Joins Quant SQL Interview Guide
Joins quant SQL interview guide for inner and outer joins, keys, duplicates, many-to-many joins, row-count checks, and examples.
Candidates querying trades, prices, users, or experiment tables.
Joins combine grains
A join combines rows from different tables, but the output grain depends on key uniqueness. Always ask whether each side has one row or many rows per key.
Join type changes missing data behavior
Inner joins drop unmatched rows, while left joins preserve rows from the left table. The right choice depends on whether unmatched records should remain visible.
Concrete example
Joining trades to daily prices by symbol and date can duplicate trades if the price table has multiple rows per symbol-date. Check row counts after the join.
Many-to-many joins are dangerous
A many-to-many join can explode row counts and inflate sums. Validate key uniqueness or aggregate to the intended grain before joining.
Common mistakes
Candidates often assume the join worked because it returned rows. In data interviews, row-count and unmatched-row checks are part of correctness.
Practice the pattern
Use the LeetQuidity curriculum and calibration to turn this topic into a focused practice plan.