In addition to the BranchPythonOperator, which lets us execute a Python function that returns the ids of the subsequent tasks that should run, we can also use a SQL query to choose a branch. Of course, we will not do it by querying the SQL database in the Python function. There is a shorter way.
We need to add a BranchSQLOperator to our DAG. This operator is a little bit different than the BranchPythonOperator. In the case of the Python operator, the function returns the ids of the tasks to run. The SQL version of the operator expects a boolean value in the first column of the first row. Optionally, it can also return a numeric. Every non-zero value is interpreted as True. Zero = False. We can also use one of the string values with a boolean equivalent (take a look at the documentation if you want to use a string value).
If the value is true, Airflow will execute the tasks specified in the follow_task_ids_if_true parameter. Otherwise, it runs the tasks defined in the follow_task_ids_if_false argument.
operator = BranchSQLOperator(
task_id="some_task_id",
conn_id="sql_connection_id",
sql="SELECT count(1) FROM a_table",
follow_task_ids_if_true="task_to_execute_if_count_at_least_one",
follow_task_ids_if_false="tasks_to_execute_if_counts_is_zero",
dag=dag
)
