Data Retrieval Test Let's test how much you have learned today! 1. What is the primary purpose of the UNION clause in SQL when it comes to data retrieval, and how does it differ from UNION ALL? Answer: The primary purpose of the UNION clause in SQL is to combine the results of two or more SELECT statements, removing duplicate rows to create a single result set. It ensures that the final list contains unique values. UNION ALL, on the other hand, combines results from multiple SELECT queries but does not remove duplicates, showing all values, even if they are repeated. 2. How does the INNER JOIN differ from a LEFT JOIN in SQL, and in what scenarios might you choose one over the other for data retrieval? Answer: The INNER JOIN retrieves rows from two tables where there is a match based on a specific condition, creating a result set by merging rows where the values in the common field are identical. A LEFT JOIN retrieves all rows from the left table and pairs them with matching rows from the right table. The key difference is that an INNER JOIN only returns matching rows, while a LEFT JOIN returns all rows from the left table and includes null values for unmatched rows on the right. You might choose an INNER JOIN when you want only matching data from both tables and a LEFT JOIN when you want all rows from the left table, regardless of matches. 3. What is the significance of the DISTINCT keyword in SQL, and when would you use it during data retrieval? Answer: The DISTINCT keyword in SQL is used to remove duplicate rows from the result set. It ensures that the final output contains only unique values, eliminating any repetition. You would use DISTINCT when you want to retrieve a list of distinct, non-repeating values from a column or set of columns. 4. When using a FULL JOIN in SQL, what happens when there's no match for a particular row from either of the joined tables, and why might you choose a FULL JOIN for data retrieval? Answer: When using a FULL JOIN in SQL, if there is no match for a particular row from either of the joined tables, that row will still appear in the result-set with NULL values for the columns from the unmatched table. A FULL JOIN combines the outcomes of both a LEFT JOIN and a RIGHT JOIN, including all rows from both tables. You might choose a FULL JOIN when you want to see the full picture of data across tables, including both matched and unmatched rows. It's useful when you need to analyze relationships between data comprehensively.