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最後の一文にさらっと書いてあるけど GROUP BY でも同じ事が起こるんだよな。自分は DISTINCT よりこっちをやってしまいがち

Don't use DISTINCT as a "join-fixer" - Simple Talk
https://www.red-gate.com/simple-talk/databases/sql-server/t-sql-programming-sql-server/dont-use-distinct-as-a-join-fixer/