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SQL practice question · Hard · Window Functions · UnitedHealth

service_contacts has one row per support call (patient_id, call_date, call_id). Count calls per patient and calendar month ('YYYY-MM'). For every patient-month whose immediately preceding calendar month also has at least one call for that patient, report the month-over-month change: pct_change = 100 * (calls this month - calls previous month) / calls previous month. Months whose previous calendar month had no calls are not reported. Output columns: patient_id, month, pct_change (rounded to 2 decimals). Order by patient_id, then month.

Table: service_contacts

Schema of service_contacts
ColumnType
patient_idINTEGER
call_dateDATE
call_idINTEGER
Sample rows of service_contacts
patient_idcall_datecall_id
1012026-01-021
1012026-01-102
1012026-02-013
1012026-02-054
1022026-01-035
1022026-02-026
Expected output on the sample data
patient_idmonthpct_change
1012026-020
1022026-020

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