UNION ALL with duplicates in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to combine two tables including duplicates using UNION ALL.

SQL

SELECT name FROM employees [1] SELECT name FROM contractors;

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

AINTERSECT

BUNION ALL

CUNION

DEXCEPT

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION instead of UNION ALL, which removes duplicates.

Using INTERSECT or EXCEPT which do not combine all rows.

The UNION ALL operator combines results from both queries and includes duplicates.

2fill in blank

medium

Complete the code to select all product IDs from two tables including duplicates.

SQL

SELECT product_id FROM sales_2023 [1] SELECT product_id FROM sales_2024;

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

AUNION ALL

BUNION

CINTERSECT

DJOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION which removes duplicates.

Using JOIN which is for combining columns, not rows.

UNION ALL returns all rows from both queries including duplicates.

3fill in blank

hard

Fix the error in the query to include duplicates from both tables.

SQL

SELECT city FROM customers [1] SELECT city FROM suppliers;

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

AUNION ALL

BUNION

CEXCEPT

DINTERSECT

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION which removes duplicates.

Using EXCEPT or INTERSECT which filter rows.

To include duplicates from both tables, use UNION ALL.

4fill in blank

hard

Fill both blanks to combine two queries including duplicates and order the results by name.

SQL

SELECT name FROM staff [1] SELECT name FROM interns ORDER BY name [2];

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

AUNION ALL

BWHERE

CASC

DGROUP BY

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION instead of UNION ALL.

Using WHERE instead of ORDER BY direction.

UNION ALL combines all rows including duplicates, and ASC orders the results ascending by name.

5fill in blank

hard

Fill all three blanks to select all emails from two tables including duplicates, filter only active users, and order by email descending.

SQL

SELECT email FROM users_2022 WHERE status = [1] [2] SELECT email FROM users_2023 WHERE status = [3] ORDER BY email DESC;

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

A'active'

BUNION ALL

DUNION

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNION which removes duplicates.

Using wrong status filter values.

Use 'active' to filter active users, and UNION ALL to combine all rows including duplicates.

Practice

1. What does the SQL statement UNION ALL do when combining results from two queries?

easy

A. It combines rows but removes duplicates.

B. It combines all rows from both queries including duplicates.

C. It only returns rows that appear in both queries.

D. It returns rows only from the first query.

Solution

1. Step 1: Understand UNION ALL behavior

   UNION ALL combines results from two queries and keeps all rows, including duplicates.

2. Step 2: Compare with UNION

   Unlike UNION, UNION ALL does not remove duplicate rows.

3. Final Answer:

   It combines all rows from both queries including duplicates. -> Option B

4. Quick Check:

   UNION ALL keeps duplicates = A [OK]

Hint: UNION ALL keeps duplicates, UNION removes them [OK]

Common Mistakes:

• Confusing UNION ALL with UNION
• Thinking duplicates are removed
• Assuming it returns only unique rows

2. Which of the following is the correct syntax to combine two SELECT queries using UNION ALL?

easy

A. SELECT col1 FROM table1 UNIONALL SELECT col1 FROM table2;

B. SELECT col1 FROM table1 UNION ALL FROM table2;

C. SELECT col1 FROM table1 UNION ALL SELECT col1 FROM table2;

D. SELECT col1 FROM table1 UNION ALL SELECT FROM table2;

Solution

1. Step 1: Check correct UNION ALL syntax

   The correct syntax is to write two SELECT statements separated by 'UNION ALL'.

2. Step 2: Identify syntax errors in options

   SELECT col1 FROM table1 UNIONALL SELECT col1 FROM table2; writes UNIONALL as one word (missing space), SELECT col1 FROM table1 UNION ALL FROM table2; misses the second SELECT keyword, SELECT col1 FROM table1 UNION ALL SELECT FROM table2; misses columns after the second SELECT. SELECT col1 FROM table1 UNION ALL SELECT col1 FROM table2; uses correct syntax.

