UNION and UNION ALL in PostgreSQL - Step-by-Step Execution

Concept Flow - UNION and UNION ALL

Start: Two SELECT queries

↓

Execute first SELECT

↓

Combine results

↓

UNION: Remove duplicates

↓

Final result set returned

↓

End

Two SELECT queries run separately, then their results combine. UNION removes duplicates; UNION ALL keeps all rows.

Execution Sample

PostgreSQL

SELECT name FROM fruits
UNION
SELECT name FROM vegetables;

Combines names from fruits and vegetables tables, removing duplicates.

Execution Table

Step	Action	First SELECT Result	Second SELECT Result	Combined Result	Duplicates Removed?	Final Output
1	Execute first SELECT	[Apple, Banana, Orange]			N/A
2	Execute second SELECT	[Apple, Banana, Orange]	[Carrot, Banana, Tomato]		N/A
3	Combine results	[Apple, Banana, Orange]	[Carrot, Banana, Tomato]	[Apple, Banana, Orange, Carrot, Banana, Tomato]	N/A
4	Apply UNION (remove duplicates)	[Apple, Banana, Orange]	[Carrot, Banana, Tomato]	[Apple, Banana, Orange, Carrot, Tomato]	Yes	[Apple, Banana, Orange, Carrot, Tomato]
5	Return final result (UNION)					[Apple, Banana, Orange, Carrot, Tomato]
6	Apply UNION ALL (keep duplicates) [alternative after step 3]			[Apple, Banana, Orange, Carrot, Banana, Tomato]	No	[Apple, Banana, Orange, Carrot, Banana, Tomato]

💡 Execution stops after combining and applying UNION or UNION ALL rules to produce final output.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4 (UNION)	After Step 6 (UNION ALL)
First SELECT Result	empty	[Apple, Banana, Orange]	[Apple, Banana, Orange]	[Apple, Banana, Orange]	[Apple, Banana, Orange]	[Apple, Banana, Orange]
Second SELECT Result	empty	empty	[Carrot, Banana, Tomato]	[Carrot, Banana, Tomato]	[Carrot, Banana, Tomato]	[Carrot, Banana, Tomato]
Combined Result	empty	empty	empty	[Apple, Banana, Orange, Carrot, Banana, Tomato]	[Apple, Banana, Orange, Carrot, Tomato]	[Apple, Banana, Orange, Carrot, Banana, Tomato]
Final Output	empty	empty	empty	empty	[Apple, Banana, Orange, Carrot, Tomato]	[Apple, Banana, Orange, Carrot, Banana, Tomato]

Key Moments - 3 Insights

Why does UNION remove some rows but UNION ALL does not?

UNION removes duplicate rows after combining both SELECT results (see step 4 in execution_table), while UNION ALL keeps all rows including duplicates (see step 6).

Are the two SELECT queries executed separately or together?

They are executed separately first (steps 1 and 2), then their results are combined (step 3).

Does the order of rows stay the same after UNION?

No, UNION may reorder rows because it removes duplicates and returns a distinct set (step 4). UNION ALL preserves all rows in combined order (step 6).

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what happens to duplicates?

A Duplicates are removed

B Duplicates are kept

C Duplicates are doubled

D Duplicates are ignored only in first SELECT

💡 Hint

Check the 'Duplicates Removed?' and 'Final Output' columns at step 4 in execution_table.

At which step do the two SELECT query results combine?

A Step 1

B Step 2

C Step 3

D Step 5

💡 Hint

Look for the step where 'Combined Result' column first shows both SELECT results together.

If we use UNION ALL instead of UNION, what changes in the final output?

A Duplicates are removed

B Duplicates remain in the output

C Only first SELECT results are shown

D Output is empty

💡 Hint

Compare 'Final Output' at step 4 (UNION) and step 6 (UNION ALL) in execution_table.

Concept Snapshot

UNION combines results of two SELECT queries and removes duplicates.
UNION ALL combines results and keeps all duplicates.
Both require same number of columns and compatible types.
UNION sorts and removes duplicates; UNION ALL is faster and keeps all rows.
Use UNION when duplicates matter; UNION ALL for full combined data.

Full Transcript

This visual execution shows how UNION and UNION ALL work in PostgreSQL. First, two SELECT queries run separately to get their results. Then, these results combine into one list. UNION removes duplicate rows from this combined list before returning it, while UNION ALL keeps all rows including duplicates. The execution table traces each step: running each SELECT, combining results, removing duplicates for UNION, and returning the final output. Variables track the state of results after each step. Key moments clarify why duplicates are removed only by UNION and how the queries execute separately before combining. The quiz tests understanding of when duplicates are removed and when results combine. The snapshot summarizes the main differences and usage tips for UNION and UNION ALL.

