Union, intersection, difference in DBMS Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the UNION operation in set theory or databases?

UNION combines all unique elements from two sets or tables, removing duplicates to form a single set.

beginner

Define INTERSECTION in the context of databases.

INTERSECTION returns only the elements that appear in both sets or tables.

beginner

What does the DIFFERENCE operation do?

DIFFERENCE returns elements that are in the first set but not in the second set.

intermediate

How does UNION handle duplicate records in database tables?

UNION removes duplicate records, so each record appears only once in the result.

intermediate

Can INTERSECTION be used to find common customers in two sales tables?

Yes, INTERSECTION returns customers present in both sales tables, showing common entries.

Which operation returns all unique records from two tables combined?

A INTERSECTION

B UNION

C DIFFERENCE

D JOIN

UNION combines all unique records from both tables, removing duplicates.

What does INTERSECTION return when applied to two sets?

A Elements only in the first set

B Elements only in the second set

C Elements common to both sets

D All elements from both sets

INTERSECTION returns only the elements that appear in both sets.

If you want to find records in Table A but not in Table B, which operation do you use?

A UNION

B INTERSECTION

C CROSS JOIN

D DIFFERENCE

DIFFERENCE returns records in the first set (Table A) that are not in the second (Table B).

Does UNION include duplicate records in the result?

A No, duplicates are removed

B Yes, all duplicates are included

C Only duplicates from the first set are included

D Only duplicates from the second set are included

UNION removes duplicates so each record appears only once.

Which operation would you use to find customers who bought products from both Store A and Store B?

A INTERSECTION

B UNION

C DIFFERENCE

D FULL OUTER JOIN

INTERSECTION finds common customers present in both Store A and Store B.

Explain the difference between UNION, INTERSECTION, and DIFFERENCE operations with examples.

Think about combining lists, finding common friends, or removing items.

How can UNION, INTERSECTION, and DIFFERENCE be useful in managing data in databases?

Consider scenarios like combining customer lists or comparing inventories.

Practice

1. Which of the following best describes the union operation in databases?

easy

A. Finds only the common records between two tables

B. Combines all unique records from two tables

C. Returns records from the first table that are not in the second

D. Deletes duplicate records from a table

Solution

1. Step 1: Understand the union operation

   The union operation combines all unique records from two sets or tables, removing duplicates.

2. Step 2: Compare with other operations

   Intersection finds common records, difference finds records in one set but not the other, and deletion removes records.

3. Final Answer:

   Combines all unique records from two tables -> Option B

4. Quick Check:

   Union = Combine unique records [OK]

Hint: Union = all unique items combined [OK]

Common Mistakes:

• Confusing union with intersection
• Thinking union includes duplicates
• Mixing difference with union

2. Which SQL keyword is used to find common records between two tables?

easy

A. UNION

B. DIFFERENCE

C. JOIN

D. INTERSECT

Solution

1. Step 1: Identify the keyword for common records

   The SQL keyword INTERSECT returns only the records common to both tables.

2. Step 2: Differentiate from other keywords

   UNION combines unique records, DIFFERENCE is not a standard SQL keyword, and JOIN combines rows based on a condition.

3. Final Answer:

   INTERSECT -> Option D

4. Quick Check:

   Common records = INTERSECT [OK]

Hint: Common records use INTERSECT keyword [OK]

Common Mistakes:

• Using UNION instead of INTERSECT
• Confusing JOIN with INTERSECT
• Assuming DIFFERENCE is a valid SQL keyword

3. Given two sets A = {1, 2, 3, 4} and B = {3, 4, 5, 6}, what is the result of A INTERSECT B?

medium

A. {3, 4}

B. {1, 2, 3, 4, 5, 6}

C. {1, 2}

D. {5, 6}

Solution

1. Step 1: Identify common elements in A and B

   The common elements between A and B are 3 and 4.

2. Step 2: Understand INTERSECT operation

   INTERSECT returns only the common elements, so the result is {3, 4}.

3. Final Answer:

   {3, 4} -> Option A

4. Quick Check:

   Intersection = common elements [OK]

Hint: Intersection = only common items [OK]

Common Mistakes:

• Listing all elements instead of common ones
• Confusing intersection with union
• Including elements only in one set

4. Consider the SQL query:
SELECT * FROM Table1
EXCEPT
SELECT * FROM Table2;
What is the purpose of the EXCEPT keyword?

medium

A. Returns records in Table1 but not in Table2

B. Returns records in Table2 but not in Table1

C. Returns records common to both tables

D. Returns all records from both tables

Solution

1. Step 1: Understand the EXCEPT keyword

   EXCEPT returns records from the first query that are not present in the second query.

2. Step 2: Apply to given tables

   So, it returns records in Table1 but not in Table2.

3. Final Answer:

   Returns records in Table1 but not in Table2 -> Option A

4. Quick Check:

   Difference = records in first not in second [OK]

Hint: EXCEPT = first table minus second table [OK]

Common Mistakes:

• Confusing EXCEPT with UNION
• Thinking EXCEPT returns common records
• Mixing order of tables in EXCEPT

5. You have two tables:
Employees_A with IDs {101, 102, 103, 104} and Employees_B with IDs {103, 104, 105, 106}.
You want to find IDs that are in either table but not in both. Which SQL operation gives this result?

hard

A. FULL OUTER JOIN with WHERE condition

B. INTERSECT

C. SYMMETRIC DIFFERENCE using UNION and EXCEPT

D. UNION

Solution

1. Step 1: Understand the requirement

   We want IDs in either table but not in both, which is called symmetric difference.

2. Step 2: Identify SQL approach

   SQL does not have a direct symmetric difference keyword, but it can be done by combining UNION and EXCEPT operations.

3. Step 3: Explain why other options are incorrect

   UNION returns all unique IDs including common ones, INTERSECT returns only common IDs, FULL OUTER JOIN with WHERE can be complex but not direct symmetric difference.

4. Final Answer:

   SYMMETRIC DIFFERENCE using UNION and EXCEPT -> Option C

5. Quick Check:

   Symmetric difference = union minus intersection [OK]

Hint: Symmetric difference = union minus intersection [OK]

Common Mistakes:

• Using UNION alone includes common IDs
• Using INTERSECT returns only common IDs
• Assuming FULL OUTER JOIN alone solves it