Concept questionMedium~7 min

Interview question

What can go wrong when filtering SQL rows that contain NULL values?

Explain SQL null semantics in filters and joins

Explains SQL three-valued logic, null-safe filters, optional relationships, and API behavior around missing values.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

In SQL, NULL means unknown or missing, so comparisons like column = NULL do not work the way normal equality does. I would use IS NULL or IS NOT NULL for explicit null checks. I would also remember that conditions involving NULL can evaluate to unknown, which means the row does not pass a WHERE filter. This matters in optional fields, left joins, and search filters.

Full answer below

Practice focus

NULL comparison behavior, three-valued logic, optional data, joins, indexes, and API-facing defaults.

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
NULL behavior is a quiet source of interview mistakes and real production bugs. A candidate who handles it well usually understands SQL as a different model from normal application booleans.
In SQL, NULL means unknown or missing, so comparisons like column = NULL do not work the way normal equality does. I would use IS NULL or IS NOT NULL for explicit null checks. I would also remember that conditions involving NULL can evaluate to unknown, which means the row does not pass a WHERE filter. This matters in optional fields, left joins, and search filters.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
I would design filters so missing data has a clear product meaning. For example, if status can be NULL, I would decide whether NULL means draft, unknown, imported legacy data, or not applicable. In left joins, filtering a related table column in WHERE can accidentally turn the query into inner-join behavior, so I would place conditions carefully. For APIs, I would map NULL to a deliberate response shape rather than relying on accidental defaults.
Finish
Advanced answer
Optional deeper angle for production judgment and nuance.
For production systems I would reduce ambiguity in the data model. If a value has a real default, I would use a default and NOT NULL constraint. If missing is meaningful, I would document it and test the filters that depend on it. I would also think about indexes because nullable columns and optional filters can produce different plans. NULL is both a query concern and a domain modeling concern.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Writing column = NULL instead of IS NULL.
Filtering a left-joined table in WHERE and losing unmatched rows.
Using NULL for several different business meanings.
Letting API behavior depend on accidental database defaults.

Red flagsCandidateRed flagsSignals that an interviewer may treat as risky.Open details

Details

Cannot explain why column = NULL is wrong.
Does not know that WHERE filters exclude unknown conditions.
Ignores null behavior in left joins.
Has no domain meaning for missing values.

Follow-up prompts

Quick interviewer prompts inside this drill.

Follow-up questionsCandidateFollow-up questionsLikely next questions after the main answer.Open details

Details

Treat these as the next interviewer turn. Answer one out loud, then save the gap in your practice notes.

When would you prefer NOT NULL plus a default value?
How can a WHERE clause change left join behavior?
How would you test null-sensitive filters?

Continue practice

No explicit flow link is set yet, so these are nearby drills from the same path or taxonomy.

Avoid Fanout TotalsCoding problem / Medium / ~25 minContinue Daily Order KPIsCoding problem / Medium / ~25 minContinue Running BalanceCoding problem / Medium / ~25 minContinue

Opening drill

Concept questionMedium~7 min

Interview question

What can go wrong when filtering SQL rows that contain NULL values?

Explain SQL null semantics in filters and joins

Explains SQL three-valued logic, null-safe filters, optional relationships, and API behavior around missing values.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

Full answer below

Practice focus

NULL comparison behavior, three-valued logic, optional data, joins, indexes, and API-facing defaults.

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Why this question matters
NULL behavior is a quiet source of interview mistakes and real production bugs. A candidate who handles it well usually understands SQL as a different model from normal application booleans.
In SQL, NULL means unknown or missing, so comparisons like column = NULL do not work the way normal equality does. I would use IS NULL or IS NOT NULL for explicit null checks. I would also remember that conditions involving NULL can evaluate to unknown, which means the row does not pass a WHERE filter. This matters in optional fields, left joins, and search filters.
Checkpoint
Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
I would design filters so missing data has a clear product meaning. For example, if status can be NULL, I would decide whether NULL means draft, unknown, imported legacy data, or not applicable. In left joins, filtering a related table column in WHERE can accidentally turn the query into inner-join behavior, so I would place conditions carefully. For APIs, I would map NULL to a deliberate response shape rather than relying on accidental defaults.
Finish
Advanced answer
Optional deeper angle for production judgment and nuance.
For production systems I would reduce ambiguity in the data model. If a value has a real default, I would use a default and NOT NULL constraint. If missing is meaningful, I would document it and test the filters that depend on it. I would also think about indexes because nullable columns and optional filters can produce different plans. NULL is both a query concern and a domain modeling concern.

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Writing column = NULL instead of IS NULL.
Filtering a left-joined table in WHERE and losing unmatched rows.
Using NULL for several different business meanings.
Letting API behavior depend on accidental database defaults.

Red flagsCandidateRed flagsSignals that an interviewer may treat as risky.Open details

Details

Cannot explain why column = NULL is wrong.
Does not know that WHERE filters exclude unknown conditions.
Ignores null behavior in left joins.
Has no domain meaning for missing values.

Follow-up prompts

Quick interviewer prompts inside this drill.

Follow-up questionsCandidateFollow-up questionsLikely next questions after the main answer.Open details

Details

Treat these as the next interviewer turn. Answer one out loud, then save the gap in your practice notes.

When would you prefer NOT NULL plus a default value?
How can a WHERE clause change left join behavior?
How would you test null-sensitive filters?

Continue practice

No explicit flow link is set yet, so these are nearby drills from the same path or taxonomy.

Avoid Fanout TotalsCoding problem / Medium / ~25 minContinue Daily Order KPIsCoding problem / Medium / ~25 minContinue Running BalanceCoding problem / Medium / ~25 minContinue

What can go wrong when filtering SQL rows that contain NULL values?

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue practice

What can go wrong when filtering SQL rows that contain NULL values?

Practice focus

Answer path

Why this question matters

Review signals

Follow-up prompts

Continue practice