difference between join and union

 oins and unions are both used in SQL to combine data from multiple tables or queries, but they serve different purposes and have distinct behaviors:

1. Join:

   • Purpose: A join is used to combine rows from two or more tables based on a related column between them.
   • Syntax: There are different types of joins, such as INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN, and CROSS JOIN, each with its own syntax and behavior.
   • Example:

     sql
     SELECT column1, column2
     FROM table1
     INNER JOIN table2 ON table1.column = table2.column;
     

   • Behavior: Joins create a result set that includes columns from both tables based on the specified join condition. Rows are matched between tables based on the join condition, and only matching rows are included in the result set.

2. Union:

   • Purpose: A union is used to combine the results of two or more SELECT statements into a single result set.
   • Syntax: The syntax for union is straightforward. You can use UNION or UNION ALL to combine queries.
   • Example:

     sql
     SELECT column1 FROM table1
     UNION
     SELECT column2 FROM table2;
     

   • Behavior: Unions combine the results of multiple queries into one result set. Duplicate rows are removed by default in UNION, while UNION ALL retains all rows, including duplicates, from each query.

Key Differences:

1. Data Combination:

   • Join: Combines rows from tables based on a common column or condition.
   • Union: Combines the results of multiple SELECT statements into one result set.

2. Result Set:

   • Join: Creates a result set with columns from multiple tables based on the join condition.
   • Union: Creates a result set with columns from individual SELECT statements, combining rows vertically.

3. Duplicate Handling:

   • Join: Does not handle duplicates directly; all matching rows based on the join condition are included.
   • Union: By default, UNION removes duplicate rows from the combined result set, while UNION ALL retains all rows, including duplicates.

4. Syntax and Usage:

   • Join: Requires specifying join conditions and types (e.g., INNER JOIN, LEFT JOIN) to combine data horizontally.
   • Union: Involves using UNION or UNION ALL between SELECT statements to combine data vertically.

In summary, joins are used to combine related data horizontally from multiple tables, while unions combine data vertically from multiple queries or tables with compatible column structures. They serve different purposes and are used in different scenarios depending on the desired outcome.