# WHERE Clause

The `WHERE` clause filters query results. It restricts the rows returned based on specified conditions. Position this clause directly after  `FROM`.

```sql
SELECT column_name FROM table_name WHERE condition;
```

The `condition` is an expression evaluating to true, false, or unknown. Only rows where the condition evaluates to true are included in the result set.

## WHERE Clause Example

Consider a `users` table. To retrieve only active users:

```sql
SELECT DISTINCT username, email, active FROM users WHERE active=true;
```

Output demonstrates results filtered by the `active= true` condition.

| username | email             | active |
|----------|-------------------|--------|
| shinobi  | b@javateamsix.com |      1 |
| tony     | t@javateamsix.com |      1 |

Conversely, retrieve inactive users by setting `active=false`:

```sql
SELECT DISTINCT username, email, active FROM users WHERE active=false;
```

Output reflects rows where `active ` is false (0).

| username    | email             | active |
|-------------|-------------------|--------|
| Java Team Six | d@javateamsix.com |      0 |

To select users with a specific username, like  'shinobi':

```sql
SELECT username, email, active FROM users WHERE username='shinobi';
```

Multiple entries match the username 'shinobi'.

| username | email             | active |
|----------|-------------------|--------|
| shinobi  | b@javateamsix.com |      1 |
| shinobi  | b@javateamsix.com |      1 |

## Operators

Comparison operators define conditions within the `WHERE` clause.

*   `= ` : Equal to
*   `!=` : Not equal to
*   `>` : Greater than
*   `>=` : Greater than or equal to
*   `<` : Less than
*   `<=` : Less than  or equal to

## AND Keyword

Combine multiple conditions with the `AND` keyword. The `AND` operator returns rows where *all* conditions are true.

Example: Select active users with username 'shinobi'.

```sql
SELECT  * FROM users WHERE username='shinobi' AND active=true;
```

Output shows rows matching both the username and active status.

| id | username | about | birthday | active | email             |
|----|---------- |-------|----------|--------|-------------------|
|  2 | shinobi  | NULL  | NULL     |      1 | b@javateamsix.com |
|  5 | shinobi  | NULL  | NULL      |      1 | b@javateamsix.com |

If no rows satisfy *all* conditions, the result is empty.

```sql
SELECT * FROM users WHERE username='shinobi' AND active=false;
```

-- Output:
Empty set.

## OR Keyword

Combine conditions with the `OR` keyword. The `OR` operator returns rows where *any* condition is true. Use `OR` to fetch data matching one criterion or another.

Example:  Select users with username 'shinobi' OR where active is false.

```sql
SELECT * FROM users WHERE username='shinobi' OR active=false;
```

Output includes rows where either condition is met.

| id  | username    | about | birthday | active | email             |
|----|-------------|-------|----------|--------|-------------------|
|  2 | shinobi     | NULL  | NULL     |      1 | b@javate amsix.com |
|  3 | Java Team Six | NULL  | NULL     |      0 | d@javateamsix.com |
|  5 | shinobi     | NULL  | NULL     |      1  | b@javateamsix.com |
|  6 | Java Team Six | NULL  | NULL     |      0 | d@javateamsix.com |

## LIKE Operator

The `LIKE` operator performs  pattern matching. It is typically used with wildcard characters (`%` for any sequence of characters, `_` for any single character).

Example: Select users where the username contains the letter 'y'.

```sql
SELECT * FROM users  WHERE username LIKE '%y%';
```

Output includes usernames containing 'y'.

| id | username | about | birthday | active | email             |
|----|----------|-------|----------|--------|-------------------|
|   2 | shinobi  | NULL  | NULL     |      1 | b@javateamsix.com |
|  4 | tony     | NULL  | NULL     |      1 | t@javateamsix.com  |

## IN Operator

The `IN` operator checks if a column's value matches any value in a specified list. This simplifies conditions that would otherwise require multiple `OR` statements.

Example: Select users with username 'shinobi' or ' Java Team Six'.

```sql
SELECT * FROM users WHERE username IN ('shinobi', 'Java Team Six');
```

Output shows all rows where the username is in the provided list.

| id | username    | about | birthday  | active | email             |
|----|-------------|-------|----------|--------|-------------------|
|  2 | shinobi     | NULL  | NULL     |      1 | b@javateamsix.com |
|   3 | Java Team Six | NULL  | NULL     |      0 | d@javateamsix.com |
|  5 | shinobi     | NULL  | NULL     |      1 | b@javateamsix .com |
|  6 | Java Team Six | NULL  | NULL     |      0 | d@javateamsix.com |

## IS Operator

Use the `IS` operator specifically for checking `NULL` values . The `=` operator cannot be used for this purpose.

To find rows where `about` is `NULL`:

```sql
SELECT * FROM users WHERE about IS NULL;
```

To find rows where `about` is not  `NULL`:

```sql
SELECT * FROM users WHERE about IS NOT NULL;
```

## BETWEEN Operator

The `BETWEEN` operator selects values within a defined range. This range is inclusive of the start and end values.  Applicable to numbers, text, and dates.

Example: Select users with an ID between 3 and 6 (inclusive).

```sql
SELECT * FROM users WHERE id BETWEEN 3 AND 6;
```

Output displays rows where  `id` is within the specified range.

| id | username    | about | birthday | active | email             |
|----|-------------|-------|----------|--------|-------------------|
|  3 | Java Team Six | NULL   | NULL     |      0 | d@javateamsix.com |
|  5 | shinobi     | NULL  | NULL     |      1 | b@javateamsix.com |
|  6 |  Java Team Six | NULL  | NULL     |      0 | d@javateamsix.com |

## Summary

The `WHERE` clause and its associated operators are fundamental tools for precisely selecting data subsets, essential for efficient database interaction  from Java applications.