CREATE TABLE table_204_639 (id NUMBER, "poll company" CLOB, "source" CLOB, "publication date" CLOB, "psuv" NUMBER, "opposition" NUMBER, "undecided" NUMBER)

which source was used before may 2010 ?
SELECT "source" FROM table_204_639 WHERE "publication date" < (SELECT "publication date" FROM table_204_639 WHERE "publication date" = 'may 2010')
SELECT "source" FROM "table_204_639" WHERE "publication date" < (SELECT "publication date" FROM "table_204_639" WHERE "publication date" = 'may 2010')