A guide to hiring an XQuery developer
XQuery is a language for asking questions of XML. If your company keeps contracts, articles, product records, orders or reference data as XML documents, XQuery is how you search them, join them and rebuild them into something new. It often sits in the background of publishing systems, document stores and data feeds, so the people who know it well are hard to find. The sections below explain the work, the signs of a strong candidate and the habits that make remote work go well. They are written for founders and team leads who inherited an XML system or need to get more out of one.
What XQuery does
A query describes what you want, and the database or processor works out how to get it. You can pull titles from every document that meets a rule, join a list of customers with a list of orders from another file, or group records and count them. The result can be a list, a report or a brand new XML document in the layout another system expects. Newer versions of the language can also read and write JSON, which helps when old and new systems have to talk.
How it relates to XPath and XSLT
XQuery uses XPath to point at parts of a document, so any XQuery developer is comfortable with paths. A XPath developer can be enough when the job is only to select values for data collection or tests. XSLT is a sibling language that turns one document into another, and it shines when the output looks like a page. XQuery shines when you search across many documents, join sources and group results. Strong candidates know all three and tell you which to use.
The jobs people hire for
One common job is writing new queries for reports, exports and search. Another is repair. A query is slow, returns too little, or fails when a document uses a namespace nobody expected. A third is migration. Teams move data from one XML database to another, or from XML to a relational store, and need someone who can read the old queries and rebuild them. Each of these needs patience with detail more than clever tricks.
When an XQuery developer makes sense
- Your content or records live in an XML database and nobody on the team knows the query language.
- Reports take too long, or only one person can change them.
- You need to convert XML into JSON, HTML or another XML layout for a partner or a new system.
- You plan to move away from an older XML platform and need its queries understood first.
- You exchange data with others in XML and want the checking and conversion to be reliable.
If your data is mostly in tables, a SQL specialist is the better start. A developer who tells you so before you spend money is a good sign.
Read the work, not only the resume
Ask a candidate to show queries they wrote, with the question each one answered. Good examples are short, named clearly and followed by a note on what they return. Pay attention to how they describe trouble. A person who can explain why a query was slow, and what changed it, has seen real data. A list of tools with no story behind it tells you much less.
Questions to ask in interviews
- How would you find all documents that contain a value in one place but not in another?
- What do you check first when a query returns nothing on a document that clearly has the data?
- How do you decide between XQuery and XSLT for a conversion job?
- What do you look at when a query is slow in the database you use?
- How do you change many documents safely?
Good answers are specific and include real cases. Be wary of anyone who only describes the case where everything works.
Try a paid mini project
Give each finalist one small piece of real work from your own project. Good examples are a report across a sample set of documents, a conversion from your XML layout to JSON, or one slow query that needs speeding up. Use sample data you are free to share. Pay for the time spent. Then compare how readable the query is, how the person handles odd documents and how well they explain their choices.
Agree how the work is kept alive
Queries tend to outlive the person who wrote them. Agree where they are stored, how they are tested and who reviews a change. Ask for a short note beside each query that says what it expects and what it returns. Keep everything in a repository your company owns, and keep a small set of sample documents that any new team member can run the queries against.
Working with your team
Walk your developer through how the data is created, who reads the results and what breaks when a document is malformed. A brief weekly meeting and one shared list of tasks keep the whole team on the same page. Share the oddest documents early, because they tend to set the real size of the job.
Common mistakes
- Testing a query only on a handful of clean documents.
- Skipping a backup before a bulk change.
- Putting all the logic in one long query that nobody dares to touch.
- Ignoring namespaces until the output comes back empty.
Looking at a wider role? Take a look at our API developers for data exchange between systems, or our back-end developers for the server side.