What is the purpose of the SQL COALESCE function?

The COALESCE function in SQL is used to return the value of the first non-null expression in the parameter list. If all parameters are null, it returns NULL.

The syntax of the COALESCE function is as follows:
COALESCE(expression1, expression2, expression3, …)

Expressions in the parameter list can be column names, constants, or functions. The COALESCE function assesses each expression in the order of the parameters, returning the value of the first non-null expression. If all expressions are null, it returns null.

The COALESCE function can be used to handle cases of null values, commonly used in scenarios such as:

1. Replace NULL value: When a field in the database could potentially be empty, you can use the COALESCE function to replace it with a default value. For example, COALESCE(column_name, ‘N/A’) means that if the field value is empty, it will return ‘N/A’.
2. Combine: You can use the COALESCE function to merge the values of multiple columns, returning the first non-null value. For example, COALESCE(column1, column2, column3) means returning the first non-null value from column1, column2, and column3.
3. Handling calculation logic: The COALESCE function can be used to handle complex calculation logic by returning a non-null value when one of the calculations results in null. For example, using COALESCE(expression1 / expression2, 0) ensures that if expression2 is 0, it will return 0 instead of producing a division by zero error.

In conclusion, the COALESCE function is used to return the value of the first non-null expression in the parameter list, and it can be used to handle empty values, replace NULL values, and deal with complex calculation logic.