Union, intersection, difference in DBMS Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the SQL query to find all unique records from two tables using union.

DBMS Theory

SELECT * FROM table1 [1] SELECT * FROM table2;

Drag options to blanks, or click blank then click option'

AJOIN

BUNION

CINTERSECT

DEXCEPT

Attempts:

3 left

💡 Hint

Common Mistakes

Using INTERSECT instead of UNION

Using JOIN which combines columns, not rows

The UNION operator combines results from two queries and removes duplicates.

2fill in blank

medium

Complete the SQL query to find common records between two tables.

DBMS Theory

SELECT * FROM table1 [1] SELECT * FROM table2;

Drag options to blanks, or click blank then click option'

AINTERSECT

BUNION

CEXCEPT

DJOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION which returns all unique rows

Using EXCEPT which returns differences

The INTERSECT operator returns only the records that appear in both tables.

3fill in blank

hard

Fix the error in the SQL query to find records in table1 but not in table2.

DBMS Theory

SELECT * FROM table1 [1] SELECT * FROM table2;

Drag options to blanks, or click blank then click option'

AEXCEPT

BINTERSECT

CUNION

DJOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INTERSECT which returns common records

Using UNION which returns all unique records

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

4fill in blank

hard

Fill both blanks to create a query that finds records in table1 but not in table2, and removes duplicates.

DBMS Theory

SELECT [1] * FROM table1 [2] SELECT * FROM table2;

Drag options to blanks, or click blank then click option'

AEXCEPT

BUNION

CDISTINCT

DINTERSECT

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION instead of EXCEPT

Not removing duplicates with DISTINCT

Use EXCEPT to find records in table1 not in table2, and DISTINCT to remove duplicates.

5fill in blank

hard

Fill all three blanks to create a query that finds records common to both tables and orders them by id.

DBMS Theory

SELECT * FROM table1 [1] SELECT * FROM table2 ORDER BY [2] [3];

Drag options to blanks, or click blank then click option'

AINTERSECT

Bid

CASC

DDESC

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION instead of INTERSECT

Ordering by wrong column or direction

INTERSECT finds common records, and ORDER BY id ASC sorts results by id in ascending order.

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