Querying into Data (DQL)

Nulls Decoded

Learning Outcome

Categorize data using CASE

Apply the CASE statement for conditional logic

Use IS NULL and IS NOT NULL in queries

Understand the concept of NULL values

lets recall

Comparison operator

Imagine you Instagram wants to:

Find users who haven’t added a bio

Show which users completed their profile

Categorize users based on followers

SQL provides:

IS NULL / IS NOT NULL
CASE Statement

Null

The system holds a placeholder - No Data

The value does not exist yet or was never provided

IS NULL

Imagine we have too
Find users who have added bio.

SELECT
User_ID,
username,
Bio
FROM users
WHERE bio IS NULL;

User ID	Username	Bio
101	mukesh	NULL
102	Rahuk	NULL
103	Ruhi	Darth Vader enthusiast
104	Salman	NULL
105	Mohan	Loves solo travel
106	akashay	NULL

Instagram identifying incomplete profiles.

User ID	Username	Bio
101	mukesh	NULL
102	Rahuk	NULL
104	Salman	NULL
106	akashay	NULL

SELECT
User_ID,
username,
Bio
FROM users
WHERE bio IS NULL;

IS NOT NUL

Imagine now we have to
Find users detail who have added bio.

User ID	Username	Bio
101	mukesh	NULL
102	Rahuk	NULL
103	Ruhi	Darth Vader enthusiast
104	Salman	NULL
105	Mohan	Loves solo travel
106	akashay	NULL

SELECT
User_ID,
username,
Bio
FROM users
WHERE bio IS NOT NULL;

Shows users with completed profiles.

User ID	Username	Bio
103	Ruhi	Darth Vader enthusiast
105	Mohan	Loves solo travel

SELECT
User_ID,
username,
Bio
FROM users
WHERE bio IS NOT NULL;

CASE Statement

Used to apply conditional logic inside SQL query.

Works like:

IF–ELSE in programming

SELECT column_name,
CASE
   WHEN condition THEN result
   ELSE result
END AS new_column
FROM table_name;

SELECT 
User_ID
username,
followers,
CASE
   WHEN followers >= 100000 THEN 'Influencer'
   WHEN followers >= 10000 THEN 'Creator'
   ELSE 'Regular User'
END AS category
FROM users;

Imagine you want to
Categorize users based on followers.

User_ID	Username	followers
101	Mukesh	9069
102	Akshay Kumar	80670
103	Ranveer Allabadia	123
104	Rahul_more	687
105	Neha_negi	123500
106	foodie_mohan	16384

Instagram categorizing users automatically.

User_ID	Username	followers	Category
101	Mukesh	9069	Regular User
102	Akshay Kumar	80670	Creator
103	Ranveer Allabadia	123	Regular User
104	Rahul_more	687	Regular User
105	Neha_negi	123500	Influencer
106	foodie_mohan	16384	Creator

SELECT
User_ID,
username,
followers,
CASE
   WHEN followers >= 100000 THEN 'Influencer'
   WHEN followers >= 10000 THEN 'Creator'
   ELSE 'Regular User'
END AS category
FROM users;

SELECT 
User_Id,
username,
Bio,
CASE
   WHEN bio IS NULL THEN 'Incomplete Profile'
   ELSE 'Complete Profile'
END AS profile_status
FROM users;

Imagine now you have to
Categorize users based on followers.

User ID	Username	Bio
101	Rahul	NULL
102	Mukesh	NULL
103	Shiwam	Darth Vader enthusiast
104	Ruhi	NULL
105	Disha	Loves solo travel
106	Mohan	NULL

Easy profile monitoring.

SELECT
User_ID,
username,
Bio,
CASE
   WHEN bio IS NULL THEN 'Incomplete Profile'
   ELSE 'Complete Profile'
END AS profile_status
FROM users;

User ID	Username	Bio	profile_status
101	Rahul	NULL	Incomplete Profile
102	Mukesh	NULL	Incomplete Profile
103	Shiwam	Darth Vader enthusiast	Complete Profile
104	Ruhi	NULL	Incomplete Profile
105	Disha	Loves solo travel	Complete Profile
106	Mohan	NULL	Incomplete Profile

Summary

Important for interviews and real-world SQL queries

Widely used in analytics dashboards

CASE works like IF–ELSE

IS NULL detects missing values

NULL represents missing data

Quiz

How do you check for NULL values correctly?

A. bio = NULL

B. bio == NULL

C. bio IS NULL

D. bio != NULL

How do you check for NULL values correctly?

A. bio = NULL

B. bio == NULL

C. bio IS NULL

D. bio != NULL

Quiz-Answer

Querying into Data (DQL)

Nulls Decoded

Nulls Decoded

More from Content ITV