SQL DISTINCT ON 取消多列重复字段值
products表里取消用户ID,产品ID重复值，，，，，distinct on(列名1，列名2，，，)
例子：select distinct on (user_id,product_id),product_price from products ...............
products表里取消用户ID,产品ID重复值，，，，，distinct on(列名1，列名2，，，)
例子：select distinct on (user_id,product_id),product_price from products ...............
