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UNION

Is used to combine the result-set two or more SELECT statements

Every SELECT statement within UNION must have same number of columns

The columns must also have similar data types

The columns in every SELECT statement must be also be in the same order

The column names in the result-set are usually equal to the column names in the first SELECT statement

SELECT column_name(s) FROM table1
UNION
SELECT column_name(s) FROM table2;
-- UNION selects only distinct values by default. To allow duplicate values, use ALL
SELECT column_name(s) FROM table1
UNION ALL
SELECT column_name(s) FROM table2;

GROUP BY

Groups rows that have the same values into summary rows

Is often used with aggregate functions(like COUNT(), AVG(),..) to group the result-set by one or more columns

SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
ORDER BY column_name(s);

HAVING

Was added to SQL because the WHERE keyword cannot be used with aggregate functions

SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY conlumn_name(s)
HAVING condition
ORDER BY column_name(s);