SQL> CREATE TABLE emp2 (
  2     id   NUMBER NOT NULL PRIMARY KEY,
  3     name VARCHAR2(30) NOT NULL,
  4     sex  CHAR(1) CHECK (sex in ('M','F'))
  5  )
  6  /

Table created.

SQL>
SQL> INSERT INTO emp2 VALUES (1, 'John', 'M');

1 row created.

SQL> INSERT INTO emp2 VALUES (2, 'Nancy', 'F');

1 row created.

SQL>
SQL> CREATE OR REPLACE FUNCTION isMale(p_id NUMBER) RETURN BOOLEAN AS
  2    v_sex emp2.sex%TYPE;
  3  BEGIN
  4     SELECT sex INTO v_sex FROM emp2 WHERE id = p_id;
  5     IF v_sex = 'M' THEN
  6        RETURN true;
  7     ELSE
  8        RETURN false;
  9     END IF;
 10  END;
 11  /

Function created.