n PostgreSQL, the term “coalesce” refers to combining multiple data rows into a single row. This is typically done using the COALESCE function, which takes a list of values and returns the first non-null value in the list.
For example, suppose you have a table with the following data:
id | first_name | last_name | email
------------------------------------
1 | John | Doe | johndoe@example.com
2 | NULL | Smith | NULL
3 | Jane | Doe | janedoe@example.com
If you want to combine the first and last names into a single column, you can use the COALESCE function like this:
SELECT COALESCE(first_name, '') || ' ' || COALESCE(last_name, '') AS full_name, email
FROM users;
This will return the following results:
full_name | email
------------------------------
John Doe | johndoe@example.com
Smith | NULL
Jane Doe | janedoe@example.com
In this example, the COALESCE function is used to combine the first_name and last_name columns into a single full_name column. This is done by using the COALESCE function to return the first non-null value in each column, and then concatenating the results using the || operator. This allows you to combine multiple columns into a single value, even if some of the values in the columns are NULL.
The COALESCE function is a useful tool in PostgreSQL for combining multiple values into a single value. It can be used in a variety of situations, such as creating a default value for a column, combining multiple columns into a single column, or creating a custom error message for a NULL value.
