Difference between EXISTS and IN in SQL?

Question

What is the difference between the EXISTS and IN clause in SQL?

When should we use EXISTS, and when should we use IN

Answer 1

If you are using the IN operator, the SQL engine will scan all records fetched from the inner query. On the other hand if we are using EXISTS, the SQL engine will stop the scanning process as soon as it found a match.

Answer 2

Based on rule optimizer:

• EXISTS is much faster than IN, when the sub-query results is very large.
• IN is faster than EXISTS, when the sub-query results is very small.

Based on cost optimizer:

• There is no difference.

Answer 3

In certain circumstances, it is better to use IN rather than EXISTS. In general, if the selective predicate is in the subquery, then use IN. If the selective predicate is in the parent query, then use EXISTS.

https://docs.oracle.com/cd/B19306_01/server.102/b14211/sql_1016.htm#i28403

Answer 4

IN:

• Works on List result set
• Doesn’t work on subqueries resulting in Virtual tables with multiple columns
• Compares every value in the result list
• Performance is comparatively SLOW for larger result set of subquery

EXISTS:

• Works on Virtual tables
• Is used with co-related queries
• Exits comparison when match is found
• Performance is comparatively FAST for larger result set of subquery

Answer 5

1. EXISTS is much faster than IN when the subquery results is very large.
   IN is faster than EXISTS when the subquery results is very small.

   CREATE TABLE t1 (id INT, title VARCHAR(20), someIntCol INT)
   GO
   CREATE TABLE t2 (id INT, t1Id INT, someData VARCHAR(20))
   GO
   
   INSERT INTO t1
   SELECT 1, 'title 1', 5 UNION ALL
   SELECT 2, 'title 2', 5 UNION ALL
   SELECT 3, 'title 3', 5 UNION ALL
   SELECT 4, 'title 4', 5 UNION ALL
   SELECT null, 'title 5', 5 UNION ALL
   SELECT null, 'title 6', 5
   
   INSERT INTO t2
   SELECT 1, 1, 'data 1' UNION ALL
   SELECT 2, 1, 'data 2' UNION ALL
   SELECT 3, 2, 'data 3' UNION ALL
   SELECT 4, 3, 'data 4' UNION ALL
   SELECT 5, 3, 'data 5' UNION ALL
   SELECT 6, 3, 'data 6' UNION ALL
   SELECT 7, 4, 'data 7' UNION ALL
   SELECT 8, null, 'data 8' UNION ALL
   SELECT 9, 6, 'data 9' UNION ALL
   SELECT 10, 6, 'data 10' UNION ALL
   SELECT 11, 8, 'data 11'
   

2. Query 1

   SELECT
   FROM    t1 
   WHERE   not  EXISTS (SELECT * FROM t2 WHERE t1.id = t2.t1id)
   

   Query 2

   SELECT t1.* 
   FROM   t1 
   WHERE  t1.id not in (SELECT  t2.t1id FROM t2 )
   

   If in t1 your id has null value then Query 1 will find them, but Query 2 cant find null parameters.

   I mean IN can't compare anything with null, so it has no result for null, but EXISTS can compare everything with null.

Answer 6

IN supports only equality relations (or inequality when preceded by NOT).
It is a synonym to =any / =some, e.g

select    * 
from      t1 
where     x in (select x from t2)
;

EXISTS supports variant types of relations, that cannot be expressed using IN, e.g. -

select    * 
from      t1 
where     exists (select    null 
                  from      t2 
                  where     t2.x=t1.x 
                        and t2.y>t1.y 
                        and t2.z like '℅' || t1.z || '℅'
                  )
;

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And on a different note -

The allegedly performance and technical differences between EXISTS and IN may result from specific vendor's implementations/limitations/bugs, but many times they are nothing but myths created due to lack of understanding of the databases internals.

The tables' definition, statistics' accuracy, database configuration and optimizer's version have all impact on the execution plan and therefore on the performance metrics.