SQL

Combine two tables with INNER JOIN and LEFT JOIN while preserving the intended rows.

Diagnose and correct unintended row multiplication by reasoning about table grain and join cardinality.

Find records with or without related matches using EXISTS and NOT EXISTS.

Structure a multi-step analysis with common table expressions while avoiding unnecessary nesting.

Group and compare time-based data using the corresponding PostgreSQL and MySQL date functions.

Handle NULL values correctly in calculations, comparisons, aggregates, and output using SQL NULL semantics.

Select the top N rows within each group using ROW_NUMBER, RANK, and DENSE_RANK.

Compare each row with prior rows using LAG to calculate changes over time.

Solve a multi-table SQL interview exercise by defining the result grain, addressing edge cases, and validating the final query.