How do the SQL "IS" and "=" operators differ?
Solution 1
You want records from Foo where Bar = @param, or if @param is null, where Bar is null. Some of the proposed solutions will give you null records with nonnull @param, which does not sound like your requirement.
Select * from Foo where (@param is null and Bar is null) or (Bar = @param)
This doesn't say whether this is Oracle or SQL Server or another RDBMS, because they each implement slightly different helper functions. SQL's ISNULL(first, second) like NVL(first, second). I like SQL Server's COALESCE() for the general applicability.
The IS comparison is only for null comparisons.
If you are using SQL Server and if you really need a different 3VL logic truth table to solve your problem (that is, if you have a specific need for "NULL=NULL" to be "true" at some point in time, and also recognize that this is deprecated and barring your reasons, not a good idea in general), within your code block you can use the directive
SET ANSI_NULLS OFF
Here's the BOL on it: http://msdn.microsoft.com/en-us/library/ms188048.aspx
Solution 2
You may be thinking about this incorrectly. If you're talking about SQL Server, for example (since that's what I have to hand), your second example will result in a syntax error. The value on the right-hand side of IS cannot be 5.
To explain, consider MSDN's explanation of these two operators in T-SQL (note that asking about "SQL" and about "SQL Server" are not necessarily the same).
Notice something important, there. There is no such thing as the "IS" operator in T-SQL. There is specifically the <expression> IS [NOT] NULL
operator, which compares a single expression to NULL.
That's not the same thing as the =
operator, which compares two expressions to each other, and has certain behavior when one or both of the expressions happens to be NULL!
Solution 3
Edit: (Update from OP: This doesn't do what I If @param is 5, then I want to see only records where Bar is 5. I want to see records where Bar is NULL if, and only if, @param is NULL. I apologize if my question didn't make that clear.)
In that case, I think you should try something like this:
SELECT * FROM Foo WHERE Bar=@param OR (Bar IS NULL AND @param IS NULL)
Previous post:
Why not simply use OR ?
SELECT * FROM "Foo" WHERE "Bar"=@param OR "Bar" IS NULL
In SQL Server, you can use ISNULL:
SELECT * FROM "Foo" WHERE ISNULL("Bar",@param)=@param
Solution 4
I don't know what version of SQL you are using but IS makes no sense in the context you just described. I get a syntax error if I try to use it the way you described. Why would you want to use it over = anyway? This is the common usage and the one software maintainers woudl expect to find.
Solution 5
What specific database are you using?
If you're doing searches based on null (or not null), using IS is the way to go. I cannot provide a technical reason but I use this syntax all the time.
SELECT * FROM Table WHERE Field IS NULL
SELECT * FROM Table WHERE Field IS NOT NULL
Dan Moulding
Updated on July 09, 2022Comments
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Dan Moulding almost 2 years
I am building some prepared statements that use parametrized values. As an example:
SELECT * FROM "Foo" WHERE "Bar"=@param
Sometimes
@param
might beNULL
. In such cases, I want the query to return records whereBar
isNULL
, but the above query will not do that. I have learned that I can use theIS
operator for this. In other words:SELECT * FROM "Foo" WHERE "Bar" IS @param
Aside from the differing treatment of
NULL
, are there any other ways in which the above two statements will behave differently? What if@param
is notNULL
, but is instead, let's say,5
? Is using theIS
operator in that case a safe (and sane) thing to do? Is there some other approach I should be taking? -
Dan J about 13 yearsNot to be a contrarian, but, while it does provide alternatives, this doesn't actually answer the OP's question, i.e. what is the difference between the IS and = operators...
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Hari Menon about 13 years"In such cases, I want the query to return records where Bar is NULL, but the above query will not do that." .. "Is using the IS operator in that case a safe (and sane) thing to do? Is there some other approach I should be taking?" I believe the OP is primarily interested in a safe way to achieve this. I really don't know the difference between those except that IS is meant for NULL and I think that that was the secondary question.
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Dour High Arch about 13 yearsI do not think this is what the OP wanted; this always returns rows where
Bar IS NULL
, while the question says "Sometimes@param
might be NULL. In such cases, I want the query to return records whereBar is NULL
". Not the same thing. -
Dan Moulding about 13 yearsThis doesn't do what I want. If
@param
is 5, then I want to see only records whereBar
is 5. I want to see records whereBar
isNULL
if, and only if,@param
isNULL
. I apologize if my question didn't make that clear. -
Dan Moulding about 13 yearsI am using SQLite and it let's me put
5
on the right hand side ofIS
and behaves as I would intuitively expect. -
Dan Moulding about 13 yearsI want to use it over
=
precisely because I need the query to return results whereBar
is NULL if@param
is NULL. The=
operator won't do that. It doesn't result in a syntax error on the system I'm using (SQLite). -
Dan Moulding about 13 yearsI am using SQLite, and it allows me to use
IS
to compare with non-null values. Can you cite a source that says that in standard SQL the "IS" operator is only for null comparisons? -
Dan Moulding about 13 yearsI am using SQLite. The problem is I don't know whether I will be comparing to NULL or not. I am comparing to
@param
. At runtime,@param
will sometimes be NULL, and at other times it will be non-NULL. -
Dan Moulding about 13 yearsCan you cite a source that says that, in standard SQL,
5
can't be put on the right hand side ofIS
? Or are you saying thatIS
isn't even standard SQL? -
Dan Moulding about 13 yearsCan you cite a source that shows that my use of
IS
is a syntax error in standard SQL? (I'm using SQLite, which allows it, but maybe that's a non-standard extension). -
user662852 about 13 yearsActually, I can: in the SQL-92 standard (free on the web), IS is a boolean operator. Both sides are be boolean, by ANSI standard. Null is a boolean value in 3VL logic. Search this document for "8.12 <search condition>": contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt,
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user662852 about 13 yearsAnd, I should point out, any RDBMS vendor is free to implement the standard or add additional elements however they like. So if SQLLite wants to provide a syntactic sugar, that's their business. It's not like SQL Server or Oracle get the full standard correct themselves.
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dburges about 13 yearsIS only works if the value is null in standard SQL. I Have never used SQL lite, but have you tried it when you have a numeric or string value insted of null?
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Dan J about 13 yearsSource for standard
IS NULL/IS NOT NULL
predicate (as opposed toIS
operator). I have yet to find reference toIS
as a standard SQL operator, though the SQLite documentation implies it is a standard reserved word. -
Dan Moulding about 13 yearsThank you for the source. So, it sounds like there is a quite a big difference between the two operators in standard SQL. In SQLite SQL (which I am using) it sounds like the only difference is the treatment of NULL. Thank you for your answer!
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Dan J about 13 yearsYeah, you've solved both the specific issue and defeated my ability to locate the ANSI SQL standard - you've got my upvote as well. :)