Mysql php private email like messaging system - php

Question

I'm building a small messaging system for my app, the primary idea is to have 2 tables.
Table1 messages
Id,sender_id,title,body,file,parent_id
Here is where messages are stored, decoupled from whom will receive it to allow for multiple recipients.
Parent I'd link to parent message if its a reply, and file is a blob to store single file attached to message
Table 2 message_users
Id,thread_id,user_id,is_read,stared,deleted
Link parent thread to target users,
Now for a single user to get count of unread messages I can do
Select count(*) from message_users where user_id = 1 and is_read is null
To get count of all messages in his inbox I can do
Select count(*) from message_users where user_id = 1;
Question is how to combine both in single optimized query ?

Answer 1

So you're trying to achieve something that will total rows that meet one condition and the total rows that meet an extra condition:
|---------|---------|
| total | unread |
|---------|---------|
| 20 | 12 |
|---------|---------|
As such will need something with a form along the lines of:
SELECT A total, B unread FROM message_users WHERE user_id=1
A is fairly straightforward, you already more-or-less have it: COUNT(Id).
B is marginally more complicated and might take the form SUM( IF(is_read IS NULL,1,0) ) -- add 1 each time is_read is not null; the condition will depend on your database specifics.
Or B might look like: COUNT(CASE WHEN is_read IS NULL THEN 1 END) unread -- this is saying 'when is_read is null, count another 1'; the condition will depend on your database specifics.
In total:
SELECT COUNT(Id) total, COUNT(CASE WHEN is_read IS NULL THEN 1 END) unread FROM message_users WHERE user_id=1
Or:
SELECT COUNT(Id) total, SUM( IF(is_read IS NULL,1,0) ) unread FROM message_users WHERE user_id=1
In terms of optimised, I'm not aware of a query that can necessarily go quicker than this. (Would love to know of it if it does exist!) There may be ways to speed things up if you have a problem with performance:
Examine your indexes: use the built in tools EXPLAIN and some reading around etc.
Use caches and/or pre-compute the value and store it elsewhere -- e.g. have a field unread_messages against user and grab this value directly. Obviously there will need to be some on-write invalidation, or indeed some service running to keep these values up to date. There are many ways of achieving this, tools in MySQL, hand roll your own etc etc.
In short, start optimising from a requirement and some real data. (My query takes 0.8s, I need the results in 0.1s and they need to be consistent 100% of the time -- how can I achieve this?) Then you can tweak and experiment with the SQL, hardware that the server runs on (maybe?), caching/pre-calculate at different points etc.

Answer 2

In MySQL, when you count a field, it only counts non null occurrences of that field, so you should be able to do something like this:
SELECT COUNT(user_id), COUNT(user_id) - COUNT(is_read) AS unread
FROM message_users
WHERE user_id = 1;
Untested, but it should point you in the right direction.

Answer 3

You can use sum with CASE WHEN clause. If is_read is null then +1 is added to the sum, else +0.
SELECT count(*),
SUM(CASE WHEN is_read IS NULL THEN 1 ELSE 0 END) AS count_unread
FROM message_users WHERE user_id = 1;