mySQL查询在相同条件下检索数据的方式有所不同

Question

I am have made a chat application in Node-JS and Socket.io with MySQL database. I am done with everything but got stuck in one thing, that I cannot solve. When a user login to their account, they are redirected to their inbox and then the list of user he chatted with are displayed. For example the current user who logged in is John and he chatted with a person name Doe when John clicks the Doe name from the list to open the chats, at that time all the messages are retrieved from the database. The data is stored as Array of Objects in a variable data . What I did in the code that, I have looped through this data and appended each message to the chat box. Where all the messages displays. There are two categories in each message Message To and Message From . These categories are because to distinguish that who texted whom. And then I change the background color of these messages. The messages sent from my side will have background Grey and the person messaged me will have Blue background.

Now the basic problem I faced is, when the length of From Messages or To Messages are less than 13 then the chat is displayed in a sequence and each of the messages are on the right place. But when the length of any of the above messages exceeds 13 then what happens is, The messages I have sent are displayed first, and then the user who sent me the messages are displayed.

Messages from and Messages to have length less than 13 so its fine now

When it exceeds the length 13 then here first my messages are shown

And then the user sent to me are shown

The data format

The Script Displaying the chat

 var ac_user = data.active_user; socket.on("get_ret_messages",(data)=>{ while (chatBody.hasChildNodes()) { chatBody.removeChild(chatBody.firstChild); } for(let i = 0; i<data.length;i++){ if(data[i].mess_to == ac_user){ mess = '<div class="bubble you">'+data[i].mess_txt+'<br><span class="time">'+data[i].mess_time+'</span></div>'; chatBody.insertAdjacentHTML("beforeend",mess); } else{ mess = '<div class="bubble me">'+data[i].mess_txt+'<br><span class="time">'+data[i].mess_time+'</span></div>'; chatBody.insertAdjacentHTML("beforeend",mess); } } chatBody.scrollTop = chatBody.scrollHeight; }); 

The Query

 socket.on("get_user_messages",(data)=>{ var GET_CLICKED_USER_MESS = "SELECT mess_txt,mess_time,mess_to FROM (select mess_to as user_id,mess_txt,mess_time,mess_id,mess_to from messages where mess_from = '"+data.active+"' AND mess_to='"+data.clicked+"' UNION select mess_to as user_id,mess_txt,mess_time,mess_id,mess_to from messages where mess_from = '"+data.clicked+"' AND mess_to='"+data.active+"' ORDER BY mess_id ASC) sq join users on users.id = sq.user_id"; con.query(GET_CLICKED_USER_MESS,(err,res)=>{ if(err) throw err; socket.emit("get_ret_messages",res); }) }); 

I am quite sure that the problem can be in the query, but I don't know what is that.

Answer 1

For the little explanation: the ORDER BY must be placed at the end for it to be effective, or else it will onlt order the sub-query, and the order was probably messed up by the JOIN

the query:

"SELECT mess_txt,mess_time,mess_to FROM (SELECT mess_to AS user_id,mess_txt,mess_time,mess_id,mess_to FROM messages WHERE mess_from = '"+data.active+"' AND mess_to='"+data.clicked+"' UNION SELECT mess_to as user_id,mess_txt,mess_time,mess_id,mess_to FROM messages WHERE mess_from = '"+data.clicked+"' AND mess_to='"+data.active+"') sq JOIN users ON users.id = sq.user_id ORDER BY mess_time ASC"