What is the difference between the IN operator and = operator in SQL? - sql

Question

I am just learning SQL, and I'm wondering what the difference is between the following lines:
WHERE s.parent IN (SELECT l.parent .....)
versus
WHERE s.parent = (SELECT l.parent .....)

Answer 1

IN
will not generate an error if you have multiple results on the subquery. Allows to have more than one value in the result returned by the subquery.
=
will generate an error if you have more than one result on the subquery.
SQLFiddle Demo (IN vs =)

Answer 2

when you are using 'IN' it can compare multiple values....like
select * from tablename where student_name in('mari','sruthi','takudu')
but when you are using '=' you can't compare multiple values
select * from tablenamewhere student_name = 'sruthi'
i hope this is the right answer

Answer 3

The "IN" clause is also much much much much slower. If you have many results in the select portion of
IN (SELECT l.parent .....),
it will be extremely inefficient as it actually generates a separate select sql statement for each and every result within the select statement ... so if you return 'Cat', 'Dog', 'Cow'
it will essentially create a sql statement for each result... if you have 200 results... you get the full sql statement 200 times...takes forever... (This was as of a few years ago... maybe imporved by now... but it was horribly slow on big result sets.)
Much more efficient to do an inner join such as:
Select id, parent
from table1 as T
inner join (Select parent from table2) as T2 on T.parent = T2.parent

Answer 4

For future visitors.
Basically in case of equals (just remember that here we are talking like where a.name = b.name), each cell value from table 1 will be compared one by one to each cell value of all the rows from table 2, if it matches then that row will be selected (here that row will be selected means that row from table 1 and table 2) for the overall result set otherwise will not be selected.
Now, in case of IN, complete result set on the right side of the IN will be used for comparison, so its like each value from table 1 will be checked on whether this cell value is present in the complete result set of the IN, if it is present then that value will be shown for all the rows of the IN’s result set, so let say IN result set has 20 rows, so that cell value from table 1 will be present in overall result set 20 times (i.e. that particular cell value will have 20 rows).
For more clarity see below screen shot, notice below that how complete result set from the right of the IN (and NOT IN) is considered in the overall result set; whole emphasis is on the fact that in case comparison using =, matching row from second table is selected, while in case of IN complete result from the second table is selected.

Answer 5

In can match a value with more than one values, in other words it checks if a value is in the list of values so for e.g.
x in ('a', 'b', 'x') will return true result as x is in the the list of values
while = expects only one value, its as simple as
x = y returns false
and
x = x returns true

Answer 6

The general rule of thumb is:
The = expects a single value to compare with. Like this:
WHERE s.parent = 'father_name'
IN is extremely useful in scenarios where = cannot work i.e. scenarios where you need the comparison with multiple values.
WHERE s.parent IN ('father_name', 'mother_name', 'brother_name', 'sister_name')
Hope this is useful!!!

Answer 7

IN
This helps when a subquery returns more than one result.
=
This operator cannot handle more than one result.
Like in this example:
SQL>
Select LOC from dept where DEPTNO = (select DEPTNO from emp where
JOB='MANAGER');
Gives ERROR ORA-01427: single-row subquery returns more than one row
Instead use
SQL>
Select LOC from dept where DEPTNO in (select DEPTNO from emp
where JOB='MANAGER');

Answer 8

1) Sometimes = also used as comparison operator in case of joins which IN doesn't.
2) You can pass multiple values in the IN block which you can't do with =. For example,
SELECT * FROM [Products] where ProductID IN((select max(ProductID) from Products),
(select min(ProductID) from Products))
would work and provide you expected number of rows.However,
SELECT * FROM [Products] where ProductID = (select max(ProductID) from Products)
and ProductID =(select min(ProductID) from Products)
will provide you 'no result'. That means, in case subquery supposed to return multiple number of rows , in that case '=' isn't useful.