Add filter with where

src/data/courses/sql/chapters/sql-basics/lessons/filter-with-where.md

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+---
+title: Filtering with WHERE
+description: Learn the basics of SQL, the language for querying databases.
+order: 130
+type: lesson-challenge
+---
+
+We can use the optional `WHERE` clause in our SQL queries to filter and only select the rows that match a certain condition.
+
+For example, let's say we have a table called `customers` that contains the information about the customers of our online bookstore.
+
+| id | name | registered_at | is_active | country |
+|----|-------------|------------|-----------|---------|
+| 1  | John Doe | 2024-01-01 | true | USA |
+| 2  | Jane Smith | 2024-01-02 | true | Canada |
+| 3  | Alice Johnson | 2024-01-03 | false | USA |
+| 4  | Bob Smith | 2024-01-04 | true | Canada |
+| 5  | Charlie Davis | 2024-01-05 | false | USA |
+
+We want to select the customers that are active i.e. `is_active` column is `true`. The SQL query to do this would be:
+
+```sql
+SELECT * 
+FROM customers 
+WHERE is_active = true;
+```
+
+When executed, our database engine will scan the `customers` table, going through each row and checking if the `is_active` column of the given row is `true`. If it is, the row is included in the result set. If it is not, the row is skipped. Output from the query above would be:
+
+| id | name | registered_at | is_active | country |
+|----|-------------|------------|-----------|---------|
+| 1  | John Doe | 2024-01-01 | true | USA |
+| 2  | Jane Smith | 2024-01-02 | true | Canada |
+| 4  | Bob Smith | 2024-01-04 | true | Canada |
+
+---
+
+## Combining Conditions
+
+We can combine multiple conditions using the `AND` and `OR` operators. Let's say we want to select the customers that are active and from Canada. The SQL query to do this would be:
+
+```sql
+SELECT * 
+FROM customers 
+WHERE is_active = true AND country = 'Canada';
+```
+
+The output from the query above would be:
+
+| id | name | registered_at | is_active | country |
+|----|-------------|------------|-----------|---------|
+| 2  | Jane Smith | 2024-01-02 | true | Canada |
+| 4  | Bob Smith | 2024-01-04 | true | Canada |
+
+As you can see, the customer `John Doe` is not included in the results because even though the customer is active, they are not from `Canada`.
+
+---
+
+## Comparison Operators
+
+We can use comparison operators to filter the rows that match a certain condition. Here is a subset of comparison operators that are most commonly used:
+
+| Operator | Description |
+|----------|-------------|
+| = | Equal |
+| != | Not equal |
+| > | Greater than |
+| < | Less than |
+| >= | Greater than or equal to |
+| <= | Less than or equal to |
+| BETWEEN | Between a range of values |
+| IN | In a list of values |
+| LIKE | Matches a pattern |
+| NOT | Negates a condition |
+
+Let's say we have another table called `orders` that contains the information about the orders of our online bookstore.
+
+| id | customer_id | order_date | total_amount |
+|----|-------------|------------|--------------|
+| 1  | 1 | 2024-01-01 | 100.00 |
+| 2  | 2 | 2024-01-02 | 150.00 |
+| 3  | 3 | 2024-01-03 | 200.00 |
+| 4  | 4 | 2024-01-04 | 250.00 |
+| 5  | 5 | 2024-01-05 | 300.00 |
+
+Given below are some questions and answers to help you practice filtering with WHERE clause.
+
+> Coding environment on the right has the `customers` and `orders` tables pre-loaded with data so go ahead and try writing the SQL queries to answer the questions.
+
+---
+
+#### Select all orders with total amount greater than 200
+
+```sql
+SELECT * 
+FROM orders 
+WHERE total_amount > 200;
+```
+
+---
+
+#### Select all orders with total amount between 100 and 200
+
+```sql
+SELECT * 
+FROM orders 
+WHERE total_amount BETWEEN 100 AND 200;
+```
+
+We can also use the `AND` operator to achieve the same result.
