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 (SELECT t.account_no FROM dual) AS account_no_temp,
                    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_temp = s.account_no_old AND t.contract_id = s.contract_id)
WHEN MATCHED THEN UPDATE SET t.account_no = s.account_no_new;