Kusto Query Language (KQL)

What's inside

What is KQL and where is it used?

Kusto Query Language (KQL) is used in Azure Monitor, Log Analytics, Microsoft Sentinel, and Application Insights to query log and telemetry data.

How is KQL different from SQL?

KQL is a read-only query language optimized for logs and telemetry data, whereas SQL supports full CRUD operations.

What is the purpose of the 'where' clause in KQL?

To filter records based on a specified condition.

What does the 'project' operator do in KQL?

It selects specific columns to return from the dataset.

What does 'summarize' do?

Aggregates data using functions like count(), avg(), sum(), and groups by specified columns.

What is the use of the 'extend' operator?

Adds calculated columns to a result set.

How do you perform joins in KQL?

Using the 'join' operator with kinds like inner, leftouter, rightouter, etc.

What is the 'let' statement in KQL?

Used to declare variables or reusable expressions.

What is 'mv-expand' used for?

To expand arrays or dynamic fields into individual records.

What is the use of the 'render' operator?

To visualize data, e.g., render timechart, barchart, piechart.

What does 'project-away' do?

Removes specified columns from the output.

How do you limit the number of rows returned?

Use the 'take' or 'limit' operator.

What does 'distinct' do?

Returns unique combinations of the selected columns.

What is the 'union' operator used for?

Combines rows from multiple tables or queries.

How do you sort results in KQL?

Use 'sort by' or 'order by'.

What is 'bin()' used for?

To group time or numeric values into buckets.

How do you filter data by time?

Use the 'where' clause with time filters like TimeGenerated > ago(1h).

What is 'parse_json()' used for?

parse_json(), also available as its alias todynamic(), converts a string containing JSON into a dynamic object.

What does 'isnull()' or 'isnotnull()' do?

Checks whether a field has or doesn't have a null value.

What does 'project-rename' do?

Renames columns in the output.

What does 'project-reorder' do?

Changes the column order in the output.

What is 'materialize()' used for?

Caches intermediate query results for reuse in the same query.

How do you use 'search' in KQL?

Performs a global text search across one or more tables.

How do you group data in KQL?

Use 'summarize' with one or more grouping fields.

What's the difference between 'has', 'contains', and '=='?

'has' checks for word presence, 'contains' for substring, '==' for exact match.

What does 'has_all()' do?

Returns rows where the text column contains all specified values.

What's the use of 'case()' in KQL?

Implements conditional branching like an if-else block.

What does 'in' do in KQL?

Checks whether a value exists in a given list.

What's the difference between 'count' and 'countif'?

'count' counts all rows; 'countif' counts rows meeting a condition.

What does the 'top' operator do?

Returns the top N rows sorted by a specified column.

What is 'range' used for in KQL?

Generates a sequence of values (e.g., for dates or numbers).

How do you merge time-series data from multiple sources?

Use 'union', then 'summarize' and 'bin' for time grouping.

How do you perform a case-insensitive search?

Use '=~' or 'tolower()' to normalize the case.

How do you detect performance anomalies?

Use 'summarize avg()', 'percentiles()', or 'stdev()', then 'render timechart'.

What does 'serialize' do?

Preserves row order across pipeline stages.

What is 'datetime_diff()'?

Returns the difference between two datetime values.

What does 'project-keep' do?

Keeps only the listed columns.

How do you parse a URL in KQL?

Use 'parse_url()' or regex with 'extract()'.

What is the difference between 'parse' and 'extract'?

'parse' is more readable for structured strings, 'extract' uses regex.

How do you check if a field exists?

Use isnotempty() or isnull(), or gettype() to inspect a dynamic field's type.

How do you join across large datasets efficiently?

Use 'hint.shufflekey' or pre-aggregate data before joining.

How do you filter for rows with missing fields?

Use 'where isnull(field)' or 'where field == ""'.

What is 'toscalar()' used for?

Extracts a single value to use in a filter or comparison.

What's the difference between 'invoke' and 'execute'?

'invoke' calls functions in KQL; 'execute' is not used in KQL.

How do you detect spikes in logs?

Use 'summarize count() by bin(TimeGenerated, x)' and 'render timechart'.

How do you visualize time series comparisons?

Use 'make-series' to prepare data, then 'render timechart'.

What is 'datatable' used for?

Creates inline tables useful for testing or examples.

How do you create a reusable KQL function?

Use the 'let' keyword with parameters.

What is 'arg_max()' and 'arg_min()'?

Returns rows with the max or min value for a group.

What does 'hint.shufflekey' do?

Optimizes large joins by distributing based on a key.

How can you trace an operation across tables?

Use 'operation_Id' or 'session_Id' to correlate across logs.

What does 'explain' do in KQL?

'explain' prefixed to a SQL statement translates it into KQL. It does not show an execution plan.

How do you find failed logins in the last 24h?

SecurityEvent | where EventID == 4625 and TimeGenerated > ago(24h)

How do you find top 10 IP addresses by request volume?

AppRequests | summarize count() by client_IP | top 10 by count_

What if a query times out?

Reduce time range, use filters early, avoid full-table scans.

How do you create a saved query?

Use 'Save' in Logs UI or define in Workbook or Analytics Rule.

How do you build alerts from queries?

Create a query > New alert rule > Define logic and threshold.

What's the difference between 'render timechart' and 'barchart'?

Timechart shows trends over time, barchart compares static values.

How do you join multiple services' logs?

Use 'join' with common fields like user_Id, session_Id.

How do you parse structured logs?

Use 'parse', 'parse_json', or 'extract'.

How do you compare app performance before and after a change?

