INTRODUCTION

The UNION operator in MySQL is a powerful tool for combining the results of multiple SELECT statements into a single result set. It's particularly useful when you want to retrieve data from different tables or different parts of the same table that share a similar structure.

Key Points:

• Combines Results: UNION merges the output from two or more SELECT queries.
• Removes Duplicates (Default): By default, UNION eliminates duplicate rows from the combined result set. This ensures you only get unique data points.
• Matching Columns: The SELECT statements used with UNION must have the same number of columns in the same order, and the data types of corresponding columns must be compatible.
• Column Names: The column names in the final result set are inherited from the first SELECT statement in the UNION chain.
• The key difference between UNION ALL and UNION is that UNION ALL retains duplicate rows in its output, while UNION automatically removes them.

Syntax:

SELECT 
  column1, 
  column2, 
FROM 
    table_name 
UNION 
     SELECT 
          column1, 
          column2, 
       FROM 
          table_name 

• SELECT ...: This represents the individual SELECT statements that retrieve data.
• FROM table1, table2, table3: These are the tables from which data is fetched.
• UNION: This keyword combines the results from the preceding SELECT statements, removing duplicates (default behavior).