Why combining result sets is useful in MySQL - Visual Breakdown

Concept Flow - Why combining result sets is useful

Start with Query 1

↓

Get Result Set 1

↓

Start with Query 2

↓

Get Result Set 2

↓

Combine Result Sets

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Final Output

Two or more queries run separately, then their results are combined into one list for easier use.

Execution Sample

MySQL

SELECT name FROM employees WHERE department = 'Sales'
UNION
SELECT name FROM employees WHERE department = 'Marketing';

This query gets names from Sales and Marketing departments and combines them into one list without duplicates.

Execution Table

Step	Query Part	Action	Resulting Rows	Notes
1	SELECT name FROM employees WHERE department = 'Sales'	Run first query	Alice, Bob	Get Sales employees
2	SELECT name FROM employees WHERE department = 'Marketing'	Run second query	Carol, Dave	Get Marketing employees
3	UNION	Combine results	Alice, Bob, Carol, Dave	Duplicates removed if any
4	Final Output	Return combined list	Alice, Bob, Carol, Dave	Single list from both departments

💡 All queries executed and results combined; no more steps.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
ResultSet1	empty	Alice, Bob	Alice, Bob	Alice, Bob	Alice, Bob
ResultSet2	empty	empty	Carol, Dave	Carol, Dave	Carol, Dave
CombinedResult	empty	empty	empty	Alice, Bob, Carol, Dave	Alice, Bob, Carol, Dave

Key Moments - 2 Insights

Why do we run two separate queries before combining results?

Each query targets a specific group (Sales or Marketing). Combining after running both lets us get all needed data in one list, as shown in steps 1 and 2 of the execution_table.

What happens if the same name appears in both departments?

Using UNION removes duplicates automatically, so the final list shows each name only once, as explained in step 3 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what names are in ResultSet1 after Step 1?

A Carol, Dave

B Alice, Bob

C Alice, Bob, Carol, Dave

D Empty

💡 Hint

Check the 'Resulting Rows' column for Step 1 in execution_table.

At which step are the two result sets combined?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for the 'Combine results' action in execution_table.

If we used UNION ALL instead of UNION, what would change in the final output?

A Duplicates would be included

B Duplicates would be removed

C Only Sales employees would show

D Only Marketing employees would show

💡 Hint

UNION ALL keeps duplicates; UNION removes them. See key_moments about duplicates.

Concept Snapshot

Combining result sets lets you get data from multiple queries as one list.
Use UNION to merge and remove duplicates.
Use UNION ALL to merge and keep duplicates.
This helps when data is spread across groups or conditions.
Run queries separately, then combine results for easy use.

Full Transcript

Combining result sets is useful because it lets you gather data from different groups or conditions into one list. For example, you can get employee names from Sales and Marketing departments separately, then combine them using UNION. This removes duplicates so each name appears once. The process runs each query, collects results, then merges them. This way, you get a single list that is easier to work with. Using UNION ALL instead keeps duplicates if needed. This method helps when data is split but you want one combined output.