Forward-Fill Missing FX Rates

SQL practice question · Hard · Data Cleaning · Oracle

The FX feed has NULL rates on some days, sometimes several days in a row. Fill each NULL with the most recent non-NULL rate of the same currency on an earlier date (last observation carried forward). NULLs before a currency's first known rate stay NULL. Return currency, rate_date, filled_rate (ROUND 4) and is_filled (1 when the original rate was NULL and a value was carried forward, else 0). Order by currency, rate_date.

Table: fx_rates

Schema of fx_rates
ColumnType
currencyTEXT
rate_dateDATE
rateREAL
Sample rows of fx_rates
currencyrate_daterate
USD2026-03-0283.1
USD2026-03-03NULL
USD2026-03-04NULL
USD2026-03-0583.25
EUR2026-03-0290.5
EUR2026-03-0390.75
EUR2026-03-04NULL
Expected output on the sample data
currencyrate_datefilled_rateis_filled
EUR2026-03-0290.50
EUR2026-03-0390.750
EUR2026-03-0490.751
USD2026-03-0283.10
USD2026-03-0383.11
USD2026-03-0483.11
USD2026-03-0583.250

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