Let’s explore the COALESCE PostgreSQL function, taking a look at what it is, what it does, its pros and cons, and when to use it to handle NULL values in some real-world examples.
NULL values is a headache. Neglecting them can result in missing data in the frontend of your application. You could address them at the application level, but that would result in performance overhead and potential data inconsistencies. Why not replace them with placeholder values directly in the queries? Well, this is what
COALESCE is all about!
In this article, you will find out everything you need to know about
COALESCE, and see how it can help you build
What Is COALESCE in PostgreSQL?
COALESCE is a PostgreSQL function that accepts multiple arguments and returns the first non-
NULL value. It returns
NULL only if all elements in the argument list are
NULL. The function is particularly useful when you want to replace missing data with meaningful alternatives or need to perform conditional operations based on the presence of
NULL values. In other words, it is a versatile tool that allows you to properly handle
NULL values in your PostgreSQL queries.
How to Use COALESCE in PostgreSQL
The COALESCE syntax in PostgreSQL is pretty straightforward:
COALESCE(value [, ...])
The function takes a list of comma-separated arguments, with at least one element. PostgreSQL will evaluate them in order, from left to right, until it finds a non-
NULL value. This means that arguments to the right of the first non-
NULL argument are not even evaluated.
Just like any other PostgreSQL function, you can use it in any query clause, including
GROUP BY, and
Let’s see it in action in an example:
SELECT COALESCE(age, 'N/A') AS age
When a record has a
NULL value in the age column, the
COALESCE function will replace it with the string
'N/A' in the result set.
Pros and Cons of COALESCE
Time to dig into the benefits and disadvantages of using the PostgreSQL function.
PostgreSQL COALESCE: Use Cases
Let's explore a few real-world scenarios where the function can be applied effectively.
Handling Missing Values
Suppose you have a
customers table and some users did not provide their full names for privacy reasons. You can use the
COALESCE function to replace those
NULL values with default strings, ensuring that the query produces consistent results.
COALESCE(full_name, 'Anonymous User') AS full_name
In the frontend, you can display these placeholder information fields in place of missing data to improve the user experience.
Performing Math Operations on NULL Values
Consider a scenario where you have a
products table storing the price and discount percentage. However, some products may not have a discount specified. In this case, the
discount column on those records is
Assume you want to calculate the final price after applying the discount. This is the query you may end up writing:
price * (1 - discount) AS final_price
Let's run it:
As you can see, this query produces wrong results.
final_price should never be
NULL but it is every time
NULL. This occurs because multiplying a
NULL by a number produces a
NULL in PostgreSQL.
You can easily avoid that and achieve the desired goal by using
COALESCE as follows:
price * (1 - COALESCE(discount, 0)) AS final_price
Run the query again:
NULL it gets replaced by the default value 0, ensuring that the calculation proceeds smoothly.
Handling Multiple Missing Values
Consider an e-commerce platform that manages product inventory. Each product has multiple warehouses, and the stock availability is tracked separately for each warehouse. However, some warehouses may not have reported their stock levels.
To address that, you can use the query below:
COALESCE(warehouse1_stock, warehouse2_stock, warehouse3_stock) AS available_stock
id = 5;
COALESCE is used with three arguments, which are the stock quantities from three different warehouses.
Running the query against a real database would produce the following result set:
available_stock has the same value as
warehouse2_stock. This is because
warehouse1_stock is NULL. In detail, that query checks each warehouse's stock level and returns the first non-
NULL value encountered, providing the user with the desired info.
In this article, you learned that
COALESCE is a powerful tool for handling NULL values directly in the database. It gives you the ability to replace
NULL values with placeholder strings, making your queries less complex.
To better appreciate its capabilities, you need a tool that helps you manage databases and visually explore query results. This is where a full-featured database client like DbVisualizer can make all the difference! In addition to being able to connect to dozens of DBMSs, this tool offers advanced query optimization functionality, and full support for all PostgreSQL features, including
COALESCE. Download DbVisualizer for free now!
Let’s answer some questions related to the topic of the guide.
What is the difference between COALESCE and IFNULL in PostgreSQL?
NULLIF are two PostgreSQL functions to handle
NULL values. The key difference between them lies in their purpose. The first is used to return the first non-
NULL value from a list of arguments. On the other hand,
NULLIF compares two expressions: NULLIF(expression1, expression2). It returns
NULL if expression1 and expression2 are equal. Otherwise, it returns expression1. It is typically used to build conditional logic based on equality.
What are the traditional methods for handling NULL values in PostgreSQL?
Traditionally, developers tended to use techniques
IS NULL or
IS NOT NULL conditions to manage NULL values in PostgreSQL. However, these produce long queries that are hard to read and maintain. For this reason, you should prefer conditional statements like the
LEAST functions to deal with
Can the COALESCE function be used in combination with conditional logic, such as CASE?
Yes, the function can be used in combination with other conditional logic, such as
CASE. This allows for more complex handling of
NULL values based on specific conditions or criteria. By leveraging the flexibility of
COALESCE and combining it with conditional logic, you can achieve more sophisticated data transformations and replacements.
Is COALESCE efficient?
The efficiency of
COALESCE depends on the size of the dataset and the complexity of the query. In general, it performs well and has a minimal performance impact. However, when dealing with large datasets or is used with a lot of parameters, it can slow down your query.
How to use COALESCE with a timestamp or date in PostgreSQL?
COALESCE can be used with timestamps or dates in PostgreSQL to replace
NULL values with strings, provided that you convert the column of type
DATE to string with a cast. Here is an example of how to do it:
COALESCE(CAST(birthdate AS TEXT), 'Missing date') AS birthdate,
COALESCE(CAST(joined_at AS TEXT), 'Missing timestamp') AS joined_at