3. Final Answer:

   SELECT col1 FROM table1 UNION ALL SELECT col1 FROM table2; -> Option C

4. Quick Check:

   Correct UNION ALL syntax = B [OK]

Hint: UNION ALL needs space and two SELECTs [OK]

Common Mistakes:

• Writing UNIONALL as one word
• Missing SELECT in second query
• Incorrect keyword order

3. Given two tables:
Table A: id
1
2
3
Table B: id
2
3
4
What is the result of:
SELECT id FROM A UNION ALL SELECT id FROM B;

medium

A. [1, 2, 3, 2, 3, 4]

B. [1, 2, 3, 4]

C. [1, 2, 3]

D. [2, 3, 4]

Solution

1. Step 1: List rows from first query

   SELECT id FROM A returns [1, 2, 3].

2. Step 2: List rows from second query

   SELECT id FROM B returns [2, 3, 4].

3. Step 3: Combine results with UNION ALL

   UNION ALL keeps duplicates, so combined list is [1, 2, 3, 2, 3, 4].

4. Final Answer:

   [1, 2, 3, 2, 3, 4] -> Option A

5. Quick Check:

   UNION ALL keeps duplicates = A [OK]

Hint: UNION ALL stacks all rows, duplicates included [OK]

Common Mistakes:

• Removing duplicates like UNION
• Listing only unique values
• Ignoring order of combined rows

4. Consider this SQL query:
SELECT name FROM employees UNION ALL SELECT name FROM departments;
It returns an error. What is the most likely cause?

medium

A. The two SELECT statements have different numbers of columns.

B. UNION ALL cannot be used with SELECT statements.

C. The keyword ALL is not allowed after UNION.

D. The tables employees and departments must have the same name.

Solution

1. Step 1: Check column counts in both SELECTs

   UNION ALL requires both SELECTs to have the same number of columns.

2. Step 2: Identify error cause

   If employees and departments tables have different columns selected, error occurs.

3. Final Answer:

   The two SELECT statements have different numbers of columns. -> Option A

4. Quick Check:

   Column count mismatch causes error = C [OK]

Hint: Both SELECTs must have same columns count [OK]

Common Mistakes:

• Thinking UNION ALL disallows duplicates only
• Assuming table names must match
• Believing ALL keyword is invalid

5. You have two tables:
Sales_2023: product_id, quantity_sold
Sales_2024: product_id, quantity_sold
You want to create a combined list of all sales including duplicates for analysis. Which query correctly uses UNION ALL and also adds a column year to identify the source year?

hard

A. SELECT product_id, quantity_sold, 'year' FROM Sales_2023 UNION ALL SELECT product_id, quantity_sold, 'year' FROM Sales_2024;

B. SELECT product_id, quantity_sold FROM Sales_2023 UNION ALL SELECT product_id, quantity_sold, '2024' AS year FROM Sales_2024;

C. SELECT product_id, quantity_sold, year FROM Sales_2023 UNION ALL SELECT product_id, quantity_sold, year FROM Sales_2024;

D. SELECT product_id, quantity_sold, '2023' AS year FROM Sales_2023 UNION ALL SELECT product_id, quantity_sold, '2024' AS year FROM Sales_2024;

Solution

1. Step 1: Add year column with literal values

   Use '2023' and '2024' as string literals to add a year column in each SELECT.

2. Step 2: Ensure both SELECTs have same columns

   Both SELECTs must have product_id, quantity_sold, and year columns for UNION ALL.

3. Step 3: Combine with UNION ALL

   SELECT product_id, quantity_sold, '2023' AS year FROM Sales_2023 UNION ALL SELECT product_id, quantity_sold, '2024' AS year FROM Sales_2024; correctly combines both tables with year column and keeps duplicates.

4. Final Answer:

   SELECT product_id, quantity_sold, '2023' AS year FROM Sales_2023 UNION ALL SELECT product_id, quantity_sold, '2024' AS year FROM Sales_2024; -> Option D

5. Quick Check:

   Matching columns with year literals + UNION ALL = D [OK]

Hint: Add year as literal in both SELECTs for UNION ALL [OK]

Common Mistakes:

• Mismatched columns count in SELECTs
• Using column name 'year' without value
• Forgetting to alias literals as year