Does a SQL join only execute the minimum number of conditions? - sql

Question

In C# if I run the following.
if(obj.a() && obj.b()){
// do something
}
Function b will only execute if a returns true. Does the same thing happen below?
select
*
from
tablea a
inner join tableb b
isnumeric(b.col1) = 1
and cast(b.col1 as int) = a.id
Will the cast only be executed when b.col1 is a numeric?

Answer 1

You can simulate short-circuit evaluation using a CASE expression.
ON CASE WHEN ISNUMERIC(b.col1) = 1
THEN CAST(b.col1 AS int)
ELSE NULL
END = a.id

Answer 2

This covers short-circuat evaluation in SQL-Server deeply:
http://www.sqlservercentral.com/articles/T-SQL/71950/
In short: the evaluation order depends on the query optimizer.
Edit: As Martin commented this does not guarantee the order since it could also be optimized. From the above link(i should have read it completely):
When run against a SQL Server 2000, no error is thrown, but SQL Server
2005 and 2008 implement an optimization to push non-SARGable
predicates into the index scan from the subquery which causes the
statement to fail.
To avoid this issue, the query can be rewritten incorporating a CASE
expression, maybe a bit obscure, but guaranteed not to fail.
So this should guarantee that ISNUMERIC will be evaluated first:
SELECT aData.*,bData.*
FROM #TableA aData INNER JOIN #TableB bData
ON aData.id = CASE ISNUMERIC(bData.col1) WHEN 1 THEN CAST(bData.col1 AS INT) END
Ignore my first approach(which might not work everytime):
You should modify your join to ensure that is gets evaluated correctly:
SELECT aData.*,bData.*
FROM #TableA aData INNER JOIN
(
SELECT col1
FROM #TableB b
WHERE ISNUMERIC(b.col1) = 1
) AS bData
ON aData.id = CAST(bData.Col1 AS int)
Sample data:
create table #TableA(id int)
create table #TableB(col1 varchar(10))
insert into #TableA values(1);
insert into #TableA values(2);
insert into #TableA values(3);
insert into #TableA values(4);
insert into #TableB values('1');
insert into #TableB values('2');
insert into #TableB values(null);
insert into #TableB values('4abc');
SELECT aData.*,bData.*
FROM #TableA aData INNER JOIN
(
SELECT col1
FROM #TableB b
WHERE ISNUMERIC(b.col1) = 1
) AS bData
ON aData.id = CAST(bData.Col1 AS int)
drop table #TableA;
drop table #TableB;
Result:
id col1
1 1
2 2

Answer 3

From HERE:
No. Where the precedence is not determined by the Formats or by parentheses, effective evaluation of expressions is generally performed from left to right. However, it is implementation-dependent whether expressions are actually evaluated left to right, particularly when operands or operators might cause conditions to be raised or if the results of the expressions can be determined without completely evaluating all parts of the expression.