MySQL 不返回相同的结果

Question

I have a table with list of customers:

customer

c_id    c_name      c_email     c_role
1       abc1        a1@abc.com  Dev
2       abc2        a2@abc.com  Dev
3       abc3        a3@abc.com  Dev
4       abc4        a4@abc.com  Dev
5       abc5        a5@abc.com  Dev
6       abc6        a6@abc.com  Dev
7       abc7        a7@abc.com  Dev
8       abc8        a8@abc.com  Dev
9       abc9        a9@abc.com  Dev

I query the table in the following way:

select * from customer where c_role = 'Dev' order by c_id limit 2;

So, I get the results with:

c_id    c_name      c_email     c_role
1       abc1        a1@abc.com  Dev
2       abc2        a2@abc.com  Dev

The business requirements says that if any records are accessed by a set of users for last 3 days, then those should not return in the subsequent query output.

So, if the user runs a query again for the next 3 days:

select * from customer where c_role = 'Dev' order by c_id limit 2;

The result should be:

c_id    c_name      c_email     c_role
3       abc3        a3@abc.com  Dev
4       abc4        a4@abc.com  Dev

Can anyone help me how to create this kind of rule in MySQL?

Answer 1

Adding a new column in current table is not going to help you.

You will have to create another table where you store all c_ids a user has accessed and the datetime when the query was executed.

CREATE TABLE IF NOT EXISTS `access_record` (
    `id` INT(11) NOT NULL AUTO_INCREMENT ,
    `c_id` INT(11) NOT NULL , // id of the record which user accessed
    `user_id` INT(11) NOT NULL , // id of the user who accessed the record
    `accessed_at` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ,
    PRIMARY KEY (`id`)
);

So whenever the user runs the next query you can use this table to know if user has already accessed a record or not and then use those c_ids to exclude them from next result set.

SELECT
    c.c_id, c.c_role,c.c_name,c.c_email
FROM
    customer AS c
WHERE
    c.c_role = 'Dev'
    AND c.c_id NOT IN (
        SELECT
            ar.c_id
        FROM
            access_record AS ar
        WHERE ar.user_id = 1  // ofcourse this will change with each user (current user in your case I assume)
            AND ar.accessed_at > DATE_SUB(NOW(), INTERVAL 3 DAY)
     )
     ORDER BY c.c_id LIMIT 2;

This will give you all records which were not accessed by specific user within last 3 days.

I hope this helps.

Answering @dang's question in comment

How do I populate access_record when a query runs?

When you have fetched all the records then you extract c_ids from those records then you insert those c_ids into the access_record table.

In MYSQL this query should do the trick

INSERT INTO access_record (c_id,user_id)

SELECT
    c.c_id, 1 // user_id of the user who is fetching records
FROM
    customer AS c
WHERE
    c.c_role = 'Dev'
    AND c.c_id NOT IN (
        SELECT
            ar.c_id
        FROM
            access_record AS ar
        WHERE ar.user_id = 1  // ofcourse this will change with each user (current user in your case I assume)
            AND ar.accessed_at > DATE_SUB(NOW(), INTERVAL 3 DAY)
     )
     ORDER BY c.c_id LIMIT 2;

You can also fetch those c_ids with one query then use second query to insert those c_ids into the access_record table.

If you have all your records fetched in $records then

$c_ids = array_column($temp, 'c_id'); // get all c_ids from fetched record array

Now run a query to insert all those c_ids.

Answer 2

I would add an extra table with users and accessdate. And make the business logic update those on access. For example:

user | accessdate | c_id

Answer 3

Your 'customer' table is data about customers. That is all it should have.

However, your selection criteria is really not what it appears to be . What business requirements want you to implement is a feed, or pipeline, with the selection acting as a consumer, being fed by un-accessed customers.

Each user (or group of users, ie: 'set of users') needs it's own feed, but that can be managed by a single table with a distinguishing field. So we need a user_group table to group your 'set of users'.

// user_group
g_id    g_data
201     abc1
202     abc2
203     abc3

We will need to populate customer_feed with the access timestamps for each customer. We can add a foreign keys to delete customers and user_groups when they are deleted, but we will need update the customer feed when we use it.

create table customer_feed (
  c_id int(11) not null,
  g_id int(11) not null,
  at timestamp not null,
  primary key (c_id, g_id),
  constraint customer_fk foreign key (c_id) references customer on delete cascade
  constraint user_group_fk foreign key (g_id) references user_group on delete cascade,
);

// customer_feed
c_id    g_id   at
101     201    2018-11-26 07:40:21
102     201    2018-11-26 07:40:21
103     201    2018-11-26 07:40:22

When we want to read the customer data, we must do three queries:

1. Update the feed for the current user-group.
2. Get the users from the feed
3. Mark the users in the feed as consumed.

So, let's say we are using user_group 201.

When we update the feed, any new users to the feed are available to be read straight away,we give them a timestamp which is very early. So we can commemorate the battle of hastings...

// 1. update the customer_feed for user_group 201
insert into customer_feed(select c.c_id,201,TIMESTAMP('1066-10-14')
from customer c left join customer_feed f 
on c.c_id = f.c_id and f.g_id=201 where f.c_id is null);

we select from the customer and the feed... We only accept records whose access dates are less than three days ago. This is the query you originally had, but with the feed restrictions.

// 2. read from feed for user_group 201
select c.* from customer c,customer_feed f where c.c_role='Dev'
and f.g_id=201 and f.c_id=c.c_id 
and f.at < date_sub(now(), interval 3 day) limit 2

..and now we need to mark the values from the feed as being consumed. So, we gather the c_id we have selected into a list of c_id, eg: '102,103', and we mark them as consumed.

// 3. Mark the feed as consumed for user_group 201
update customer_feed set at=now() where g_id=201 and c_id in '102,103'

Answer 4

Add a new column in your customer table like start_accessing and then you can run the query:

SELECT * 
    FROM   customer 
    WHERE  c_role = 'Dev' 
           AND Date_add(start_accessing, INTERVAL 3 day) >= Curdate() 
    ORDER  BY c_id 
    LIMIT  2; 

start_accessing will be the column that will save when the user started accessing the resource.

Answer 5

Add a datetime stamp to the table and query from that.

Answer 6

There might be a way to get a 3 day rotation without having to change the tables.

By calculating batches of devs.
And calculate the current dev batch based on the current date.

The example below is for MySql 7.x (no window functions)

set @date = current_date;
-- set @date = date('2020-07-04');

set @dayslimit = 3;
set @grouplimit = 2;
set @devcnt = (select count(*) cnt 
       from test_customer 
        where c_role = 'Dev');
set @curr_rnk = ((floor(datediff(@date, date('2000-01-01')) / @dayslimit)%floor(@devcnt / @dayslimit))+1);

select c_id, c_name, c_email, c_role
-- , rnk
from
(
   select t.*, 
    case when @rn := @rn +1 then @rnk := ceil((@rn/@grouplimit)%(@devcnt+1/@grouplimit)) end as rnk
   from test_customer t
   cross join (select @rn:=0, @rnk:= null) vars
   where c_role = 'Dev'
   order by c_id
) q
where rnk = @curr_rnk
order by c_id
limit 2;

A test on rextester here