Overlapping Licence Subscriptions

SQL practice question · Hard · Joins · Microsoft

Find double-billing risk: pairs of subscriptions of the SAME tenant whose periods overlap. Periods are inclusive of both start_date and end_date; end_date NULL means still active and is treated as '2026-12-31' (the reporting date). Return tenant_id, sub_id_1 (smaller id), sub_id_2 (larger id) and overlap_days (number of calendar days covered by both, inclusive). Order by tenant_id, sub_id_1, sub_id_2.

Table: m365_subscriptions

Schema of m365_subscriptions
ColumnType
sub_idINTEGER
tenant_idTEXT
planTEXT
start_dateDATE
end_dateDATE
Sample rows of m365_subscriptions
sub_idtenant_idplanstart_dateend_date
1T1E32026-01-012026-03-31
2T1E52026-03-152026-06-30
3T1Teams Essentials2026-07-01NULL
4T2E32026-02-012026-02-28
5T2E32026-03-012026-03-31
6T3Business Basic2026-01-10NULL
7T3Business Premium2026-05-012026-05-20
Expected output on the sample data
tenant_idsub_id_1sub_id_2overlap_days
T11217
T36720

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