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SQL percentiles for skewed delivery times
A percentile summarizes a position in a distribution, but its exact value depends on the calculation convention. On a small dataset, two legitimate percentile definitions can give visibly different answers. State the met
SQL recursive CTEs for an organization hierarchy
A recursive common table expression walks relationships repeatedly: start with an anchor row, find its children, then find their children until no further eligible rows remain. An employee-to-manager table is a useful ex
SQL row grain: stop double-counting orders before you query
A SQL row's grain is the real-world thing that one row represents. An order table may contain one row per order; an order-item table contains one row per item line. Joining those tables changes the rows you are adding. I
SQL running totals: choose the correct window frame
A running total depends on both ordering and the window frame: the set of rows included for each result. A row-by-row cumulative amount is different from a cumulative amount that includes every peer sharing the current d
SQL slowly changing dimensions in a historical customer report
A customer can change region while their earlier purchases remain unchanged. Joining every purchase to today's customer record reallocates historical sales to the current region. If the report asks where the customer bel
SQL subqueries versus CTEs: make an audit-friendly query
A subquery is a query used within another SQL statement. A common table expression, or CTE, gives a query result a name within a statement's WITH clause. Both can express intermediate steps. Choose the form that makes th
SQL text cleaning: normalize categories without merging different people
Text cleaning should preserve the meaning required by the analysis. Trimming whitespace and mapping approved category variants can improve a region report. Applying the same aggressive transformation to customer names or
SQL versus pandas versus Spark: choose from measured constraints
SQL, pandas and Spark overlap in transformation capability but run in different environments. Choose from data size, existing platform, concurrency, latency, reliability and team skill. Distributed syntax is not evidence
Standard deviation versus standard error with a simulation
Standard deviation describes variation among observations. The standard error of a mean describes variation in the estimated mean across repeated samples under a specified sampling model. They have the same units but ans
Stationarity: what differencing changes and what it loses
Many time-series models assume that relevant statistical behavior is stable over time. Differencing is one way to remove a changing level: replace each observation with the change from the previous observation. It can ma
Stemming versus lemmatization for a specific task
Stemming removes character suffixes using rules; lemmatization maps inflected forms to a dictionary base form, often using part of speech. Both reduce vocabulary, but both can merge distinctions or create awkward outputs
Stockout analysis with incomplete availability data
Zero sales do not prove a stockout. Customers may not have wanted the item, the store may have been closed, or the sales feed may be incomplete. Likewise, a missing availability record is not evidence that stock was avai
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