How do the SQL “IS” and “=” operators differ?

问题:

I am building some prepared statements that use parametrized values. As an example:

SELECT * FROM "Foo" WHERE "Bar"=@param

Sometimes @param might be NULL. In such cases, I want the query to return records where Bar is NULL, but the above query will not do that. I have learned that I can use the IS 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 not NULL, but is instead, let's say, 5? Is using the IS operator in that case a safe (and sane) thing to do? Is there some other approach I should be taking?

回答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

回答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).

Equals (=) operator

IS NULL operator

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!

回答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

回答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.

回答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