What's the difference of SELECT and SELECT IN in sql? - sql

Question

The difference of the two statements?
Is the second statemnet faster than the first statement?
First statement :
SELECT * FROM students WHERE id = 2197176;
SELECT * FROM students WHERE id = 74877;
Second statement:
SELECT * FROM students WHERE id IN(2197176, 74877, ...)
UPDATE:
If the time complexity of first statement is m*n, will the second statement be m*n?
m: the time complexity of SELECT * FROM students WHERE id = 2197176;.
n: the amount of ids.
UPDATE:
In the following two cases, which case is faster? And why?
Assuming the table is as follows:
| ID | FLAG |
| ----------|:------:|
| 2197176 | true |
| 74877 | false |
First case:
List ids = getIds();
for(id in ids){
result = getResultFromFirstStatement(id); //one sql statement
if(result.flag) { do sth ...}
}
Second case:
List ids = getIds();
results = getResultFromSecondStament(ids); //`n` sql statements
for(r in results){
if(r.flag) { do sth ...}
}

Answer 1

I ran execution plan on 3 different queries.
First query: Using UNION
Second query: Using UNION ALL
Third query: Using IN
USE AdventureWorksLT2012
-- First query using UNION
SELECT ProductID, Name FROM SalesLT.Product WHERE ProductID = 716
UNION
SELECT ProductID, Name FROM SalesLT.Product WHERE ProductID = 727
UNION
SELECT ProductID, Name FROM SalesLT.Product WHERE ProductID = 770
-- Second query using UNION ALL
SELECT ProductID, Name FROM SalesLT.Product WHERE ProductID = 716
UNION ALL
SELECT ProductID, Name FROM SalesLT.Product WHERE ProductID = 727
UNION ALL
SELECT ProductID, Name FROM SalesLT.Product WHERE ProductID = 770
-- Third query using IN
SELECT ProductID, Name FROM SalesLT.Product WHERE ProductID IN(716, 727, 770)
As you can see the UNION is using 53% (Because UNION tries to delete duplicates), UNION ALL is costing 34% and IN costs 14% of whole batch

Answer 2

First query
SELECT * FROM students WHERE id = 2197176 ..
returns rows with an id column value equal with specific value in this case 2197176 multiple select returns union of results.
In the second query
SELECT * FROM students WHERE id IN (2197176, 74877, ...);
returns rows where the id column value equals with 2197176 or 74877 or ... .
In equal parameter result of both query are the same records, but in readability and performance second query are better.

Answer 3

IN (val1,val2,val3, ...) is an abbreviated form of filtering predicates inside a WHERE clause and not directly related to the select keyword.
SELECT
column,list, ...
FROM
table
JOIN othertables ON ...
WHERE
table.id IN (1,2,3)
This could be re-written as:
SELECT
column,list, ...
FROM
table
JOIN othertables ON ...
WHERE
( table.id = 1
OR table.id = 2
OR table.id = 3
)

Answer 4

If the time complexity of first statement is m*n, will the second statement be m*n?
m: the time complexity of SELECT * FROM students WHERE id = 2197176;.
n: the amount of ids.
No. The second statement will be the same complexity as the first, but with a larger constant for n.
HOWEVER, if you have an index on the table for ID then complexity of the first is 1 and the second is n. (Which is still the same since n = 1 in the first case.)