--1
SELECT
    es.FIRST_NAME || ' ' || es.LAST_NAME
FROM
    "HR".EMPLOYEES es
WHERE
    LENGTH(es.FIRST_NAME) < 5 OR
    LENGTH(es.LAST_NAME) < 5
ORDER BY
    LENGTH(es.FIRST_NAME) + LENGTH(es.LAST_NAME),
    LENGTH(es.LAST_NAME),
    es.LAST_NAME,
    es.FIRST_NAME

--2
SELECT
    es.DEPARTMENT_ID, COUNT(es.EMPLOYEE_ID)
FROM
    "HR".EMPLOYEES es
GROUP BY
    es.DEPARTMENT_ID
HAVING
    COUNT(es.EMPLOYEE_ID) <= 3

--3
SELECT
    es.FIRST_NAME || ' ' || es.LAST_NAME, es.DEPARTMENT_ID, es2.DEPARTMENT_ID
FROM
    "HR".EMPLOYEES es
LEFT JOIN
    "HR".EMPLOYEES es2
    ON
        es.MANAGER_ID = es2.EMPLOYEE_ID AND
        es.DEPARTMENT_ID <> es2.DEPARTMENT_ID

--4
SELECT
    es.FIRST_NAME || ' ' || es.LAST_NAME
FROM
    "HR".EMPLOYEES es
WHERE es.EMPLOYEE_ID IN (
    SELECT DISTINCT
        es_inn.MANAGER_ID
    FROM
        "HR".EMPLOYEES es_inn
)
UNION
SELECT --it doesn't work!
    es.FIRST_NAME || ' ' || es.LAST_NAME
FROM
    "HR".EMPLOYEES es
WHERE es.EMPLOYEE_ID NOT IN (
    SELECT DISTINCT
        es_inn.MANAGER_ID
    FROM
        "HR".EMPLOYEES es_inn
)

--5
SELECT
    *
FROM
    "HR".LOCATIONS l
WHERE
    REGEXP_LIKE(l.STREET_ADDRESS, '[^0-9][0-9]{4}|^[0-9]{4}[^0-9]')

--6
SELECT DISTINCT
    REGEXP_SUBSTR(l.STREET_ADDRESS, '\S+-\S+')
FROM
    "HR".LOCATIONS l
WHERE
    REGEXP_LIKE(l.STREET_ADDRESS, '-')