How Do The SQL "Is" and "=" Operators Differ

How do the SQL IS and = operators differ?

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

Difference between = and is in sql server

Nothing equals null.

Not even null equals null.

null is not a value, it is more like a concept, or a mark, meaning unknown value.

As such, you need two operators for this, one for equality, and one for checking the concept of null.

Once you start to think of null as "unknown value" a lot of the other behavior also makes sense.

10 + null? Add an unknown value to 10? Obviously you will have another unknown value as a result.

For more information, please check the documentation of the equality operator in T-SQL.

Additionally, see the documentation for SET ANSI_NULL.

Note that the documentation is in conflict about the behavior of x = null between the equality operator (documentation says it will always be false if x is non-null) whereas SET ANSI_NULLS documentation says that x = null will behave equivalent to x is null when the option is turned on.

What is the difference between the IN operator and = operator in SQL?

IN

will not generate an error if you have multiple results on the subquery. Allows to have more than one value in the result returned by the subquery.

=

will generate an error if you have more than one result on the subquery.

  • SQLFiddle Demo (IN vs =)

Difference between and != operators in MSSQL Server

!= is not ANSI compliant.

That's all.

Use <>

UPD. Oh, here

Difference between and != in SQL

None whatsoever, syntactically.

Both are inequality operators, <> is the SQL-92 standard, however its interchangable on some platforms with != (E.g. SQL Server)

SQLite - Difference between IS and = (equals) in WHERE clause. (using JDBC PreparedStatement)

The "IS" keyword is strictly to be used with NULL value. (eg. IS NULL or IS NOT NULL).

If you are looking for a match you need to use =.

If you are concerned about handling null column values then you should wrap the columns of concern with the isnull() function.

SELECT Col1
, Col2
, isnull(Col3,'') AS Col3
FROM myTable
Where col1 = 'somevalue'

What is the difference between != and operators?

From the manual:

Note: The != operator is converted to <> in the parser stage. It is not possible to implement != and <> operators that do different things.

So no, there is no difference between the two.

What is the difference between = and := in MySQL?

Both of them are assignment operators but one thing I can find their differences is that = can be used to perform boolean operation while := cannot.

valid: SUM(val = 0)

Invalid: SUM(val := 0)

FROM User-Defined Variables

One more thing, You can also assign a value to a user variable in statements other than SET. In this case, the assignment operator must be := and not = because the latter is treated as the comparison operator = in non-SET statements.

mysql> SET @t1=1, @t2=2, @t3:=4;
mysql> SELECT @t1, @t2, @t3, @t4 := @t1+@t2+@t3;
+------+------+------+--------------------+
| @t1 | @t2 | @t3 | @t4 := @t1+@t2+@t3 |
+------+------+------+--------------------+
| 1 | 2 | 4 | 7 |
+------+------+------+--------------------+

Performance differences between equal (=) and IN with one literal value

There is no difference between those two statements, and the optimiser will transform the IN to the = when IN has just one element in it.

Though when you have a question like this, just run both statements, run their execution plan and see the differences. Here - you won't find any.

After a big search online, I found a document on SQL to support this (I assume it applies to all DBMS):

If there is only one value inside the parenthesis, this commend [sic] is equivalent to,

WHERE "column_name" = 'value1

Here is the execution plan of both queries in Oracle (most DBMS will process this the same):

EXPLAIN PLAN FOR
select * from dim_employees t
where t.identity_number = '123456789'

Plan hash value: 2312174735
-----------------------------------------------------
| Id | Operation | Name |
-----------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | TABLE ACCESS BY INDEX ROWID| DIM_EMPLOYEES |
| 2 | INDEX UNIQUE SCAN | SYS_C0029838 |
-----------------------------------------------------

And for IN() :

EXPLAIN PLAN FOR
select * from dim_employees t
where t.identity_number in('123456789');

Plan hash value: 2312174735
-----------------------------------------------------
| Id | Operation | Name |
-----------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | TABLE ACCESS BY INDEX ROWID| DIM_EMPLOYEES |
| 2 | INDEX UNIQUE SCAN | SYS_C0029838 |
-----------------------------------------------------

As you can see, both are identical. This is on an indexed column. Same goes for an unindexed column (just full table scan).

Standard SQL boolean operator IS vs. equals (=) operator

That's a new one for me.

If I read that correctly the <boolean value expression> grammar defines three predicates solely for use with the boolean datatype IS TRUE, IS FALSE, IS UNKNOWN.

These differ from their equality counterparts in that they only evaluate to True or False. Never to Unknown. i.e. UNKNOWN = TRUE would evaluate to UNKNOWN but UNKNOWN IS TRUE evaluates to False.

The full truth tables for IS and = are below.

+---------+-------+-------+---------+
| IS | TRUE | FALSE | UNKNOWN |
+---------+-------+-------+---------+
| TRUE | TRUE | FALSE | FALSE |
| FALSE | FALSE | TRUE | FALSE |
| UNKNOWN | FALSE | FALSE | TRUE |
+---------+-------+-------+---------+

As opposed to

+---------+---------+---------+---------+
| = | TRUE | FALSE | UNKNOWN |
+---------+---------+---------+---------+
| TRUE | TRUE | FALSE | UNKNOWN |
| FALSE | FALSE | TRUE | UNKNOWN |
| UNKNOWN | UNKNOWN | UNKNOWN | UNKNOWN |
+---------+---------+---------+---------+


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