SQL Session for Beginners:
The third part of SQL Series where we will cover following topics:
Under SQL Basics we will see - Union, Union All, Group By, Having, & Exist
The UNION operator is used to combine the result-set of two or more SELECT statements. The UNION operator selects only distinct values by default. To allow duplicate values, use UNION ALL.
The GROUP BY statement groups rows that have the same values into summary rows, like "find the number of customers in each country".
The HAVING clause was added to SQL because the WHERE keyword could not be used with aggregate functions.
The EXISTS operator is used to test for the existence of any record in a subquery.
