initial commit

lab10.sql

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+SET search_path TO lab5;
+
+-- TASK 1 – Enforce Section Capacity
+
+-- 1. Trigger function: ensure no course offering exceeds capacity
+CREATE OR REPLACE FUNCTION check_offering_capacity()
+RETURNS TRIGGER AS $$
+DECLARE
+    v_capacity INTEGER;
+    v_current  INTEGER;
+BEGIN
+    SELECT off_capacity
+    INTO v_capacity
+    FROM course_offering
+    WHERE off_id = NEW.off_id;
+
+    SELECT COUNT(*)
+    INTO v_current
+    FROM enrollment
+    WHERE off_id = NEW.off_id
+      AND enr_status = 'Active';
+
+    IF v_current >= v_capacity THEN
+        RAISE EXCEPTION
+            'Cannot enroll student %. Section % is full (capacity=%).',
+            NEW.stu_num, NEW.off_id, v_capacity;
+    END IF;
+
+    RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+
+-- 2. Trigger
+CREATE OR REPLACE TRIGGER trg_check_capacity
+BEFORE INSERT ON enrollment
+FOR EACH ROW
+EXECUTE FUNCTION check_offering_capacity();
+
+-- TASK 2 – Enrollment Audit Logging
+
+CREATE TABLE IF NOT EXISTS enrollment_audit (
+    audit_id      SERIAL PRIMARY KEY,
+    enr_id_stu    VARCHAR(20),
+    enr_id_off    INTEGER,
+    action_type   VARCHAR(10),
+    old_status    VARCHAR(8),
+    new_status    VARCHAR(8),
+    action_ts     TIMESTAMP DEFAULT NOW(),
+    action_user   TEXT DEFAULT current_user
+);
+
+CREATE OR REPLACE FUNCTION audit_enrollment_changes()
+RETURNS TRIGGER AS $$
+BEGIN
+    IF TG_OP = 'INSERT' THEN
+        INSERT INTO enrollment_audit(
+            enr_id_stu,
+            enr_id_off,
+            action_type,
+            new_status
+        )
+        VALUES (
+            NEW.stu_num,
+            NEW.off_id,
+            'INSERT',
+            NEW.enr_status
+        );
+
+    ELSIF TG_OP = 'UPDATE' THEN
+        INSERT INTO enrollment_audit(
+            enr_id_stu,
+            enr_id_off,
+            action_type,
+            old_status,
+            new_status
+        )
+        VALUES (
+            NEW.stu_num,
+            NEW.off_id,
+            'UPDATE',
+            OLD.enr_status,
+            NEW.enr_status
+        );
+    END IF;
+
+    RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE OR REPLACE TRIGGER trg_enrollment_audit
+AFTER INSERT OR UPDATE ON enrollment
+FOR EACH ROW
+EXECUTE FUNCTION audit_enrollment_changes();
+
+-- TASK 3 – Validate Enrollment Status
+
+ALTER TABLE enrollment DROP CONSTRAINT IF EXISTS enr_status_ck;
+
+ALTER TABLE enrollment
+ADD CONSTRAINT enr_status_ck
+CHECK (enr_status IN ('Active', 'Dropped', 'Completed', 'Failed'));
+
+CREATE OR REPLACE FUNCTION validate_enrollment_status()
+RETURNS TRIGGER AS $$
+BEGIN
+    IF NEW.enr_status NOT IN ('Active','Dropped','Completed','Failed') THEN
+        RAISE EXCEPTION 'Invalid enrollment status: %', NEW.enr_status;
+    END IF;
+
+    RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE OR REPLACE TRIGGER trg_validate_status
+BEFORE INSERT OR UPDATE OF enr_status ON enrollment
+FOR EACH ROW
+EXECUTE FUNCTION validate_enrollment_status();
+
+-- TESTS
+
+INSERT INTO enrollment VALUES ('S100', 1, 'Active');
+INSERT INTO enrollment VALUES ('S101', 1, 'Active');
+INSERT INTO enrollment VALUES ('S102', 1, 'Active');
+
+INSERT INTO enrollment VALUES ('S200', 2, 'Active');
+UPDATE enrollment SET enr_status = 'Dropped'
+WHERE stu_num = 'S200' AND off_id = 2;
+
+UPDATE enrollment
+SET enr_status = 'Potato'
+WHERE stu_num = 'S200' AND off_id = 2;
+
+-- OUTPUT
+
+-- SELECT * FROM enrollment;
+
+
+-- SELECT * FROM enrollment_audit;
+
+
+-- REFLECTION
+-- Storing business rules directly inside the database ensures that
+-- data remains correct and consistent no matter what application
+-- or user interacts with it. Trigger-based validation centralizes
+-- logic so errors are prevented at the source instead of being caught
+-- later in the application.