使用 SQL 查询从表中选择数据

Question

I have a table name "chat_details" I wanna access only data with green underline according to time, I use the following query

//suppose $user_id = 1;

"SELECT * 
FROM chat_details WHERE from_user_id='$user_id' OR to_user_id='$user_id' ORDER BY time DESC"

It fetch all the rows because all rows contain user_id = 1 in one of the column, but i need only green underline rows as compare to red one because green one are latest according to time(column), how can i fetch only these green underlines?

Answer 1

This is one posible query, that should solve your question.

  SELECT * 
  FROM chat_details c 
  WHERE (c.from_user_id='$user_id' OR c.to_user_id='$user_id') 
  AND NOT EXISTS (
        SELECT 1 FROM chat_details d 
        WHERE d.from_user_id = c.from_user_id 
        AND d.to_user_id = c.to_user_id 
        AND d.time > c.time)
  ORDER BY c.time DESC"

Actually I could not test, hope I haven't made a mistake.

The query selects all data as in your query, but only thoose which haven't a newer chat between the two users. For the EXISTS keyword see http://www.mysqltutorial.org/mysql-exists/