Question: How many times did  li ju win the womens singles and wang liqin win the mens singles?
Schema: create table table_28138035_6 (womens_doubles varchar, womens_singles varchar, mens_singles varchar, PRIMARY KEY (womens_doubles))
SQL: select count(womens_doubles) from table_28138035_6 where womens_singles = "li ju" and mens_singles = "wang liqin"