difference between where_in and find_in_set

问题

I am working on a join query in which I have to get the data from a column which contain the comma separated values.like allowed_activity contain 1,2,3,4,5,6 this the activity_id which is allowed.

So In the query, I am checking that current activity is allowed or not and for that, I have used where_in and also tried find_in_set in where condition.

Here is that part of the query:

$this->db->where_in('a.allowed_activity',$activity_id);
//And With FIND_IN_SET
$this->db->where("FIND_IN_SET($activity_id, a.allowed_activity) !=",0);

Problem or Confusion

When I use where_in it doesn't give me the result. But if I use FIND_IN_SET then it will return the desired result. I thought that where_in can do the task but it didn't.

Just Want to know why find_in_set working differently than where_in Because of both the function work same as finding the data is in comma separated list.

Here is the complete query:

SELECT mycolumns
FROM `table1` as `ea`
LEFT OUTER JOIN `table2` as `a` ON `ea`.`package_id` = `a`.`id`
LEFT OUTER JOIN `table3` as `e` ON `ea`.`employer_id` = `e`.`id`
WHERE `ea`.`card_id` > 0
AND FIND_IN_SET(6, a.allowed_activity) !=0
AND `ea`.`id` = '6'
ORDER BY `ea`.`creation_date` DESC

I am using Codeigniter Active record.

回答1:

WHERE IN requires the set of values to be specified literally in the query, not as a single value containing a comma-delimited string. If you write:

WHERE 6 IN (a.allowed_activity)

it will treat a.allowed_activity as just a single value, and compare it with 6, not as a set of multiple values to search.

FIND_IN_SET searches a comma-delimited string for the value.

Another way to view it is that IN is a shortcut for a bunch of = tests combined with OR:

WHERE x IN (a, b, c, d)

is short for

WHERE x = a OR x = b OR x = c OR x = d

When you rewrite it like this, you can see clearly why it won't work with a column containing a comma-delimited string. It simply translates

WHERE 6 IN (a.allowed_activity) 

to:

WHERE 6 = a.allowed_activity

回答2:

The following code is incorrect:

$this->db->where("FIND_IN_SET($activity_id, a.allowed_activity) !=",0);

I'm surprised this even runs without error, but one possibility is that it is generating a WHERE clause which compares a string against 0. Of course, they will never be equal, and so this will always be true.

If you want to call the native FIND_IN_SET() function, then you would need to use whereRaw():

$this->db->whereRaw("FIND_IN_SET($activity_id, a.allowed_activity) !=0");

Using WHERE IN (...) is incorrect here, because you have all the values in a single row, separated from commas. WHERE IN would make sense if you wanted to filter for certain values from many rows, each column having a single value.

回答3:

FIND_IN_SET search for a value in a array and return the index of the value then if the function is different than 0 this mean that the value in present in the list . so the code

  this->db->where("FIND_IN_SET(6, a.allowed_activity) !=",0);

mean return the index of the value 6 in a.allowed_activity
and if the value return is different that 0 then the value 6 is present in a.allowed_activity

WHERE ... IN check if the value in the set but don't return the index .. just return true or false if the value is present or not

来源：https://stackoverflow.com/questions/45729241/difference-between-where-in-and-find-in-set