+
+```sql
+SELECT * 
+FROM orders 
+WHERE total_amount >= 100 AND total_amount <= 200;
+```
+
+---
+
+#### Select all the orders where customer id is 1, 2 or 3
+
+You can use the `IN` operator which has the following syntax:
+
+```sql
+SELECT * 
+FROM table_name 
+WHERE column_name IN (value1, value2, value3);
+```
+
+Since we want to select all the orders where the `customer_id` is either `1`, `2` or `3`, the SQL query to do this would be:
+
+```sql
+SELECT * 
+FROM orders 
+WHERE customer_id IN (1, 2, 3);
+```
+
+We can also use the `OR` operator to achieve the same result.
+
+```sql
+SELECT * 
+FROM orders 
+WHERE 
+    customer_id = 1 OR 
+    customer_id = 2 OR 
+    customer_id = 3;
+```
+
+---
+
+#### Select all customers whose last name is Smith
+
+We can use the `LIKE` operator to filter based on a pattern. The syntax for the `LIKE` operator is:
+
+```sql
+SELECT * 
+FROM table_name 
+WHERE column_name LIKE '%pattern%';
+```
+
+`%` is the wildcard character and matches any sequence of characters. The table below is the description of what the given pattern matches.
+
+| Pattern | Description |
+|---------|-------------|
+| `%Smith` | Any string that ends with `Smith` |
+| `Smith%` | Any string that starts with `Smith` |
+| `%Smith%` | Any string that contains `Smith` anywhere in the string |
+
+In our case, we want to select all customers whose last name is `Smith`. The SQL query to do this would be:
+
+```sql
+SELECT * 
+FROM customers 
+WHERE name LIKE '%Smith';
+```
+
+To select all customers whose name starts with `S`, the SQL query would be:
+
+```sql
+SELECT * 
+FROM customers 
+WHERE name LIKE 'S%';
+```
+
+---
+
+#### Negating a condition
+
+We can negate a condition using the `NOT` operator. For example, to select all orders where the customer id is not 1 or 3, the SQL query would be:
+
+```sql
+SELECT * 
+FROM orders 
+WHERE customer_id NOT IN (1, 3);
+```
+
+To select all customers whose name does not start with `S`, the SQL query would be:
+
+```sql
+SELECT * 
+FROM customers 
+WHERE name NOT LIKE 'S%';
+```
+
+To select all the orders where amount is not between 100 and 200, the SQL query would be:
+
+```sql
+SELECT * 
+FROM orders 
+WHERE total_amount NOT BETWEEN 100 AND 200;
+```
+
+To select all customers whose name does not contain the letter `i` in their name, the SQL query would be:
+
+```sql
+SELECT * 
+FROM customers 
+WHERE name NOT LIKE '%i%';
+```
+
+#### Select customers with name starting with 'A' and ending with 'n'
+
+```sql
+SELECT * 
+FROM customers 
+WHERE name LIKE 'A%n';
+```
+
+---
+
+### Operator Precedence
+
+Considering the same `orders` table, let's say we want to select all orders with total amount greater than 200 and customer id is 1 or 3. We can write the query as:
+
+```sql
+SELECT * 
+FROM orders 
+WHERE total_amount > 200 AND customer_id IN (1, 3);
+```
+
+We can also write the same query without using the `IN` operator:
+
+```sql
+SELECT * 
+FROM orders 
+WHERE 
+    total_amount > 200 AND 
+    (customer_id = 1 OR customer_id = 3);
+```
+
+If you look closely, you will see that we have used parentheses to group the conditions. This is because of the operator precedence: `AND` has higher precedence than `OR`. If we don't use parentheses, the query will be evaluated as:
+
+```sql
+SELECT * 
+FROM orders 
+WHERE 
+    (total_amount > 200 AND customer_id = 1) 
+    OR customer_id = 3;
+```
+
+Which is not the same as the query we wrote earlier.
+
+#### What is operator precedence?
+
+Operator precedence specifies the order in which conditional operators are evaluated when two or more operators with different precedence are adjacent in an expression. 
+
+To understand precedence, let's take an example for a mathematical expression.
+
+```
+1 + 2 * 3
+```
+
+Since multiplication `*` has higher precedence than addition `+`, the expression is evaluated as 
+
+```
+1 + (2 * 3) = 1 + 6 = 7
+``` 
+
+If we want to force the addition to be evaluated first, we can use parentheses:
+
+```
+(1 + 2) * 3 = 3 * 3 = 9
+```
+
+Similarly, in SQL different operators have different precedence. Thus, if you want to ensure that a certain condition is evaluated first, you should use parentheses to group the conditions.