Set operations with ORDER BY in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are set operations in SQL?

Set operations combine results from two or more SELECT queries into a single result. Common set operations include UNION, INTERSECT, and EXCEPT.

beginner

How does ORDER BY work with set operations?

ORDER BY sorts the final combined result of the set operation. It must be placed after all SELECT statements and set operators.

intermediate

Can you use ORDER BY inside each SELECT of a set operation?

No, ORDER BY inside individual SELECTs is ignored unless you use TOP or LIMIT. The final ORDER BY after the set operation controls sorting.

beginner

What happens if you use UNION without ORDER BY?

The combined result will have unique rows but the order is not guaranteed. To see sorted results, use ORDER BY after the UNION.

beginner

Write a simple SQL query using UNION and ORDER BY.

Example: SELECT name FROM students UNION SELECT name FROM teachers ORDER BY name ASC;

Where should ORDER BY be placed when using set operations like UNION?

A Between the SELECT statements

B After the last SELECT statement and set operation

C Before the first SELECT statement

D Inside each SELECT statement

ORDER BY must come after all SELECT statements and set operations to sort the final combined result.

What does UNION do in SQL?

A Combines results and removes duplicates

B Combines results and keeps duplicates

C Returns only common rows

D Returns rows from the first query only

UNION combines results from two queries and removes duplicate rows.

Can ORDER BY inside individual SELECT statements affect the final output order in a UNION?

A Yes, it controls the final order

B No, ORDER BY is ignored everywhere

C Yes, but only in some databases

D No, only the final ORDER BY after UNION affects order

ORDER BY inside individual SELECTs is ignored for final sorting; only the ORDER BY after the set operation matters.

Which set operation returns only rows common to both queries?

A INTERSECT

B UNION

C EXCEPT

D JOIN

INTERSECT returns only rows that appear in both queries.

What will happen if you omit ORDER BY after a UNION?

A Query will fail

B Result rows will be sorted by default

C Result rows will not be sorted

D Duplicates will appear

Without ORDER BY, the combined result has no guaranteed order.

Explain how to use ORDER BY with set operations like UNION in SQL.

Think about where the sorting happens after combining results.

Describe the difference between UNION and INTERSECT and how ORDER BY affects their results.

Focus on what rows each set operation returns and when sorting applies.

Practice

1. What does the UNION operation do when combining results from two SELECT queries?

easy

A. Sorts the results in ascending order

B. Combines results and removes duplicate rows

C. Combines results and keeps all duplicate rows

D. Joins tables based on a common column

Solution

1. Step 1: Understand UNION operation

   The UNION operation combines results from two SELECT queries into one result set.

2. Step 2: Check duplicate handling

   UNION removes duplicate rows, unlike UNION ALL which keeps duplicates.

3. Final Answer:

   Combines results and removes duplicate rows -> Option B

4. Quick Check:

   UNION removes duplicates = C [OK]

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

Common Mistakes:

• Confusing UNION with UNION ALL
• Thinking UNION sorts results automatically
• Mixing UNION with JOIN operations

2. Which of the following is the correct syntax to combine two SELECT queries with UNION and sort the final result by column name?

easy

A. SELECT name FROM table1 UNION ORDER BY name SELECT name FROM table2;

B. SELECT name FROM table1 ORDER BY name UNION SELECT name FROM table2;

C. SELECT name FROM table1 UNION SELECT name FROM table2 ORDER BY name;

D. SELECT name FROM table1 UNION SELECT name FROM table2 SORT BY name;

Solution

1. Step 1: Understand correct UNION syntax

   The UNION combines two SELECT queries; ORDER BY applies after the last SELECT.

2. Step 2: Check placement of ORDER BY

   ORDER BY must come after the entire UNION, not between or before queries.

