i have this simple mysql code:
SELECT count(avs_id) as gesamtNat FROM sv_erfassung.erfasst WHERE DATUM between '2016-05-01' AND '2016-05-02' and rechtsform = 'natürliche Person';
SELECT count(avs_id) as gesamtNat FROM sv_erfassung.erfasst WHERE DATUM between '2016-04-29' AND '2016-05-02' and rechtsform = 'natürliche Person';
You didn't associate time with date try this,
SELECT count(avs_id) as gesamtNat FROM sv_erfassung.erfasst WHERE DATUM between '2016-05-01' AND '2016-05-02 23:59:59' and rechtsform = 'natürliche Person';