COALESCE SQL Function

COALESCE() is one of the handiest functions in SQL. Read this tutorial to learn how to master it.

What is the `COALESCE()`function?

Coalesce returns the first non-null value in a list. If all the values in the list are NULL, then the function returns null.

When to use `COALESCE()`?

This function is useful when combining the values from several columns into one.

For example, a table called users contains values of users' work_email and personal_email.

Using the COALESCE() function, we can create a column called email, which shows the user's work_email if it is not null. Otherwise, it shows personal_email.

`user_id`	`work_email`	`personal_email`	`COALESCE(work_email, personal_email)`
1	[email protected]	null	[email protected]
2	null	[email protected]	[email protected]
3	[email protected]	[email protected]	[email protected]

`COALESCE()` syntax

COALESCE(value_1, value_2, ...., value_n)

The COALESCE() function takes in at least one value (value_1). It will return the first value in the list that is non-null.

For example, it will first check if value_1 is null. If not, then it returns value_1. Otherwise, it checks if value_2 is null. The process goes on until the list is complete.

`COALESCE()` examples

Example 1: `COALESCE()` a column with a constant

Consider the table countries with a list of countries and their national days. Some national days are empty. For rows with empty national days, we can fill it in with the value 'Unknown'.

The query and the result are as follows.

DataFrameas

df

variable

SELECT
   country_id,
   name, 
   national_day,
   COALESCE(national_day, 'Unknown') AS national_day_coalesced
FROM countries
ORDER BY country_id

Notice how the null value in national_day is replaced by a constant Unknown.

Example 2: `COALESCE()` two columns

We have a table named products. It contains the product name and its description. Some descriptions are too long (more than 60 characters). In that case, we replace the description with the product name.

The query and the result are as follows.

DataFrameas

df

variable

SELECT DISTINCT
   product_name,
   description,
   COALESCE(
      CASE WHEN 
         LENGTH(description) >= 60 
         THEN NULL 
         ELSE description 
         END, 
      product_name) product_name_or_description
FROM products

Notice how the column product_name_or_description displays the product_name if the description is long. Otherwise, it displays the description.

Example 3: `COALESCE()` three columns or more

We can take example 2 one step further. Assume that there are currently two requirements.

If the length of the description is less than 60, then display the description.
Otherwise, check if the length of the product_name is less than 20. If so, we display the product_name
Otherwise, display product

DataFrameas

df

variable

SELECT DISTINCT
   product_name,
   description,
   COALESCE(
      CASE 
         WHEN LENGTH(description) > 50
         THEN NULL
         ELSE description
      END,
   CASE 
      WHEN LENGTH(product_name) > 14
      THEN NULL
      ELSE product_name
   END,
'product') AS product_name_or_description
FROM products
ORDER BY product_name

Notice how the column product_name_or_description displays either the product_name or the description depending on the lengths of the product_name or description.

Technical requirements

COALESCE() works in SQL Server (starting with 2008), Azure SQL Database, Azure SQL Data Warehouse, Parallel Data Warehouse, BigQuery, and Amazon RedShift.