difference between where_in and find_in_set

Question

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<

Answer 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