How would you distinguish between left join, union, and right join in the context of SQL queries?

Question Analysis

This question is asking you to differentiate between three types of SQL operations: LEFT JOIN, UNION, and RIGHT JOIN. These are used to combine or relate data from multiple tables, but they serve different purposes and are used in different contexts. Understanding these differences is crucial for performing the correct data manipulation when working with databases.

Answer

In SQL, LEFT JOIN, UNION, and RIGHT JOIN are used to combine data from two or more tables, but they achieve this in distinct ways:

• LEFT JOIN:

  • Purpose: Returns all records from the left table and the matched records from the right table. If there is no match, the result is NULL on the side of the right table.
  • Usage Example: Useful when you want all records from one table regardless of whether there is a matching record in another table.
  • Syntax:

    SELECT columns
    FROM table1
    LEFT JOIN table2 ON table1.common_field = table2.common_field;
    

• UNION:

  • Purpose: Combines the result sets of two or more SELECT queries. It removes duplicate rows between the various SELECT statements.
  • Usage Example: Useful when you want to merge data from different queries that have a similar structure.
  • Syntax:

    SELECT column1, column2 FROM table1
    UNION
    SELECT column1, column2 FROM table2;
    

  • Note: All SELECT statements within the UNION must have the same number of columns in the result sets with similar data types.

• RIGHT JOIN:

  • Purpose: Returns all records from the right table and the matched records from the left table. If there is no match, the result is NULL on the side of the left table.
  • Usage Example: Useful when you want all records from the right table regardless of whether there is a matching record in another table.
  • Syntax:

    SELECT columns
    FROM table1
    RIGHT JOIN table2 ON table1.common_field = table2.common_field;
    

Understanding these operations helps you to manipulate and retrieve data efficiently, ensuring that you get the desired results from your database queries.