SQL query for all grouped records in joined table

Let’s consider a toy example. There is a table employees and table tasks, where each task is assigned to one employee. One employee can have multiple tasks.

I want to query employees filtering them by some columns in their tasks. I group the employees to display each one only once. The query would be something like this:

SELECT * 
FROM employees emp JOIN tasks tsk on emp.id = tsk.assigned_emp
WHERE tsk.deadline = today
GROUP BY employees

This works fine, but now let’s say that I’d like to select the employees that don’t have any tasks which deadline is due today. My first try was:

tsk.deadline != today

but then is finds employees with at least one task that is not due today. How to write a query to include all possible tasks for a given employee?

Answer

You want the LEFT JOIN ... IS NULL pattern for this.

SELECT * 
  FROM employees emp 
  LEFT JOIN tasks tsk   ON emp.id = tsk.assigned_emp
                       AND tsk.deadline = today
 WHERE tsk.deadline IS NULL
 GROUP BY employees

JOIN removes rows from its first table that don’t match your ON condition. By contrast, LEFT JOIN leaves them in place putting NULL values into the result columns from the second table.