Explain the difference between UNION and UNION ALL.

Explanation:

When you're working with SQL to manipulate and retrieve data, you'll often need to combine results from multiple queries. This is where UNION and UNION ALL come in. Both are used to combine the results of two or more SELECT statements. However, they handle duplicates differently:

1. UNION: Combines the results of two queries and removes any duplicate rows.
2. UNION ALL: Combines the results of two queries and includes all duplicate rows.

Key Talking Points:

• UNION removes duplicate entries from the result set.
• UNION ALL retains all duplicates in the result set.
• Both require the number and order of columns to be the same in the queries being combined.
• UNION may have slightly higher computational cost due to duplicate elimination.

NOTES:

Reference Table:

Pseudocode:

-- Example using UNION
SELECT column1, column2 FROM table1
UNION
SELECT column1, column2 FROM table2;

-- Example using UNION ALL
SELECT column1, column2 FROM table1
UNION ALL
SELECT column1, column2 FROM table2;

Follow-Up Questions and Answers:

1. Why might you use UNION instead of UNION ALL?

   Answer: You would use UNION when you need to ensure that there are no duplicate records in your result set. This is useful when you want a distinct list of entries across multiple datasets.

2. What are some performance considerations when choosing between UNION and UNION ALL?

   Answer: UNION generally has a higher computational cost because it checks and removes duplicates, which involves sorting and comparison operations. UNION ALL is faster as it directly appends the results of the queries without additional processing for duplicates.

3. Can UNION and UNION ALL be used with ORDER BY?

   Answer: Yes, ORDER BY can be used with both UNION and UNION ALL, but it should be applied to the final result set. You cannot use ORDER BY within each individual SELECT statement unless it's a subquery.