How to Short-Circuit SQL Where Clause - sql

Question

I am trying to perform the following query in SQL server:
declare #queryWord as nvarchar(20) = 'asdas'
SELECT * FROM TABLE_1
WHERE (ISDATE(#queryWord) = 1)
AND TABLE_1.INIT_DATE = CONVERT(Date, #queryWord)
This obviously causes an error because 'asdas' cannot be converted to Date. Although, I was expecting a different behavior. That is, because ISDATE(#queryWord) = 1 is false, I was expecting SQL to not check the second condition, but apparently, it does.
I know there are some other ways to perform this query but this is not my question. I wonder if there is some way to do not check the second condition is the first one does not satisfy. I am curious because I thought that SQL already did this.

Answer 1

SQL Server does not do short-circuiting (nor should it).
If you need it to not try something under some circumstances, you need to force that in the way that you write your query.
For this query the easiest fix would be to use a CASE expression in your WHERE clause.
declare #queryWord as nvarchar(20) = 'asdas'
SELECT * FROM TABLE_1
WHERE TABLE_1.INIT_DATE = (CASE WHEN ISDATE(#queryWord) = 1
THEN CONVERT(Date, #queryWord)
ELSE NULL END)
Off-hand, CASE and query-nesting are the only two supported ways that I can think of to force an order of evaluation for dependent conditions in SQL.

Answer 2

I Guess you could do it in 2 passes:
declare #queryWord as nvarchar(20) = 'asdas'
select
*
from
(
SELECT * FROM TABLE_1
WHERE (ISDATE(#queryWord) = 1) ) t1
where t1.INIT_DATE = CONVERT(Date, #queryWord)
So your inner query runs the first test and the outer query the second. In a single query, I don't believe there is any way to force any order of evaluating conditions.

Answer 3

Why not do a CASE in the WHERE condition?
DECLARE #tester TABLE (
theDate DATE,
theValue INT
)
INSERT INTO #tester VALUES ('2013-10-17', 35)
INSERT INTO #tester VALUES ('2013-10-16', 50)
INSERT INTO #tester VALUES ('2013-10-15', 2)
declare #queryWord as nvarchar(20) = 'asdas'
SELECT *
FROM #tester
WHERE theDate =
CASE
WHEN ISDATE(#queryWord) = 1 THEN CONVERT(Date, #queryWord)
ELSE theDate
END
SET #queryWord = '2013-10-17'
SELECT *
FROM #tester
WHERE theDate =
CASE
WHEN ISDATE(#queryWord) = 1 THEN CONVERT(Date, #queryWord)
ELSE theDate
END

Answer 4

It can be "simulated" with a CASE statement. But you have to make the first condition giving a TRUE value to avoid checking of the 2nd condition :
declare #queryWord as nvarchar(20) = 'asdas'
SELECT *
FROM TABLE_1
WHERE (CASE
WHEN ISDATE(#queryWord) = 0 THEN 0
WHEN TABLE_1.INIT_DATE = CONVERT(Date, #queryWord) THEN 1
ELSE 0 END) = 1

Answer 5

There is no defined evaluation order in a SQL statement -- except in the case of case expressions, and even there the order isn't so much defined as the result guaranteed. The conditions in your where clause could theoretically be done in parallel or alternating order.
Case expressions differ not by having a defined order, but by having a guaranteed result. IOW, case when 1=1 then 0 When longrunningfunction() = 1 then 2 end is guaranteed to return zero, but there is no promise not to run the longrunningfunction.