diff --git a/advlabdb/models.py b/advlabdb/models.py
index 16c44ee..b4f5716 100644
--- a/advlabdb/models.py
+++ b/advlabdb/models.py
@@ -58,8 +58,7 @@ class PartStudent(db.Model):
     id = db.Column(db.Integer, primary_key=True)
     final_part_mark = db.Column(
         db.Integer,
-        db.CheckConstraint(f"final_part_mark >= {MIN_MARK}"),
-        db.CheckConstraint(f"final_part_mark <= {MAX_MARK}"),
+        db.CheckConstraint(f"final_part_mark BETWEEN {MIN_MARK} AND {MAX_MARK}"),
         nullable=True,
     )
 
@@ -274,22 +273,19 @@ class SemesterExperiment(db.Model):
     id = db.Column(db.Integer, primary_key=True)
     oral_weighting = db.Column(
         db.Float,
-        db.CheckConstraint("oral_weighting >= 0"),
-        db.CheckConstraint("oral_weighting <= 1"),
+        db.CheckConstraint("oral_weighting BETWEEN 0 AND 1"),
         default=0.5,
         nullable=False,
     )
     protocol_weighting = db.Column(
         db.Float,
-        db.CheckConstraint("protocol_weighting >= 0"),
-        db.CheckConstraint("protocol_weighting <= 1"),
+        db.CheckConstraint("protocol_weighting BETWEEN 0 AND 1"),
         default=0.5,
         nullable=False,
     )
     final_weighting = db.Column(
         db.Float,
-        db.CheckConstraint("final_weighting >= 0"),
-        db.CheckConstraint("final_weighting <= 1"),
+        db.CheckConstraint("final_weighting BETWEEN 0 AND 1"),
         default=1.0,
         nullable=False,
     )
@@ -436,10 +432,8 @@ class Part(db.Model):
 
 class Semester(db.Model):
     id = db.Column(db.Integer, primary_key=True)
-    label = db.Column(db.String(10), nullable=False)
-    year = db.Column(
-        db.Integer, db.CheckConstraint(f"year >= {MIN_YEAR}"), db.CheckConstraint(f"year <= {MAX_YEAR}"), nullable=False
-    )
+    label = db.Column(db.String(10), db.CheckConstraint("label IN ('WS', 'SS')"), nullable=False)
+    year = db.Column(db.Integer, db.CheckConstraint(f"year BETWEEN {MIN_YEAR} AND {MAX_YEAR}"), nullable=False)
 
     parts = db.relationship("Part", backref="semester", lazy=True)
     semester_experiments = db.relationship("SemesterExperiment", backref="semester", lazy=True)
@@ -448,14 +442,8 @@ class Semester(db.Model):
 
     __table_args__ = (db.UniqueConstraint(label, year),)
 
-    def customInit(label, year):
-        if label not in ("WS", "SS"):
-            raise DataBaseException("The semester label has to be WS or SS!")
-
-        return Semester(label=label, year=year)
-
     def customInitFromOldSemester(label, year, oldSemester, transferParts, transferAssistants):
-        semester = Semester.customInit(label=label, year=year)
+        semester = Semester(label=label, year=year)
 
         if transferParts:
             semester.transferPartsFrom(oldSemester)
@@ -513,21 +501,18 @@ class ExperimentMark(db.Model):
 
     oral_mark = db.Column(
         db.Integer,
-        db.CheckConstraint(f"oral_mark >= {MIN_MARK}"),
-        db.CheckConstraint(f"oral_mark <= {MAX_MARK}"),
+        db.CheckConstraint(f"oral_mark BETWEEN {MIN_MARK} AND {MAX_MARK}"),
         nullable=True,
     )
     protocol_mark = db.Column(
         db.Integer,
-        db.CheckConstraint(f"oral_mark >= {MIN_MARK}"),
-        db.CheckConstraint(f"oral_mark <= {MAX_MARK}"),
+        db.CheckConstraint(f"oral_mark BETWEEN {MIN_MARK} AND {MAX_MARK}"),
         nullable=True,
     )
 
     final_experiment_mark = db.Column(
         db.Integer,
-        db.CheckConstraint(f"oral_mark >= {MIN_MARK}"),
-        db.CheckConstraint(f"oral_mark <= {MAX_MARK}"),
+        db.CheckConstraint(f"oral_mark BETWEEN {MIN_MARK} AND {MAX_MARK}"),
         onupdate=final_experiment_mark_update,
         nullable=True,
     )