PostgreSQL COALESCE Guide

In PgSQL, the COALESCE function retrieves the first non-NULL value from a list of parameters. If all parameters are NULL, then COALESCE will return NULL. This function is useful for handling NULL values and ensuring a non-NULL value is returned for further processing in a query.

For example, suppose there is a table called “employees” with columns “id”, “name”, and “salary”. If you want to retrieve the salary of each employee, but some employees have empty salary fields, you can use the COALESCE function to replace NULL values.

Choose the id and name of employees, with their salary displayed as 0 if it is null.

In the above query, the COALESCE function will return the value of the “salary” field. If the value is NULL, it will return 0. This ensures that the query will return a non-NULL value even if some employees have an empty salary field.

In conclusion, the purpose of the COALESCE function in PgSQL is to return the first non-NULL value, making it very useful when dealing with queries that may contain NULL values.