Untitled

-- Обновляем client_name в #KP_CommonClientsOkved из #reg
WITH CTE_UpdateClientName AS (
    SELECT 
        kpc.client_inn_dil, 
        reg.KONTRAGENT_NAIMENOVANIE AS client_name
    FROM #KP_CommonClientsOkved kpc
    LEFT JOIN #reg reg ON kpc.client_inn_dil = reg.KONTRAGENT_INN
    WHERE reg.KONTRAGENT_NAIMENOVANIE IS NOT NULL
)
UPDATE kpc
SET kpc.client_name_dil = cte.client_name
FROM #KP_CommonClientsOkved kpc
INNER JOIN CTE_UpdateClientName cte ON kpc.client_inn_dil = cte.client_inn_dil;

-- проверка 
SELECT * FROM #KP_CommonClientsOkved WHERE #KP_CommonClientsOkved.client_inn_dil = '0275918765'
SELECT * FROM #reg WHERE #reg.KONTRAGENT_INN = '0275918765'