Database Administrator exam mistakes to avoid

Updated 17 September 2026 · 3 min read

Database candidates usually know more than their marks suggest. The losses come from reading SQL casually, assuming an index will be used, forgetting what a transaction leaves behind, and treating backup as a schedule rather than a recovery. Each has a fix.

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Reading joins carelessly

An inner join hides rows that do not match, a left join keeps them with nulls, and moving a condition from the join clause to the where clause turns a left join into an inner one. Questions are built on exactly these differences. Sketch two small tables and trace the rows before answering.

Assuming the index will be used

Creating an index does not mean the planner will use it. A function around the column, a leading wildcard, a mismatched data type, or statistics that no longer describe the data will all send the query back to a scan. When a question says a query is slow despite an index, the answer is usually one of those.

Forgetting what a transaction holds

Long transactions hold locks, and locks block other work. Candidates answer questions about blocking by tuning the blocked query rather than shortening the blocking transaction. Ask what the transaction is holding and for how long before you look at the victim.

Confusing isolation levels

Read committed, repeatable read and serializable differ in what they prevent and what they cost. Questions describe a symptom, such as a report that sees different totals when run twice within one transaction, and expect you to name the level that fixes it without locking the whole system.

Treating a backup as protection

A backup on the same disk as the database, or a log chain broken by a manual truncation, or a schedule nobody has tested, is not protection. Test answers consistently favour verified, off-server restores over a more frequent but unverified schedule.

Confusing delete, truncate and drop

The three remove data in different ways and leave different things behind, and questions exploit that constantly. One removes rows and can be filtered and rolled back inside a transaction, one empties a table quickly and resets far less gracefully, and one removes the table itself along with its structure. Know which of them can be undone, which fires triggers and which resets an identity column, because a scenario about recovering ten rows has a very different answer from one about recovering a table.

Optimising before measuring

Options that add indexes, change hardware or rewrite queries before anyone looks at a plan are traps. The expected first step is to measure: which query, which plan, how many rows, and what changed.

Questions where these mistakes cost marks

A technician has just finished replacing a faulty power supply in a user's PC and verified that it boots successfully into Windows. What is the FINAL step of the CompTIA troubleshooting methodology?
1. Document findings, actions, and outcomes.
2. Implement the solution.
3. Test the theory.
4. Establish a theory of probable cause.
Show the answer
A. Document findings, actions, and outcomes.
The final step in the troubleshooting process is always to document the issue, what was done to fix it, and the final outcome for future reference.
Original practice question, written against published exam objectives. Open this question
You run sfc /scannow and it reports Windows Resource Protection found corrupt files but was unable to fix some of them. What is the NEXT best step?
1. Run the DISM tool to repair the Windows image.
2. Boot into Safe Mode and run it again.
3. Reinstall Windows completely.
4. Run chkdsk /r.
Show the answer
A. Run the DISM tool to repair the Windows image.
DISM (Deployment Image Servicing and Management) can repair the underlying Windows component store that SFC uses to fix corrupted files.
Original practice question, written against published exam objectives. Open this question
Which of the following is considered a strong password?
1. P@ssw0rd!
2. password
3. qwerty
4. 12345678
Show the answer
A. P@ssw0rd!
Of the four, only P@ssw0rd! mixes character classes. In real use it is still weak, because it is a dictionary word with predictable substitutions - length and unpredictability matter more than symbol count.
Real past-paper question; answer cross-checked against the original session. Open this question

Common questions

How can I get better at reading execution plans?

Read plans for queries you wrote yourself on your practice database. Change one thing at a time, add an index or rewrite a join, and see the plan respond. Plans make sense much faster when you caused the change.

How much of the test is SQL writing?

It varies, but expect to produce at least simple queries. Practise on paper, because a blank page is harder than an editor.