There is no difference between those two statements, and the optimiser will transform the IN to the = when IN has just one element in it.
Though when you have a question like this, just run both statements, run their execution plan and see the differences. Here – you won’t find any.
After a big search online, I found a document on SQL to support this (I assume it applies to all DBMS):
If there is only one value inside the parenthesis, this commend [sic] is equivalent to,
WHERE "column_name" = 'value1
Here is the execution plan of both queries in Oracle (most DBMS will process this the same):
EXPLAIN PLAN FOR
select * from dim_employees t
where t.identity_number="123456789"
Plan hash value: 2312174735
-----------------------------------------------------
| Id | Operation | Name |
-----------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | TABLE ACCESS BY INDEX ROWID| DIM_EMPLOYEES |
| 2 | INDEX UNIQUE SCAN | SYS_C0029838 |
-----------------------------------------------------
And for IN() :
EXPLAIN PLAN FOR
select * from dim_employees t
where t.identity_number in('123456789');
Plan hash value: 2312174735
-----------------------------------------------------
| Id | Operation | Name |
-----------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | TABLE ACCESS BY INDEX ROWID| DIM_EMPLOYEES |
| 2 | INDEX UNIQUE SCAN | SYS_C0029838 |
-----------------------------------------------------
As you can see, both are identical. This is on an indexed column. Same goes for an unindexed column (just full table scan).