-- 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)];

---------------------------------------------------------------
--1. Return the number of events by genders.

--2. Return the number of events by names.

--3. Return the number of countries by continents.

--4. Return the number of countries by continents. Omit the NULL value from the output.

--5. Return the number of countries by continents. Return a continent only if it has more than 30 countries.

--6. Return the name of each continent, and the minimum, maximum and average of the area.

--7. Return a continent only if the average population is less than 500000.

--8. Return the name of each continent and the number of countries. Return a continent only if the average population is less than 500000.


--9. Return the number of events of each name and gender combination.


---------------------------
--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'.

--3. Return the name, the population and the continent of each country. Replace the missing continent names with string 'unknown'.

--4. Return the name, the population and the continent of each country. Replace the missing continent names with string 'unknown' and the missing population values with 0.


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

--examples


--CONCATENATION

--examples



--CONCAT()
--example


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

--example




-------------------------------------
--DUAL Table:
--The DUAL is special one row, one column table present by default in all Oracle databases. The owner of DUAL is SYS (SYS owns 
--the data dictionary, therefore DUAL is part of the data dictionary.) but DUAL can be accessed by every user. The table has a 
--single VARCHAR2(1) column called DUMMY that has a value of 'X'.
--There may be a situation where we want to query something that is not from a table. For example, getting the current date or 
--querying a simple arithmetic expression like 2+2.

Examples: 
select 2*3 
from dual;

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


--example


--------------------------------
--INSTR()

--example



-----------------------------------------
--Date functions
--SYSDATE

--example

SELECT SYSDATE
FROM dual;

--MONTHS_BETWEEN()

--example

--ADD_MONTHS()

--example


--NEXT_DAY()

--example


--LAST_DAY()

--example


--EXTRACT()
--example


----------------------------
-- TO_CHAR function converts DATETIME or DATE values to character string values.




-- TO_DATE function converts date strings in various formats to a date integer value, with data type DATE.