mysql select only not null values

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## Selecting Only NOT NULL Values in MySQL: A Comprehensive Tutorial

This tutorial provides a deep dive into selecting only NOT NULL values in MySQL, covering various scenarios, optimization techniques, and best practices. We'll explore how to use the `IS NOT NULL` operator effectively, handle different data types, and address common pitfalls.

**1. The `IS NOT NULL` Operator: The Foundation**

The cornerstone of selecting NOT NULL values is the `IS NOT NULL` operator. It's used in the `WHERE` clause of your `SELECT` statement to filter rows based on whether a specific column contains a NULL value.

**Syntax:**
**Explanation:**

* `SELECT column1, column2, ...`: Specifies the columns you want to retrieve from the table. You can select all columns using `SELECT *`.
* `FROM table_name`: Indicates the table from which you are retrieving data.
* `WHERE column_name IS NOT NULL`: This is the crucial part. It filters the result set to include only rows where the value in `column_name` is NOT NULL. Note the `IS` keyword is required; using `=` or `!=` with NULL will not work as intended.

**Code Example:**

Let's assume we have a table named `employees` with the following structure:
Now, to select all employees who have a valid phone number (i.e., `phone_number` is NOT NULL):
This query will return:
Notice that employees with NULL `phone_number` values (Jane, Michael, and Jessica) are excluded from the result.

**2. Selecting NOT NULL Values for Multiple Columns**

You can combine multiple `IS NOT NULL` conditions in your `WHERE` clause using logical operators like `AND` and `OR`.

**Example (AND): Select employees who have both a valid email and a valid phone number.**
This query will return:
**Example (OR): Select employees who have either a valid email or a valid phone number (or both).**
This query will return:
**3. Handling Different Data Types and NULL Values**

The `IS NOT NULL` operator works consistently across different data types in My ...

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