KQL Where Clause: Filter Records from Last 30 Minutes
Exhibit
KQL draft: Heartbeat | __________ | summarize LastSeen = max(TimeGenerated) by Computer Requirement: show only records from the last 30 minutes before summarizing.
Based on the exhibit, which KQL clause should replace the blank to show only heartbeat records from the last 30 minutes?
Quick Answer
The answer is `where TimeGenerated >= ago(30m)`. This KQL where clause filter for the last 30 minutes works by using the `ago()` function, which dynamically calculates a datetime exactly 30 minutes before the current moment, and the `>=` operator ensures only records with a `TimeGenerated` timestamp at or after that point are returned. On the AZ-104 exam, this tests your ability to write time-based filters in Kusto Query Language, often appearing in log analytics scenarios where you must isolate recent heartbeat or health data. A common trap is confusing `ago()` with `now()` or forgetting the comparison operator—using `>` instead of `>=` would exclude records exactly 30 minutes old. Memory tip: think of `ago(30m)` as “30 minutes ago from now,” and always pair it with `>=` to catch everything from that moment forward.
⚠ Common exam trap
Candidates often confuse filtering (`where`) with projection (`project`), sorting (`sort`), or extending (`extend`), and may choose a clause that manipulates the output format or order instead of actually restricting the rows based on a time condition.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
where TimeGenerated >= ago(30m)
The KQL clause `where TimeGenerated >= ago(30m)` filters the results to include only records where the `TimeGenerated` timestamp is within the last 30 minutes. The `ago()` function calculates a datetime value relative to the current time, and the `>=` operator ensures only records from that point forward are returned. This directly satisfies the requirement to show heartbeat records from the last 30 minutes.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
project Computer, TimeGenerated
Why it's wrong here
Project changes the columns returned, but it does not limit the records by time.
When this WOULD be correct
This option would be correct in a question that asks: 'Which KQL clause should be used to display only the Computer and TimeGenerated columns from heartbeat records?' where the goal is column selection, not time filtering.
- ✓
where TimeGenerated >= ago(30m)
Why this is correct
The where clause filters rows before summarizing, and ago(30m) is the KQL function that represents the last 30 minutes from the current time. This is the correct way to restrict the Heartbeat table to recent records before calculating the most recent check-in for each computer. It is a standard operational troubleshooting pattern in Log Analytics.
- ✗
extend TimeWindow = 30m
Why it's wrong here
Extend creates a new calculated column, but it does not filter the table to recent records.
When this WOULD be correct
A question asking to add a column showing a 30-minute time window for each record, such as 'Add a column named TimeWindow that contains the value 30m for all records.'
- ✗
sort by TimeGenerated desc
Why it's wrong here
Sorting changes the order of results, but it does not exclude older heartbeat records from the query.
When this WOULD be correct
If the question asked 'Which clause should replace the blank to display heartbeat records in descending order of time?' then `sort by TimeGenerated desc` would be correct.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The AZ-104 exam frequently reuses these exact scenarios with slightly different constraints.
✓where TimeGenerated >= ago(30m)Correct answer▾
Why this is correct
The where clause filters rows before summarizing, and ago(30m) is the KQL function that represents the last 30 minutes from the current time. This is the correct way to restrict the Heartbeat table to recent records before calculating the most recent check-in for each computer. It is a standard operational troubleshooting pattern in Log Analytics.
✗project Computer, TimeGeneratedWrong answer — click to see why▾
Why this is wrong here
The 'project' operator only selects columns to display, it does not filter records by time. The question requires filtering heartbeat records to those from the last 30 minutes, which requires a 'where' clause with a time condition.
★ When this WOULD be the correct answer
This option would be correct in a question that asks: 'Which KQL clause should be used to display only the Computer and TimeGenerated columns from heartbeat records?' where the goal is column selection, not time filtering.
Why candidates choose this
Candidates may confuse the 'project' operator with filtering, thinking that selecting specific columns implicitly limits the data, or they may misread the question as asking about which columns to show rather than which records to keep.
✗extend TimeWindow = 30mWrong answer — click to see why▾
Why this is wrong here
The `extend` operator adds a calculated column but does not filter records; it would not limit results to the last 30 minutes.
★ When this WOULD be the correct answer
A question asking to add a column showing a 30-minute time window for each record, such as 'Add a column named TimeWindow that contains the value 30m for all records.'
Why candidates choose this
Candidates may confuse adding a time-related column with filtering by time, or think `extend` can implicitly filter data.
✗sort by TimeGenerated descWrong answer — click to see why▾
Why this is wrong here
The `sort by` clause only orders results but does not filter them; it cannot limit records to the last 30 minutes.
★ When this WOULD be the correct answer
If the question asked 'Which clause should replace the blank to display heartbeat records in descending order of time?' then `sort by TimeGenerated desc` would be correct.
Why candidates choose this
Candidates may confuse sorting with filtering, thinking that sorting by time and then taking the top results implicitly filters to recent data.
Analysis generated from the official AZ-104blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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Same concept, more angles
1 more way this is tested on AZ-104
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Based on the exhibit, which KQL operator should replace the blank to return only those columns?
easy- A.where, because it filters rows and also selects the visible columns.
- B.summarize, because it groups the failed records into a smaller result set.
- ✓ C.project, because it returns only the named columns in the result.
- D.extend, because it creates new output columns for the selected fields.
Why C: The `project` operator in Kusto Query Language (KQL) is specifically designed to select a subset of columns from the input table, returning only the named columns in the result set. This matches the requirement to 'return only those columns,' making option C correct.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This AZ-104 practice question is part of Courseiva's free Microsoft certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the AZ-104 exam.