUP BY i.id;
+```
+
+10. Find the average IQ of students in batches with batch ID `1` and `2`.
+
+```sql
+SELECT 
+    batch_id, AVG(iq)
+FROM
+    students
+WHERE batch_id IN (1, 2)
+GROUP BY batch_id;
+```
+
+11. Find count of students that are part of batches that have average IQ greater than `120`.
+
+```sql
+SELECT 
+    batch_id, AVG(iq) as avg_iq, COUNT(iq)
+FROM
+    students
+GROUP BY batch_id
+HAVING avg_iq > 130;
+```