UNION ALL with duplicates in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SQL UNION ALL operator do?

UNION ALL combines the results of two or more SELECT queries and includes all rows, even duplicates.

beginner

How does UNION ALL differ from UNION?

UNION removes duplicate rows from the combined result, while UNION ALL keeps all duplicates.

beginner

Given two tables with overlapping data, which operator would you use to keep all rows including duplicates?

Use UNION ALL to keep all rows including duplicates from both tables.

beginner

Write a simple SQL query using UNION ALL to combine two tables named 'A' and 'B' with the same columns.

SELECT * FROM A UNION ALL SELECT * FROM B;

intermediate

Why might you choose UNION ALL over UNION in a query?

Because UNION ALL is faster since it does not check for duplicates, and you want to keep all rows including duplicates.

What happens to duplicate rows when using UNION ALL?

A Duplicates are kept

B Duplicates are removed

C Only one duplicate is kept

D Query fails if duplicates exist

UNION ALL keeps all rows including duplicates.

Which SQL operator removes duplicate rows when combining results?

A UNION ALL

B INTERSECT

C UNION

D EXCEPT

UNION removes duplicates, UNION ALL does not.

Which operator is generally faster when combining two result sets?

A UNION ALL

B UNION

C INTERSECT

D EXCEPT

UNION ALL is faster because it does not check for duplicates.

If you want to combine two tables and keep all rows including duplicates, which query is correct?

A SELECT * FROM A UNION SELECT * FROM B;

B SELECT * FROM A UNION ALL SELECT * FROM B;

C SELECT * FROM A INTERSECT SELECT * FROM B;

D SELECT * FROM A EXCEPT SELECT * FROM B;

UNION ALL keeps all rows including duplicates.

What is a key reason to use UNION ALL instead of UNION?

A To filter rows

B To remove duplicates

C To sort results automatically

D To improve query speed

UNION ALL is faster because it skips duplicate removal.

Explain the difference between UNION and UNION ALL in SQL.

Think about how duplicates are handled and speed.

Describe a scenario where using UNION ALL is better than UNION.

Consider when duplicates are important and speed matters.

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