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landing/docs/Introduction-To-Databases/017-having-clause_aggregate-functions.md

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+# `HAVING` Clause
+
+Unlike where clause which imposes conditions on columns `Having` clause enables you to specify conditions that filter which group results appear in the results.
+
+## Syntax
+
+```sql
+SELECT column_name(s)
+FROM table_name
+WHERE condition
+GROUP BY column_name(s)
+HAVING condition
+ORDER BY column_name(s);
+```
+
+## Description
+
+- Used with `aggregate functions`
+- Must follow `GROUP BY` clause in the query
+
+## Aggregate Functions
+ - SQL aggregation is the task of collecting a set of values to return a single value.
+ - An aggregate function is a function where the values of multiple rows are grouped together as input on certain criteria to form a single value of more significant meaning.
+    
+## Aggregate Functions Examples
+
+Suppose this are the table given to us
+
+|Students | table||
+|--------|-----------|--------|
+| rollno | name      | class |
+|      1 | Sanskriti | TE    |
+|      1 | Shree     | BE    |
+|      2 | Harry     | TE    |
+|      3 | John      | TE    |
+|      3 | Shivani   | TE    |
+
+
+|purchase | table||
+|------------|-------|---------------|
+| item       | price | customer_name |
+| Pen          |    10 | Sanskriti     |
+| Bag          |  1000 | Sanskriti     |
+| Vegetables   |   500 | Sanskriti     |
+| Shoes        |  5000 | Sanskriti     |
+| Water Bottle |   800 | XYZ           |
+| Mouse        |   120 | ABC           |
+| Sun Glasses  |  1350 | ABC           |
+
+
+### AVG function 
+  
+  Calculates `average` of the given column of values
+
+```sql
+SELECT AVG(price) AS Avg_Purchase, customer_name
+FROM purchase
+GROUP BY customer_name;
+```
+
+| Avg_Purchase | customer_name |
+|--------------|---------------|
+|    1627.5000 | Sanskriti     |
+
+### SUM function
+  
+   Calculates `sum` of values of given column.
+
+```sql
+SELECT SUM(price) AS Total_Bill, customer_name
+FROM purchase
+GROUP BY customer_name;
+```
+
+| Total_Bill | customer_name |
+|------------|---------------|
+|       6510 | Sanskriti     |
+
+### COUNT function 
+
+  Gives `count` of entries/ values in given column.
+
+```sql
+SELECT COUNT(item) AS Total_Items, customer_name
+FROM purchase
+GROUP BY customer_name;
+```
+
+| Total_Items | customer_name |
+|-------------|---------------|
+|           4 | Sanskriti     |
+
+### MAX function
+
+  Return `maximum` value from the number of values in the column.
+
+```sql
+SELECT MAX(price) AS Highest_Purchase, customer_name
+FROM purchase
+GROUP BY customer_name;
+```
+
+| Highest_Purchase | customer_name |
+|-----------------|---------------|
+|             5000 | Sanskriti     |
+
+### MIN function
+  
+   Return `minimum` value from the number of values in the column.
+
+```sql
+SELECT MIN(price) AS Lowest_Purchase, customer_name
+FROM purchase
+GROUP BY customer_name;
+```
+| Lowest_Purchase | customer_name |
+|-----------------|---------------|
+|              10 | Sanskriti     |
+
+
+## Having clause Examples
+
+### Example 1
+
+```sql
+SELECT COUNT(class) AS strength, class
+FROM Students
+GROUP BY class
+HAVING COUNT(class) > 2;
+```
+
+Above query gives number of students in a class `having` number of students > 2 
+
+| strength | class |
+|----------|-------|
+|        4 | TE  |
+
+### Example 2
+
+```sql
+SELECT customer_name, MIN(price) AS MIN_PURCHASE
+FROM purchase
+GROUP BY customer_name
+HAVING MIN(price) > 10;
+```
+Above query finds `minimum` price which is > 10
+
+| customer_name | MIN_PURCHASE |
+|---------------|------------|
+| XYZ           |        800 |
+| ABC           |        120 |
+
+### Example 3
+
+```sql
+SELECT customer_name, AVG(price) AS Average_Purchase
+FROM purchase
+GROUP BY customer_name
+HAVING AVG(price) > 550
+ORDER BY customer_name DESC;
+```
+Above query calculates `average` of price and prints customer name and average price which is greater than 550 with descending `order` of customer names.
+
+| customer_name | Average_Purchase |
+|---------------|------------------|
+| XYZ           |         800.0000 |
+| Sanskriti     |        1627.5000 |
+| ABC           |         735.0000 |
+
+### Example 4
+
+```sql
+SELECT customer_name, SUM(price) AS Total_Purchase
+FROM purchase
+WHERE customer_name
+LIKE "S%"
+GROUP BY customer_name
+HAVING SUM(price) > 1000;
+```
+Calculates `SUM` of price and returns customer name and sum > 1000.
+
+| customer_name | Total_Purchase |
+|---------------|----------------|
+| Sanskriti     |           6510 |
+
+