Try

SELECT 
    string, 
    CASE 
        WHEN COUNT(*) = 0 THEN 0  -- Handle division by zero
        ELSE (SUM(CASE WHEN adm = 2 THEN 1 ELSE 0 END)::decimal / COUNT(*)) * 100 
    END AS perc_adm
FROM 
    your_table
GROUP BY 
    string;