The Meaning of Coalesce is to come together to form one larger group, substance, etc. 

In SQL, the COALESCE() function is a powerful tool used to handle NULL values in queries. The function takes multiple arguments and returns the first non-NULL value. This function is extremely useful when dealing with NULL values in database tables. 

What are NULL values in SQL? 

NULL is a special value in SQL that represents missing or unknown data. A NULL value is different from a zero value or a blank value. NULL values can be difficult to work with because they are not considered equal to any other value, even another NULL value. 

Why use the COALESCE() function in SQL? 

The COALESCE() function is used to handle NULL values in SQL queries. When querying data from a database table, NULL values can cause issues because they do not evaluate to any value or data type. The COALESCE() function helps to avoid these issues by replacing NULL values with a specified default value. 

Syntax of the COALESCE() function in SQL 

The syntax of the COALESCE() function in SQL is as follows: 

COALESCE(expression1, expression2, expression3, …) 

The function takes multiple arguments, separated by commas. It returns the first non-NULL value in the list of arguments. If all the arguments are NULL, the function returns NULL. 

Examples of the COALESCE() function in SQL 

Let’s take a look at some examples to understand how the COALESCE() function is used in SQL. Example 1: Simple use of the COALESCE() function 

Suppose we have a table called employees that contains information about employees in a company. The table has columns for employee ID, name, salary, and department. Some of the employees do not have a department assigned yet. We can use the COALESCE() function to replace the NULL value in the department column with a default value, such as “unassigned”. 

SELECT employee_id, name, salary, COALESCE(department, 'unassigned') 

FROM employees;

In this example, the COALESCE() function is used to replace the NULL value in the department column with the string ‘unassigned’ for any employee who does not have a department assigned. 

Example 2: Using COALESCE() with multiple columns 

Suppose we have a table called products that contain information about products in a store. The table has columns for product ID, name, price, and discount price. The discount price column may contain NULL values. We want to create a query that shows the discounted price if it exists, otherwise, it should show the regular price. 

SELECT product_id, name, COALESCE(discount_price, price) AS price 

FROM products; 

In this example, the COALESCE() function is used to show the discount price if it exists, otherwise, it shows the regular price. 

Example 3: Using COALESCE() with subqueries 

Suppose we have two tables: customers and orders. The customer table contains information about customers, and the orders table contains information about orders placed by customers. We want to create a query that shows the total number of orders placed by each customer, even if they have not placed any orders yet. 

SELECT c.customer_id, c.name, COALESCE(SUM(o.quantity), 0) AS total_orders FROM customers c 

LEFT JOIN orders o ON c.customer_id = o.customer_id 

GROUP BY c.customer_id; 

In this example, the COALESCE() function is used with a subquery to show the total number of orders placed by each customer. If a customer has not placed any orders, the COALESCE() function returns a value of 0. 

COALESCE() function in WHERE clause 

When used in a WHERE clause, the COALESCE() function can be used to search for a specific value across multiple columns or tables. If a specific column or table does not have the desired value, the function can move to the next column or table in the list until it finds a non-null value. 

For example, let’s say we have a table called “Customers” that contains the following columns: “customer_id”, “first_name”, “last_name”, and “email”. If we want to search for all customers whose email address is either “xyz@email.com” or “abc@email.com”, we could use the COALESCE() function in the WHERE clause like this:

SELECT * FROM Customers WHERE COALESCE(email, ”) IN (‘xyz@email.com’, ‘abc@email.com’); 

In this example, the COALESCE() function is used to check if the “email” column is equal to either “xyz@email.com” or “abc@email.com” If the “email” column is null, it will return an empty string (”), and then check if that empty string is equal to either email address. This allows us to search for the desired values across multiple columns and handle null values more efficiently. 

Conclusion 

The COALESCE() function is an essential tool in SQL for handling NULL values in queries. It helps to avoid issues that can arise when dealing with NULL values.