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

Concept Flow - DISTINCT for unique values

Start Query

↓

Read Table Data

↓

Apply DISTINCT

↓

Remove Duplicate Rows

↓

Return Unique Rows

↓

End Query

The query reads data from a table, applies DISTINCT to remove duplicates, and returns only unique rows.

Execution Sample

SQL

SELECT DISTINCT city FROM customers;

This query selects unique city names from the customers table, removing duplicates.

Execution Table

Step	Action	Input Data	Resulting Data
1	Read all city values from customers	[New York, Boston, New York, Chicago, Boston]	[New York, Boston, New York, Chicago, Boston]
2	Apply DISTINCT to remove duplicates	[New York, Boston, New York, Chicago, Boston]	[New York, Boston, Chicago]
3	Return unique city list	[New York, Boston, Chicago]	[New York, Boston, Chicago]
4	End query	-	-

💡 All duplicate city names removed, only unique cities returned

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
city_list	empty	[New York, Boston, New York, Chicago, Boston]	[New York, Boston, Chicago]	[New York, Boston, Chicago]

Key Moments - 2 Insights

Why does DISTINCT remove duplicate rows but not change the order of unique values?

DISTINCT scans all rows and keeps only the first occurrence of each unique value, preserving their order as they appear in the data (see execution_table step 2).

Does DISTINCT affect all columns or just the selected columns?

DISTINCT applies to the entire row of selected columns. Here, since only 'city' is selected, duplicates are removed based on city values only (execution_table step 2).

Visual Quiz - 3 Questions

Look at the execution_table, what is the city_list after step 2?

A [New York, Boston, Chicago]

B [New York, Boston, New York, Chicago, Boston]

C [Boston, Chicago]

D [Chicago]

💡 Hint

Check the 'Resulting Data' column in step 2 of execution_table

At which step does the query remove duplicate city names?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column describing when DISTINCT is applied

If the query selected two columns, city and state, how would DISTINCT behave?

A Remove duplicates based on city only

B Return all rows without removing duplicates

C Remove duplicates based on both city and state together

D Remove duplicates based on state only

💡 Hint

DISTINCT applies to all selected columns as a combined row (see key_moments explanation)

Concept Snapshot

DISTINCT keyword removes duplicate rows from query results.
Syntax: SELECT DISTINCT column_name FROM table;
It returns only unique values of the selected columns.
Duplicates are removed based on all selected columns combined.
Useful to find unique entries in a column or set of columns.

Full Transcript

This visual execution trace shows how the SQL DISTINCT keyword works. The query reads all city values from the customers table, including duplicates. Then DISTINCT is applied to remove duplicate city names, keeping only unique cities. The final result returns the unique city list. The variable city_list changes from all cities to only unique cities after DISTINCT is applied. Key moments clarify that DISTINCT removes duplicates based on all selected columns combined and preserves the order of first occurrences. The visual quiz tests understanding of when duplicates are removed and how DISTINCT behaves with multiple columns.