DISTINCT for unique values in MySQL - Step-by-Step Execution

Concept Flow - DISTINCT for unique values

Start Query

↓

Scan Table Rows

↓

Check Each Row Value

↓

Is Value Already Seen?

Yes→Skip Row

No↓

Add Value to Result Set

↓

More Rows?

Yes→Check Each Row Value

No↓

Return Unique Values

The DISTINCT keyword scans each row, keeps only unique values, and skips duplicates to return a list of unique results.

Execution Sample

MySQL

SELECT DISTINCT city FROM customers;

This query returns a list of unique city names from the customers table.

Execution Table

Step	Row Value (city)	Already Seen?	Action	Result Set
1	New York	No	Add to result	[New York]
2	Los Angeles	No	Add to result	[New York, Los Angeles]
3	New York	Yes	Skip row	[New York, Los Angeles]
4	Chicago	No	Add to result	[New York, Los Angeles, Chicago]
5	Los Angeles	Yes	Skip row	[New York, Los Angeles, Chicago]
6	Houston	No	Add to result	[New York, Los Angeles, Chicago, Houston]
7	Chicago	Yes	Skip row	[New York, Los Angeles, Chicago, Houston]
End	-	-	No more rows	[New York, Los Angeles, Chicago, Houston]

💡 All rows processed; duplicates skipped; unique cities returned.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	Final
Result Set	[]	[New York]	[New York, Los Angeles]	[New York, Los Angeles]	[New York, Los Angeles, Chicago]	[New York, Los Angeles, Chicago]	[New York, Los Angeles, Chicago, Houston]	[New York, Los Angeles, Chicago, Houston]	[New York, Los Angeles, Chicago, Houston]

Key Moments - 2 Insights

Why does the query skip some rows even though they exist in the table?

Rows with city values already in the result set are skipped to avoid duplicates, as shown in steps 3, 5, and 7 in the execution_table.

Does DISTINCT change the order of the results?

DISTINCT keeps the order of first occurrences as they appear in the table scan, so the first time a city appears it is added, preserving that order.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Result Set after step 4?

A [New York, Los Angeles, Chicago]

B [New York, Los Angeles]

C [New York, Chicago]

D [Los Angeles, Chicago]

💡 Hint

Check the Result Set column at step 4 in the execution_table.

At which step does the city 'Houston' get added to the Result Set?

A Step 5

B Step 6

C Step 7

D Step 3

💡 Hint

Look for 'Houston' in the Row Value column and see the corresponding action.

If the city 'Miami' was added as a new row after step 7, what would happen?

A The query would stop immediately.

B It would be skipped because it's a duplicate.

C It would be added to the Result Set.

D It would replace an existing city in the Result Set.

💡 Hint

DISTINCT adds new unique values to the Result Set as shown in the execution_table.

Concept Snapshot

DISTINCT keyword returns unique values from a column.
Syntax: SELECT DISTINCT column FROM table;
It scans rows, adds unseen values to results, skips duplicates.
Order of first occurrences is preserved.
Useful to find unique entries without repeats.

Full Transcript

The DISTINCT keyword in SQL helps to get unique values from a column. When you run a query like SELECT DISTINCT city FROM customers, the database looks at each city in the customers table one by one. If it sees a city for the first time, it adds it to the result list. If the city has already been added before, it skips that row. This way, the final output contains only unique city names without any duplicates. The order of cities in the result matches the order they first appear in the table. This process continues until all rows are checked, then the unique list is returned.