--PRACTICE PART:
--OLYMPICS DATABASE 

--1.Return the ID and the number of medals of each country 
--  which gained exactly the same number of gold, silver and bronze medals.

--2.Return the ID and the number of medals of each country which number of gold medals is between 5 and 9.

--3.Return each country which name ends with 'istan'.

--4.Return each country which name starts with 'A' and at least 8 letters long.

--5.Return each country which area starts with '3'.

--6.Return the ID and the number of medals of each country. 
--  Sort the countries by the number of gold medals in descending order.

--7.Return all the data of each event which name is missing.

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--DATE FUNCTIONS-----------------------------------------------
---------------------------------------------------------------
--TO_DATE()

--TO_CHAR()

--EXTRACT()

--MONTHS_BETWEEN()

--ADD_MONTHS()

--NEXT_DAY()

--LAST_DAY()

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--DISTINCT

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--ALIASES

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--NVL()
-- The NVL() function allows you to replace null
-- with a more meaningful alternative in the results of a query.

--1. Return the name, the population and the continent of country Cook Islands.

--2. Return the name, the population and the continent of country Cook Islands. 
--Replace the missing continent name with string 'unknown'.

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--LOWER(), UPPER(), INITCAP()
-- lower()  - Convert the text to lower-case
-- upper()  - Convert the text to upper-case

---------------------------------------------------------------
--SUBSTR()

--syntax:
--substr(column_name, start_position, number_of_char.)

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--REPLACE()
-- The REPLACE() function replaces all occurrences 
-- of a substring within a string, with a new substring.
-- Syntax:
-- REPLACE(string, old_string, new_string)

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--INSTR()
--syntax: instr(string, substring)

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-- GROUP BY
-- The SQL GROUP BY Statement
-- The GROUP BY statement groups rows that have the same values into 
-- summary rows, like "find the number of customers in each country".

-- The GROUP BY statement is often used with 
-- aggregate functions (COUNT(), MAX(), MIN(), SUM(), AVG()) to group 
-- the result-set by one or more columns.

-- GROUP BY Syntax
--	SELECT column_name(s)
--	FROM table_name
--	[WHERE condition]
--	[GROUP BY column_name(s)]
--	[ORDER BY column_name(s)];

---------------------------------------------------------------
-- HAVING

-- The SQL HAVING Clause
-- The HAVING clause was added to SQL because the WHERE keyword 
-- cannot be used with aggregate --functions.


-- HAVING Syntax
--	SELECT column_name(s)
--	FROM table_name
--	[WHERE condition]
--	[GROUP BY column_name(s)]
--	[HAVING condition]
--	[ORDER BY column_name(s)];

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