Practice

1. What is the main difference between UNION and UNION ALL in PostgreSQL?

easy

A. UNION removes duplicate rows, while UNION ALL keeps all rows including duplicates.

B. UNION requires matching column names, UNION ALL does not.

C. UNION ALL removes duplicates, UNION keeps duplicates.

D. UNION can combine different numbers of columns, UNION ALL cannot.

Solution

1. Step 1: Understand the behavior of UNION

   UNION combines results from two queries and removes duplicate rows to give unique results.

2. Step 2: Understand the behavior of UNION ALL

   UNION ALL combines results but keeps all rows, including duplicates.

3. Final Answer:

   UNION removes duplicates, UNION ALL keeps duplicates. -> Option A

4. Quick Check:

   Duplicates removed = UNION, duplicates kept = UNION ALL [OK]

Hint: UNION removes duplicates; UNION ALL keeps all rows [OK]

Common Mistakes:

• Thinking UNION ALL removes duplicates
• Believing UNION allows different column counts
• Confusing column name requirements

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

easy

A. SELECT name FROM students UNION ALL SELECT name FROM teachers;

B. SELECT name FROM students UNIONALL SELECT name FROM teachers;

C. SELECT name FROM students UNION ALL FROM teachers;

D. SELECT name FROM students UNION ALL SELECT name FROM teachers WHERE;

Solution

1. Step 1: Check the correct keyword spacing

   The correct keyword is UNION ALL with a space between UNION and ALL.

2. Step 2: Verify the SELECT statements are complete

   Both queries must be valid SELECT statements with matching column counts and types.

3. Final Answer:

   SELECT name FROM students UNION ALL SELECT name FROM teachers; -> Option A

4. Quick Check:

   Correct syntax uses 'UNION ALL' with space [OK]

Hint: Use space between UNION and ALL, both queries must be valid [OK]

Common Mistakes:

• Writing UNIONALL as one word
• Missing SELECT in second query
• Adding WHERE without condition

3. Given two tables:
Table A:
id | value
1 | 'apple'
2 | 'banana'

Table B:
id | value
2 | 'banana'
3 | 'cherry'

What is the result of this query?

SELECT value FROM A UNION SELECT value FROM B ORDER BY value;

medium

A. apple, banana, banana, cherry

B. apple, banana, cherry, banana

C. banana, cherry

D. apple, banana, cherry

Solution

1. Step 1: Combine values from both tables using UNION

   UNION removes duplicates, so 'banana' appears only once.

2. Step 2: Order the combined results alphabetically

   Ordering by value gives: 'apple', 'banana', 'cherry'.

3. Final Answer:

   apple, banana, cherry -> Option D

4. Quick Check:

   UNION removes duplicates, so only one 'banana' [OK]

Hint: UNION removes duplicates; check ordering carefully [OK]

Common Mistakes:

• Including duplicate 'banana' with UNION
• Ignoring ORDER BY effect
• Confusing UNION with UNION ALL

4. Consider this query:

SELECT id FROM table1 UNION ALL SELECT id FROM table2;

It returns an error. What is the most likely cause?

medium

A. The tables have different names.

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

C. UNION ALL cannot be used with SELECT statements.

D. The query is missing a semicolon.

Solution

1. Step 1: Check column counts in both SELECT statements

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

2. Step 2: Identify error cause

   If column counts differ, PostgreSQL throws an error.

3. Final Answer:

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

4. Quick Check:

   Column count mismatch causes UNION ALL error [OK]

Hint: Ensure both SELECTs have same column count for UNION ALL [OK]

Common Mistakes:

• Thinking table names must match
• Believing UNION ALL disallows SELECT
• Ignoring column count mismatch

5. You want to combine two queries:

SELECT id, name FROM employees WHERE department = 'Sales'
SELECT id, name FROM contractors WHERE department = 'Sales'

and keep all rows including duplicates. Which query correctly does this and why?

hard

A. SELECT id, name FROM employees UNION ALL SELECT id, name FROM contractors; Because WHERE is not needed in second query.

B. SELECT id, name FROM employees UNION SELECT id, name FROM contractors WHERE department = 'Sales'; Because UNION removes duplicates.

C. SELECT id, name FROM employees UNION ALL SELECT id, name FROM contractors WHERE department = 'Sales'; Because UNION ALL keeps duplicates.

D. SELECT id, name FROM employees UNION SELECT id, name FROM contractors; Because UNION ALL is invalid here.

Solution

1. Step 1: Understand requirement to keep duplicates

   To keep duplicates, use UNION ALL because UNION removes duplicates.

2. Step 2: Check WHERE clause usage

   Both queries filter department = 'Sales', so WHERE must be in both queries.

3. Final Answer:

   SELECT id, name FROM employees UNION ALL SELECT id, name FROM contractors WHERE department = 'Sales'; Because UNION ALL keeps duplicates. -> Option C

4. Quick Check:

   Keep duplicates = UNION ALL with correct WHERE clauses [OK]

Hint: Use UNION ALL to keep duplicates; apply WHERE in each SELECT [OK]

Common Mistakes:

• Using UNION instead of UNION ALL
• Omitting WHERE in second query
• Thinking UNION ALL is invalid