3. Final Answer:

   SELECT name FROM table1 UNION SELECT name FROM table2 ORDER BY name; -> Option C

4. Quick Check:

   ORDER BY after UNION = A [OK]

Hint: Place ORDER BY after all UNION queries [OK]

Common Mistakes:

• Putting ORDER BY before UNION
• Using SORT BY instead of ORDER BY
• Placing ORDER BY between SELECTs

3. Given two tables:
table1 with values (1, 'Alice'), (2, 'Bob')
table2 with values (2, 'Bob'), (3, 'Charlie')
What is the result of this query?

SELECT id, name FROM table1 UNION SELECT id, name FROM table2 ORDER BY id;

medium

A. (1, 'Alice'), (2, 'Bob'), (3, 'Charlie')

B. (1, 'Alice'), (2, 'Bob'), (2, 'Bob'), (3, 'Charlie')

C. (2, 'Bob'), (3, 'Charlie')

D. (1, 'Alice'), (3, 'Charlie')

Solution

1. Step 1: Combine rows with UNION

   UNION merges rows from both tables and removes duplicates, so (2, 'Bob') appears once.

2. Step 2: Sort combined results by id

   Ordering by id gives rows in order: 1, 2, 3.

3. Final Answer:

   (1, 'Alice'), (2, 'Bob'), (3, 'Charlie') -> Option A

4. Quick Check:

   UNION removes duplicates, ORDER BY sorts = B [OK]

Hint: UNION removes duplicates; ORDER BY sorts final list [OK]

Common Mistakes:

• Expecting duplicates with UNION
• Ignoring ORDER BY sorting
• Confusing UNION with UNION ALL

4. Identify the error in this SQL query:

SELECT name FROM table1 UNION ALL ORDER BY name SELECT name FROM table2;

medium

A. SELECT statements must have different column names

B. UNION ALL cannot be used with ORDER BY

C. Missing semicolon after first SELECT

D. ORDER BY is placed incorrectly between UNION ALL and second SELECT

Solution

1. Step 1: Check UNION ALL syntax

   UNION ALL combines two SELECT queries; ORDER BY must come after both queries, not between.

2. Step 2: Identify ORDER BY placement error

   ORDER BY is incorrectly placed between UNION ALL and second SELECT, causing syntax error.

3. Final Answer:

   ORDER BY is placed incorrectly between UNION ALL and second SELECT -> Option D

4. Quick Check:

   ORDER BY after UNION ALL queries = D [OK]

Hint: ORDER BY must follow all UNION ALL queries [OK]

Common Mistakes:

• Placing ORDER BY between UNION ALL and second SELECT
• Thinking UNION ALL disallows ORDER BY
• Assuming column names must differ

5. You have two tables:
employees with columns (id, name, department)
contractors with columns (id, name, department)
Write a query to list all unique names from both tables sorted alphabetically, but include duplicates if the same name appears in both tables.
Which query achieves this?

hard

A. SELECT name FROM employees UNION ALL SELECT name FROM contractors ORDER BY name;

B. SELECT name FROM employees UNION SELECT name FROM contractors ORDER BY name;

C. SELECT name FROM employees INTERSECT SELECT name FROM contractors ORDER BY name;

D. SELECT name FROM employees EXCEPT SELECT name FROM contractors ORDER BY name;

Solution

1. Step 1: Understand requirement for duplicates

   The query should include duplicates if the same name appears in both tables, so duplicates must be kept.

2. Step 2: Choose correct set operation

   UNION removes duplicates, so it is not suitable. UNION ALL keeps duplicates from both tables.

3. Step 3: Confirm sorting

   ORDER BY name sorts the combined result alphabetically.

4. Final Answer:

   SELECT name FROM employees UNION ALL SELECT name FROM contractors ORDER BY name; -> Option A

5. Quick Check:

   UNION ALL keeps duplicates, ORDER BY sorts = A [OK]

Hint: Use UNION ALL to keep duplicates, ORDER BY to sort [OK]

Common Mistakes:

• Using UNION which removes duplicates
• Confusing INTERSECT or EXCEPT with UNION
• Placing ORDER BY incorrectly