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scripts/check-duplicate-user-emails.sql

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+-- Run this BEFORE applying migration 20260817000000_case_insensitive_user_email.
+--
+-- Finds User rows whose emails are identical except for casing (e.g.
+-- "Name@x.de" and "name@x.de"). The migration creates a unique index on
+-- lower(email) and will fail to apply if any such duplicates exist.
+--
+-- Usage:
+--   npx prisma db execute --file scripts/check-duplicate-user-emails.sql --schema prisma/schema.prisma
+-- or paste directly into psql / your DB console.
+--
+-- If this returns any rows, resolve them manually first — e.g. decide which
+-- account is canonical, merge or reassign its CourseBooking/UserTraining
+-- rows to that account, then delete or rename the duplicate — before
+-- applying the migration.
+
+SELECT
+  LOWER(email) AS normalized_email,
+  array_agg(id) AS user_ids,
+  array_agg(email) AS original_emails,
+  count(*) AS duplicate_count
+FROM "User"
+GROUP BY LOWER(email)
+HAVING count(*) > 1;