Added age_calc function (P7).

Practical/Practical Exam/PL-SQL/P7 - Age_calc.md

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+# P7 - Age_calc function
+
+## Creating function
+
+```sql
+CREATE OR REPLACE FUNCTION Age_calc (p_dob IN DATE)
+RETURN VARCHAR IS
+-- declare
+  ageOP VARCHAR2(255);
+  years NUMBER;
+  months NUMBER;
+  days NUMBER;
+
+BEGIN
+
+  years := EXTRACT(YEAR FROM SYSDATE) - EXTRACT(YEAR FROM p_dob);
+  months := EXTRACT(MONTH FROM SYSDATE) - EXTRACT(MONTH FROM p_dob);
+  days := EXTRACT(DAY FROM SYSDATE) - EXTRACT(DAY FROM p_dob);
+
+  IF (days < 1) THEN
+    months := months - 1;
+    days := days + EXTRACT(DAY FROM LAST_DAY(ADD_MONTHS(SYSDATE, -1)));
+  END IF;
+
+  IF (months < 1) THEN
+    years := years - 1;
+    months := months + 12;
+  END IF;
+
+  ageOP := 'Given person is ' || years || ' years ' || months || ' months ' || days || ' days old.';
+
+RETURN ageOP;
+END Age_calc;
+/
+
+```
+
+## Calling function
+
+```sql
+BEGIN
+  DBMS_OUTPUT.PUT_LINE(Age_calc(TO_DATE('2004-04-28', 'YYYY-MM-DD')));
+END;
+/
+
+```
+
+---