# WHERE ?

> 📌 Note: This tutorial uses the dvdrental database. If you dont have the dvdrental, please go through this <https://courses.parottasalna.com/database-engineering/sample-databases/dvd-rental-database>

### Intro

The [`SELECT`](/database-engineering/querying-data/select-query.md) statement returns all rows from one or more columns in a table. To retrieve rows that satisfy a specified condition, you use a `WHERE` clause.

The syntax of the PostgreSQL `WHERE` clause is as follows

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

In this syntax, you place the `WHERE` clause right after the `FROM` clause of the `SELECT` statement.

The `WHERE` clause uses the `condition` to filter the rows returned from the `SELECT` clause. The `condition` is a boolean expression that evaluates to true, false, or unknown. The query returns only rows that satisfy the `condition` in the `WHERE` clause. In other words, the query will include only rows that cause the `condition` evaluates to true in the result set.

> ⚠️ Warning: If you are using the column aliases , you can't use them in the Query.&#x20;

To form the condition in the `WHERE` clause, you use comparison and logical operators

| Operator | Description                                         |
| -------- | --------------------------------------------------- |
| =        | Equal                                               |
| >        | Greater than                                        |
| <        | Less than                                           |
| >=       | Greater than or equal                               |
| <=       | Less than or equal                                  |
| <> or != | Not equal                                           |
| AND      | Logical operator AND                                |
| OR       | Logical operator OR                                 |
| IN       | Return true if a value matches any value in a list  |
| BETWEEN  | Return true if a value is between a range of values |
| LIKE     | Return true if a value matches a pattern            |
| IS NULL  | Return true if a value is NULL                      |
| NOT      | Negate the result of other operators                |

### Using WHERE clause with the equal (=) operator example <a href="#id-1-using-where-clause-with-the-equal-operator-example" id="id-1-using-where-clause-with-the-equal-operator-example"></a>

The following statement uses the `WHERE` clause to find customers with the first name is `Jamie,`

```sql
SELECT first_name, last_name FROM customer WHERE first_name = 'Jamie';
```

<figure><img src="/files/GUEsWJikb5jfbJfvuyXt" alt=""><figcaption></figcaption></figure>

### Using the WHERE clause with the AND operator example <a href="#id-2-using-the-where-clause-with-the-and-operator-example" id="id-2-using-the-where-clause-with-the-and-operator-example"></a>

The following example uses a `WHERE` clause with the `AND` logical operator to find customers whose first name and last names are `Linda` and `Williams`

```sql
SELECT first_name, last_name FROM customer WHERE first_name = 'Linda' AND last_name = 'Williams';
```

<figure><img src="/files/WJlUKOelxh021ZmX6LeT" alt=""><figcaption></figcaption></figure>

### Using the WHERE clause with the OR operator example <a href="#id-3-using-the-where-clause-with-the-or-operator-example" id="id-3-using-the-where-clause-with-the-or-operator-example"></a>

The following example uses a WHERE clause with an OR operator to find the customers whose last name is `Rice` or first name is Linda.

```sql
SELECT first_name, last_name FROM customer WHERE last_name = 'Rice' OR first_name = 'Linda';
```

<figure><img src="/files/5nVsaIbbMh9FXN8rN19V" alt=""><figcaption></figcaption></figure>

### Using the WHERE clause with the IN operator example <a href="#id-4-using-the-where-clause-with-the-in-operator-example" id="id-4-using-the-where-clause-with-the-in-operator-example"></a>

If you want to find a value in a list of values, you can use the `IN` operator.

The following example uses the WHERE clause with the IN operator to find the customers with first names in the list Ann, Anne, and Annie

```sql
SELECT first_name, last_name 
FROM customer
WHERE first_name IN ('Ann', 'Anne', 'Annie');
```

<figure><img src="/files/ERHG4irBT8a898osbdGU" alt=""><figcaption></figcaption></figure>

### Using the WHERE clause with the LIKE operator example <a href="#id-5-using-the-where-clause-with-the-like-operator-example" id="id-5-using-the-where-clause-with-the-like-operator-example"></a>

To find a string that matches a specified pattern, you use the `LIKE` operator.

The following example uses the LIKE operator in the WHERE clause to find customers whose first names start with the word Ann.

```sql
SELECT first_name, last_name
FROM customer
WHERE first_name
LIKE 'Ann%';
```

<figure><img src="/files/1omcEtwyh1fS4pzcRf7B" alt=""><figcaption></figcaption></figure>

The `%` is called a wildcard that matches any string. The `'Ann%'` pattern matches any strings that start with `'Ann'`.

### Using the WHERE clause with the BETWEEN operator example <a href="#id-6-using-the-where-clause-with-the-between-operator-example" id="id-6-using-the-where-clause-with-the-between-operator-example"></a>

The following example finds customers whose first names start with the letter `A` and contains 3 to 5 characters by using the `BETWEEN` operator.

The `BETWEEN` operator returns true if a value is in a range of values.

```sql
SELECT
  first_name,
  LENGTH(first_name) name_length
FROM
  customer
WHERE
  first_name LIKE 'A%'
  AND LENGTH(first_name) BETWEEN 3
  AND 5
ORDER BY
  name_length
LIMIT 5;
```

<figure><img src="/files/dNHEUsmNkZkoDhPjJcsm" alt=""><figcaption></figcaption></figure>

### Using the WHERE clause with the not equal operator (<>) example <a href="#id-7-using-the-where-clause-with-the-not-equal-operator-lessgreater-example" id="id-7-using-the-where-clause-with-the-not-equal-operator-lessgreater-example"></a>

This example finds customers whose first names start with `Linda` and last names are not `Motley`

```sql
SELECT
  first_name,
  last_name
FROM
  customer
WHERE
  first_name LIKE 'Linda%'
  AND last_name <> 'Motley';
```

<figure><img src="/files/b5YvTBaQUGtA45gu3pG8" alt=""><figcaption></figcaption></figure>

Note that you can use the `!=` operator and `<>` operator interchangeably because they are equivalent.


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