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.

Practice

1. What does the SQL keyword DISTINCT do in a query?

easy

A. It sorts the results in ascending order.

B. It returns only unique rows by removing duplicates.

C. It counts the number of rows in a table.

D. It updates duplicate rows with new values.

Solution

1. Step 1: Understand the purpose of DISTINCT

   The DISTINCT keyword is used to filter out duplicate rows in the result set, so only unique rows remain.

2. Step 2: Compare with other options

   Sorting is done by ORDER BY, counting by COUNT(), and updating by UPDATE statement, none of which are related to DISTINCT.

3. Final Answer:

   It returns only unique rows by removing duplicates. -> Option B

4. Quick Check:

   DISTINCT = unique rows [OK]

Hint: DISTINCT removes duplicates, showing only unique rows [OK]

Common Mistakes:

• Confusing DISTINCT with ORDER BY
• Thinking DISTINCT counts rows
• Assuming DISTINCT updates data

2. Which of the following is the correct syntax to select unique city names from a table called Customers?

easy

A. SELECT DISTINCT city FROM Customers;

B. SELECT UNIQUE city FROM Customers;

C. SELECT city UNIQUE FROM Customers;

D. SELECT city FROM Customers DISTINCT;

Solution

1. Step 1: Recall correct DISTINCT syntax

   The correct syntax to get unique values is SELECT DISTINCT column_name FROM table_name.

2. Step 2: Check each option

   SELECT DISTINCT city FROM Customers; matches the correct syntax. The other options use incorrect keyword placement or invalid keywords.

3. Final Answer:

   SELECT DISTINCT city FROM Customers; -> Option A

4. Quick Check:

   Correct syntax uses DISTINCT after SELECT [OK]

Hint: Use SELECT DISTINCT column FROM table for unique values [OK]

Common Mistakes:

• Using UNIQUE instead of DISTINCT
• Placing DISTINCT after FROM
• Writing keywords in wrong order

3. Given the table Orders with a column Product containing values: ['Pen', 'Pen', 'Book', 'Notebook', 'Book'], what will the query SELECT DISTINCT Product FROM Orders; return?

medium

A. Pen, Book, Notebook

B. Pen, Book

C. Pen, Pen, Book, Notebook, Book

D. Notebook

Solution

1. Step 1: Identify duplicates in Product column

   The values are Pen (twice), Book (twice), and Notebook (once).

2. Step 2: Apply DISTINCT to remove duplicates

   DISTINCT returns each unique value once: Pen, Book, Notebook.

3. Final Answer:

   Pen, Book, Notebook -> Option A

4. Quick Check:

   DISTINCT removes duplicates, leaving unique values [OK]

Hint: DISTINCT returns each unique value once [OK]

Common Mistakes:

• Listing duplicates as separate rows
• Missing some unique values
• Confusing DISTINCT with COUNT

4. What is wrong with this query to get unique customer names from Customers table?
SELECT DISTINCT name, age FROM Customers;
Assuming you want unique names only.

medium

A. The query needs GROUP BY instead of DISTINCT.

B. DISTINCT cannot be used with multiple columns.

C. DISTINCT should be replaced with UNIQUE.

D. DISTINCT applies to all selected columns, so duplicates in name may remain if age differs.

Solution

1. Step 1: Understand how DISTINCT works with multiple columns

   DISTINCT treats the combination of all selected columns as one unit, so rows are unique only if all columns differ.

2. Step 2: Analyze the effect on name uniqueness

   If the same name appears with different ages, both rows appear because the combination is unique, not just the name.

3. Final Answer:

   DISTINCT applies to all selected columns, so duplicates in name may remain if age differs. -> Option D

4. Quick Check:

   DISTINCT on multiple columns = unique row combos [OK]

Hint: DISTINCT on multiple columns checks combined uniqueness [OK]

Common Mistakes:

• Thinking DISTINCT applies to each column separately
• Using UNIQUE keyword instead of DISTINCT
• Confusing DISTINCT with GROUP BY

5. You have a table Sales with columns Region and SalesPerson. You want to find all unique salespersons regardless of region. Which query gives the correct result?

hard

A. SELECT SalesPerson FROM Sales GROUP BY Region;

B. SELECT DISTINCT Region, SalesPerson FROM Sales;

C. SELECT DISTINCT SalesPerson FROM Sales;

D. SELECT UNIQUE SalesPerson FROM Sales;

Solution

1. Step 1: Identify the goal

   You want unique salespersons regardless of region, so only SalesPerson column matters.

2. Step 2: Choose query that returns unique SalesPerson values

   SELECT DISTINCT SalesPerson FROM Sales returns unique salespersons. SELECT DISTINCT Region, SalesPerson FROM Sales; returns unique pairs of Region and SalesPerson, which may repeat salespersons. SELECT SalesPerson FROM Sales GROUP BY Region; groups by Region, not SalesPerson, so it won't give unique salespersons. SELECT UNIQUE SalesPerson FROM Sales; uses UNIQUE which is invalid syntax.

3. Final Answer:

   SELECT DISTINCT SalesPerson FROM Sales; -> Option C

4. Quick Check:

   Use DISTINCT on the column you want unique values for [OK]

Hint: DISTINCT only on needed column for unique values [OK]

Common Mistakes:

• Selecting multiple columns with DISTINCT causing duplicates
• Using GROUP BY on wrong column
• Using UNIQUE keyword instead of DISTINCT