Pure-reference catalog of row/tenant isolation mechanisms across four database engines: MySQL and MariaDB (no native RLS - views with SQL SECURITY INVOKER plus app-layer enforcement), CockroachDB (native RLS via ALTER TABLE ENABLE ROW LEVEL SECURITY and CREATE POLICY, matching Postgres semantics), Vitess (keyspace sharding + vindexes route tenant writes to dedicated shards without a policy layer), and SQL Server (CREATE SECURITY POLICY with inline table-valued function filter/block predicates). Covers the isolation mechanism, tenant-context pattern, bypass risks, and test patterns for each engine. Use when designing or auditing tenant isolation on MySQL, MariaDB, CockroachDB, Vitess, or SQL Server.