Interview and refresher notes
Explain the term before you explain the syntax.
A compact reference for students, experienced analysts refreshing a topic, and applicants preparing for technical interviews. The aim is to help you describe a concept clearly in your own words before showing code.
Python
Key terms worth being able to explain clearly in an interview or technical discussion.
- Mutable vs immutable
- Mutable objects can change after creation, such as lists and dictionaries. Immutable objects, such as strings, integers and tuples, cannot be changed in place.
- List vs tuple
- A list is mutable and is used when values may change. A tuple is immutable and is useful for fixed collections or records.
- == vs is
- == compares values. is compares object identity, meaning whether two names refer to the same object.
- Iterable vs iterator
- An iterable can produce values one at a time. An iterator is the object that keeps track of the current position during iteration.
- Generator
- A generator produces values lazily, usually with yield, so large sequences do not have to be stored fully in memory.
- Shallow vs deep copy
- A shallow copy copies the outer container but shares nested objects. A deep copy recursively copies nested objects too.
- loc vs iloc
- pandas loc selects by labels; iloc selects by integer positions.
- Overfitting
- A model overfits when it learns training data too closely and performs poorly on new data.
R
Key terms worth being able to explain clearly in an interview or technical discussion.
- Vector
- The basic one-dimensional R structure. All elements in an atomic vector have the same basic type.
- List
- A flexible object that can contain items of different types, including vectors, data frames and other lists.
- Factor
- A categorical variable stored with defined levels. Useful when categories have meaning or ordering.
- Data frame vs tibble
- Both store tabular data. Tibbles are a tidyverse-friendly form with clearer printing and stricter behaviour.
- NA vs NULL vs NaN
- NA means a missing value; NULL usually means absence of an object or element; NaN is a special numeric value meaning 'not a number'.
- Vectorisation
- Applying an operation to an entire vector or column instead of writing an explicit loop.
- Pipe
- |> passes the result on the left into the next function, making multi-step workflows easier to read.
- Reproducibility
- The ability for someone else, or future you, to rerun the same analysis and obtain the same result.
SQL
Key terms worth being able to explain clearly in an interview or technical discussion.
- View
- A named SELECT query that can be queried like a virtual table. A normal view stores the query definition rather than a separate copy of the rows; some database systems also support materialised or indexed views.
- Stored procedure
- A named executable routine stored in the database. It can accept parameters and contain multiple SQL statements and control-flow logic. Exact syntax and capabilities depend on the database system.
- CTE
- A Common Table Expression is a named result set defined with WITH and used within the scope of the statement that follows. In SQL Server it is not materialised as a separate table by default.
- Temporal table
- A table designed to preserve row history over time. In SQL Server, a system-versioned temporal table keeps current data plus historical row versions so you can query an earlier point in time.
- Normalisation
- A relational database design process that reduces unnecessary duplication and update anomalies by putting facts into appropriate related tables.
- Denormalisation
- An intentional choice to combine or repeat data to reduce joins or improve some read-heavy workloads, accepting more duplication and maintenance risk.
- ROW_NUMBER
- Assigns a unique sequential number to each row within a partition according to the ORDER BY in the OVER clause.
- RANK
- Gives tied rows the same rank and leaves gaps afterwards: 1, 2, 2, 4.
- DENSE_RANK
- Gives tied rows the same rank but does not leave gaps: 1, 2, 2, 3.
- Primary key
- A column or combination of columns chosen to uniquely identify each row in a table.
- Foreign key
- A column or combination of columns that references a key in another or the same table and can enforce referential integrity.
- Index
- A database structure that can speed up retrieval for suitable query patterns. It uses storage and adds maintenance work when data changes.
Excel
Key terms worth being able to explain clearly in an interview or technical discussion.
- Workbook
- The Excel file that can contain one or more worksheets.
- Worksheet
- A single sheet inside a workbook.
- Range
- A group of cells, for example A2:D100.
- Excel Table
- A structured range with headers, automatic expansion and structured references.
- Relative reference
- A reference such as A2 that changes when a formula is copied.
- Absolute reference
- A reference such as $A$2 that stays fixed when copied.
- PivotTable
- A tool for interactively summarising and grouping data.
- Power Query
- A repeatable data-import and transformation tool.
- Power Pivot
- Excel's Data Model layer for relationships between tables and DAX measures.
- Volatile function
- A function that recalculates frequently, such as NOW, TODAY, RAND, OFFSET or INDIRECT, which can affect performance in large workbooks.
Power BI
Key terms worth being able to explain clearly in an interview or technical discussion.
- Fact table
- A table of measurable events, such as sales transactions, survey responses or appointments.
- Dimension table
- A descriptive table used to group and filter facts, such as Date, Product, Region or Customer.
- Star schema
- A model where fact tables connect to dimension tables in a simple, predictable pattern.
- Import mode
- Data is loaded into the Power BI model, usually giving fast interactive performance.
- DirectQuery
- Queries are sent back to the source at report time instead of importing all data into the model.
- Measure
- A DAX calculation evaluated at query time in the current filter context.
- Calculated column
- A DAX expression computed row by row and stored in the model.
- RLS
- Row-level security restricts which rows different users can see.
- Workspace
- A collaborative area in Power BI Service where content is developed and managed.
- App
- A curated package of Power BI content distributed to consumers from a workspace.
DAX
Key terms worth being able to explain clearly in an interview or technical discussion.
- Filter context
- The set of filters affecting a calculation, created by visuals, slicers, filters and DAX expressions.
- Row context
- The current row being evaluated, common in calculated columns and iterator functions.
- CALCULATE
- Evaluates an expression after modifying filter context. It is the central context-changing function in DAX.
- Context transition
- When CALCULATE converts row context into filter context.
- Iterator
- A function such as SUMX or AVERAGEX that evaluates an expression row by row over a table.
- Measure branching
- Building advanced measures from simpler base measures instead of repeating the same logic.
- ALL / REMOVEFILTERS
- Functions used to clear filters. REMOVEFILTERS is often clearer when the intention is simply to remove filtering.
- RELATED
- Returns a value from the one-side of an existing relationship while row context is available.
- SELECTEDVALUE
- Returns a single visible value when exactly one exists, otherwise an alternate result.
- Time intelligence
- Calculations that compare or accumulate values across dates, usually using a dedicated Date table.
No matching glossary terms were found. Try a broader search term.