Adds sub-queries and functions.

database/notes/07-subqueries-functions-worksheet-answers.md

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+# Queries
+
+## Sub-queries
+
+1. Get all the students whose IQ is greater than the average IQ.
+
+```sql
+SELECT 
+    first_name, last_name, iq
+FROM
+    students
+WHERE
+    iq > (SELECT 
+            AVG(iq)
+        FROM
+            students);
+```
+
+2. Get all the students whose IQ is greater than the highest IQ of the batch_id `2` students.
+
+```sql
+SELECT 
+    *
+FROM
+    students
+WHERE
+    iq > (SELECT 
+            MAX(iq)
+        FROM
+            students
+        WHERE
+            batch_id = 2);
+```
+
+3. Get all the students whose IQ is greater than `all` the IQs of the batch_id `2` students.
+
+```sql
+SELECT 
+    *
+FROM
+    students
+WHERE
+    iq > ALL (SELECT 
+            iq
+        FROM
+            students
+        WHERE
+            batch_id = 2);
+```
+
+4. Find all the students who are in batches that start with the word `Jedi` (Without JOIN)
+
+```sql
+SELECT 
+    *
+FROM
+    students
+WHERE
+    batch_id IN (SELECT 
+            id
+        FROM
+            batches
+        WHERE
+            name LIKE 'Jedi%');
+```
+
+5. Find all the students whose IQ is greater than the average IQ of their batch.
+
+```sql
+SELECT 
+    *
+FROM
+    students;
+
+SELECT 
+    first_name, last_name, batch_id, iq
+FROM
+    students s
+WHERE
+    s.iq > (SELECT 
+            AVG(iq)
+        FROM
+            students e
+        WHERE
+            e.batch_id = s.batch_id);
+```
+
+6. Get all the instructors that have at least one batch (Without using joins)
+
+```sql
+SELECT 
+    *
+FROM
+    instructors i
+WHERE
+    EXISTS( SELECT 
+            id
+        FROM
+            batches
+        WHERE
+            instructor_id = i.id);
+```
+
+7. Print all the names, batch ID and average IQ of the batch for each student
+
+```sql
+SELECT 
+    id,
+    first_name,
+    last_name,
+    iq,
+    batch_id,
+    (SELECT 
+            AVG(iq)
+        FROM
+            students
+        WHERE batch_id = s.batch_id     
+		)
+FROM
+    students s;
+```
+
+## Built-in Functions
+1. Get the average IQ of all the students rounded to the nearest integer.
+
+```sql
+SELECT 
+    ROUND(AVG(iq))
+FROM
+    students;
+```
+2. Find all batches whose name is longer than 10 characters.
+
+```sql
+SELECT 
+    id, name
+FROM
+    batches
+WHERE
+    LENGTH(name) > 10;
+```
+
+3. Find all batches whose name's first 10 characters contains the string `sher`
+
+```sql
+SELECT 
+    id, name
+FROM
+    batches
+WHERE
+    LOCATE('sher', SUBSTR(name, 1, 10)) > 0;
+```
+
+4. Get all batches that have started on a Sunday
+
+```sql
+SELECT 
+    id, name
+FROM
+    batches
+WHERE
+    DAYNAME(start_date) = 'Sunday';
+```
+
+5. Get all batches that have been running for more than 10 years
+
+```sql
+SELECT 
+    id, name, start_date
+FROM
+    batches
+WHERE
+	(DATEDIFF(NOW(), start_date) / 365) > 10;
+```
+6. Print the name and the instructor's id for each batch. If no instructor is assigned, print `NO INSTRUCTOR`.
+
+```sql
+SELECT
+    name, IFNULL(instructor_id, 'NO INSTRUCTOR')
+FROM
+    batches;
+```
+
+7. Print the name and IQ of each student. If the IQ of the student is less than 100, print `LOW IQ` instead.
+
+```sql
+SELECT 
+    first_name, last_name, IF(iq < 100, 'LOW IQ', iq)
+FROM
+    students;
+```
+
+8. For each student print the name and their IQ category.
+
+| IQ        | Category |
+| --------- | -------- |
+| < 100     | LOW IQ   |
+| 100 - 150 | MEDIUM   |
+| > 150     | HIGH     |
+
+```sql
+SELECT 
+    first_name,
+    last_name,
+    CASE
+        WHEN iq > 150 THEN 'HIGH IQ'
+        WHEN iq BETWEEN 100 AND 150 THEN 'MEDIUM IQ'
+        ELSE 'LOW IQ'
+    END
+FROM
+    students;
+```
+