SELECT ... FROM training_plan_anwesenheit LEFT JOIN training_plan_kopf ... WHERE training_plan_anwesenheit.tplan_anwesenheit_saison_id = 2018/2019'If you replace the literal '2018/2019' with the column name training_plan_kopf.saison_id you are basically saying "use whatever value you find in column training_plan_kopf.saison_id", and as there is nothing in that query that limits that value to '2018/2019' it will use whatever values it finds.]]>
tplan_anwesenheit_saison_id = '2018/2019'