What does COALESCE do in SQL

In SQL, the `COALESCE` function is used to return the first non-null expression in a list of expressions. It is often used to handle situations where you have multiple expressions or columns, and you want to retrieve the first non-null value from them. 

The basic syntax of the `COALESCE` function is as follows:

COALESCE(expression1, expression2, expression3, ...)

- `expression1`, `expression2`, `expression3`, etc.: These are the expressions or values that you want to evaluate in order. The function returns the first non-null expression in the list.

Here's an example to illustrate how #COALESCE works:

Suppose you have a table called `employee` with two columns, `first_name` and `middle_name`, and you want to retrieve the full name of each employee. However, some employees may not have a middle name, so you want to use their first name as their full name in that case. You can use `COALESCE` like this:

SELECT COALESCE(first_name || ' ' || middle_name, first_name) AS full_name
FROM employee;

In this example, `COALESCE` evaluates the concatenation of `first_name` and `middle_name`. If `middle_name` is not null, it returns the full name by concatenating both names. If `middle_name` is null, it simply returns `first_name` as the full name.

So, in summary, `COALESCE` is a useful SQL function for handling null values by returning the first non-null expression in a list of expressions.