Summarize by time range (e.g., avg(duration)) and compare time bins.

How do you validate query accuracy?

Test with known values, validate against source, and use sample datasets.

How do you optimize queries for speed?

Use 'summarize' early, filter as early as possible, reduce joins.

What's your approach to debugging KQL queries?

Start small, add filters incrementally, and verify intermediate output.

How does KQL fit into observability?

Helps monitor system health, detect anomalies, and perform root cause analysis.

How do you stay current with KQL developments?

Microsoft Docs, GitHub samples, KQL tutorials, and hands-on labs.

What are the most powerful features of KQL?

Time-series aggregation, fast queries, dynamic parsing, joins.

How would you explain KQL to a non-technical person?

A way to search through logs using filters and rules.

What are common KQL mistakes?

Incorrect joins, not filtering enough, large dataset queries without bins.

What is the 'parse' operator used for?

To extract structured fields from unstructured text.

How do you concatenate strings in KQL?

Use strcat() or strcat_array() functions.

How do you convert data types in KQL?

Use toint(), tostring(), todatetime(), etc.

What does 'mv-apply' do?

Applies a subquery to each element of a multi-value array.

How do you calculate percentiles in KQL?

Use percentile(column, percent) or percentiles() functions.

What is 'make-series'?

Creates a series of data points grouped by time or other dimensions.

How do you pivot data in KQL?

Use 'evaluate pivot()' operator.

How do you perform a regex match in KQL?

Use 'matches regex' operator.

How do you handle time zones in KQL?

KQL datetimes are UTC. Use datetime_utc_to_local() and datetime_local_to_utc() for zone conversion, and format_datetime() for display.

How do you limit resource usage in complex queries?

Use 'set query_take_max_records' or optimize filters.

What is the use of 'bag_unpack()'?

Unpacks dynamic objects into separate columns.

How do you use 'row_number()' in KQL?

Assigns unique sequential numbers to rows.

How do you parameterize KQL queries?

Use 'let' statements and query parameters in workbooks or apps.

How do you extract nested fields from JSON?

Use dot notation like dynamicField.subField.

How do you find anomalies in data?

Use 'series_decompose_anomalies()' or statistical functions.

How do you write recursive or iterative queries in KQL?

KQL does not support recursion; use multiple queries or functions.

How do you use 'invoke' to call user-defined functions?

Invoke calls a function by name with parameters.

How do you detect repeated errors across services?

summarize count() by error_message | where count_ > 1

How do you handle dynamic or semi-structured logs?

Use dynamic data types and parsing functions.

How do you export query results from KQL?

Use Azure Portal export options or APIs.

How do you optimize queries for cost?

Filter early, reduce data scanned, use summarize and bins.

How do you identify slow-running queries?

Use the portal's query diagnostics, or .show queries in Azure Data Explorer, and filter by duration.

How do you schedule KQL queries?

Use Azure Monitor Workbooks, Logic Apps, or Automation.

How do you detect brute force attacks using KQL?

Look for multiple failed logins from same IP in short time.

How do you track user behavior over time?

Use session or user IDs with summarize and time bins.

How do you correlate alerts from different sources?

Join on common IDs or timestamps, aggregate alert types.

How do you analyze multi-tenant data securely?

Filter by tenant ID and apply RBAC on queries.

How do you use KQL with Azure Sentinel?

Use KQL to create analytic rules, hunting queries, and workbooks.

How do you monitor API calls in logs?

Filter API operation names and analyze counts or latencies.

How do you troubleshoot failed queries?

Check syntax, resource limits, and query execution plan.

How do you count the elements of a dynamic array in KQL?

Use array_length(arr). There is no mv-count() function.

How do you extract parts of a string using KQL?

Use substring(), extract(), or parse operators.

How do you compare two timestamps for ordering?

Use datetime comparison operators like <, >, ==.

What does 'hasprefix' do?

'hasprefix' matches a term prefix inside the text; 'startswith' matches the start of the whole string.

How do you filter data by IP address range?

Use 'ipv4_is_in_range()' function.

What is the difference between 'parse' and 'extractjson()'?

'parse' extracts data with pattern matching; 'extractjson()' extracts JSON values.

How do you create custom columns with calculations?

Use 'extend' operator with expressions.

How do you find unique users per day?

Summarize count of distinct user IDs grouped by bin(TimeGenerated, 1d).

How do you identify sessions that lasted longer than X minutes?

Calculate session duration using datetime_diff() and filter accordingly.

How do you use 'prev()' and 'next()' functions?

Access previous or next row values for trend or pattern detection.

How do you detect missing data points in time series?

make-series emits a point for every interval, using its default value where data is missing; series_fill_linear() or series_fill_const() fill the gaps.

How do you handle large result sets?

KQL has no skip operator. Page with serialize plus row_number() ranges, narrow the time window, or export the results.

What is the difference between 'top' and 'top-nested'?

'top' returns top rows by a column; 'top-nested' returns top N per group.

How do you work with hierarchical data in KQL?

Recursive queries are not supported; flatten the data or use dynamic objects.

What is 'row_cumsum()' used for?

Calculates cumulative sum over ordered rows.

How do you convert UNIX timestamps to datetime?

Use 'unixtime_seconds_todatetime()' or similar functions.

How do you calculate moving averages?

KQL has no moving_avg(). Build a series with make-series, then apply series_fir() for a moving average.

How can you anonymize data in KQL?

Use hashing functions like 'hash_sha256()'.

How do you monitor log ingestion status?

Query ingestion logs or monitoring tables for ingestion success or failures.