Data Analytics Mock Interview: 20 Questions in 50 Minutes

COALESCE returns the first non-null value from its arguments, for example COALESCE(phone, alt_phone, 'Not given') fills missing phone numbers. NULLIF(a, b) returns NULL when a equals b, otherwise a. Its classic use is avoiding division by zero: revenue / NULLIF(orders, 0) returns NULL instead of an error when orders is zero. Together they handle most null-related reporting problems cleanly.

Flash Fill (Ctrl+E) detects a pattern from one or two examples you type and fills the rest of the column, for example extracting first names or reformatting phone numbers. It is fast for one-time cleanup. However, it produces static values, not formulas, so it will not update when source data changes, and it can guess wrongly on irregular rows. For repeatable reports, use formulas or Power Query instead.

The population is the whole group of interest; a sample is a subset used to estimate population values.

A primary key uniquely identifies each row in its table; a foreign key references another table's primary key to enforce relationships.

match_mode decides how a value is matched: 0 exact (default), -1 exact or next smaller, 1 exact or next larger, and 2 wildcard. search_mode decides direction: 1 searches first to last, -1 searches last to first, and 2 or -2 use binary search on sorted data. Searching last to first is a quick way to fetch the latest entry for a customer without sorting, and next-smaller matching handles tax slabs or commission bands.

Goal Seek changes one input to hit a target output; Solver optimises with several variables and constraints, for example maximising profit under budget limits.

Columnar storage keeps all values of one column together on disk instead of storing full rows together. Analytical queries usually read a few columns across millions of rows, so the engine reads only those columns. Similar values sit side by side and compress very well, which cuts storage and scan cost. The trade-off is slower single-row inserts and updates, which is why OLTP databases stay row-based.

A table auto-expands, supports structured references, banded rows and slicers, and keeps formulas consistent as rows are added.

A chi-square test of independence checks whether two categorical variables are related, using a contingency table of counts. Example: is preferred payment method related to city tier? It compares observed counts with the counts expected if there were no relationship. Use it with counts, not percentages, and make sure expected counts are not too small, roughly at least 5 per cell. A significant result shows association, not causation.

A transaction is a group of SQL statements that succeed or fail together, such as debiting one account and crediting another. ACID: Atomicity means all or nothing; Consistency means rules and constraints stay valid; Isolation means concurrent transactions do not see each other's partial work; Durability means committed data survives crashes. Analysts mostly read data, but knowing ACID explains why reports should read committed data.

Grouping users by a shared start date or trait and tracking behaviour over time, for example monthly retention per signup month.

I would compare channels on incremental return, not last-click ROAS. Gather spend, attributed conversions under two models, customer quality such as repeat rate and margin, and any lift test or mix model results. Look at diminishing returns: the last rupee in a saturated channel is worth less than the average. Then recommend cutting where marginal return is lowest, ideally phased with a holdout to confirm the impact.

Use RFM scoring or clustering on behaviour and value, validate segments with business teams and attach an action to each segment.

First check whether it is a tracking problem: did the analytics tag break on some templates, did consent settings change, does server log or backend order data show the same fall? If the drop is real, check the search console for impressions and rankings, recent site releases such as changed URLs, missing redirects, noindex tags or robots.txt changes, and whether a search algorithm update or seasonal pattern matches. Then break the drop down by page group.

Start from the business questions, show 4–6 top KPIs with trend vs target, allow drill-downs, keep colours consistent, remove clutter, add time/region filters and give one clear insight per visual.

Separate input cells from calculation cells and colour inputs clearly. Unlock only the input cells through Format Cells, Protection, then protect the sheet so formulas cannot be edited, and optionally hide formulas. Add data validation on inputs to block wrong values such as negative quantities. Protect workbook structure so sheets cannot be deleted, keep a version history, and add a short instructions sheet so users know where to type.

Situation: while building a returns report I noticed one warehouse had zero returns for three months. Task: confirm whether it was real. Action: I compared with the source system and found its return codes had changed after a software update, so my filter missed them. I informed the data owner, updated the mapping and back-filled the history. Result: the corrected report showed that warehouse had the highest return rate, which led to a packaging review.

Situation: a regional manager asked for a salary-level export of all employees to plan hiring. Task: help without breaking data policy. Action: I clarified the actual need, which was headcount cost by grade and location, checked with HR on access rules, and built a Power BI view with aggregated costs and row-level security limiting each manager to their region. Result: the manager got what was needed and no individual salaries were exposed.

Situation: our team moved 15 Excel and older BI reports into Tableau. Task: I owned the sales set. Action: I listed every metric with its definition, rebuilt the shared data source first, then the dashboards, and ran side-by-side number checks for two weeks with the old reports. I held short demos for users. Result: all reports went live with matching numbers, and refresh effort fell from a day to an automatic schedule.

Situation: a teammate's incentive calculation sheet was about to go to payroll. Task: I was asked to review it. Action: I noticed totals did not match the sales extract, traced it to a SUM range that stopped at row 500 while data had grown to 640 rows, and told the teammate privately with the fix. Result: we converted the data to an Excel Table so ranges expand, and payouts went out correctly.