Oracle 11g Release 2

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CREATE TABLE v (
  contract_id INT NOT NULL PRIMARY KEY,
  tenant_id INT NOT NULL
);
CREATE TABLE t (
  t_id INT NOT NULL PRIMARY KEY,
  -- tenant_id column is missing here
  account_no INT NOT NULL,
  contract_id INT NOT NULL REFERENCES v
);
CREATE TABLE u (
  u_id INT NOT NULL PRIMARY KEY,
  tenant_id INT NOT NULL,
  account_no_old INT NOT NULL,
  account_no_new INT NOT NULL,

  UNIQUE (tenant_id, account_no_old)
);
    MERGE INTO (SELECT t.account_no, t.contract_id 
                    FROM t) t
        USING (
          SELECT u.account_no_old, u.account_no_new, v.contract_id
          FROM u, v
          WHERE v.tenant_id = u.tenant_id
        ) s
        ON ((t.account_no, 'dummy') = ((s.account_no_old, 'dummy')) AND t.contract_id = s.contract_id) 
        WHEN MATCHED THEN UPDATE SET t.account_no = s.account_no_new;
ORA-38104: Columns referenced in the ON Clause cannot be updated: "T"."ACCOUNT_NO"