SELECT AVG("Social Democratic Party") FROM table_39090 WHERE "Green" < '0'
Which Social Democratic Party has a Green smaller than 0?
CREATE TABLE table_39090 (
    "Election" real,
    "Labour" real,
    "Conservative" real,
    "Liberal" real,
    "Social Democratic Party" real,
    "Social and Liberal Democrats/ Liberal Democrats" real,
    "Independent" real,
    "Green" real,
    "Other" text,
    "Control" text
)