WHERE Clause

The where clause is used with a relational statement to isolate the object element or row
Lets you restrict the rows returned
If you restrict rows based on the result of a group function you must have a GROUP BY as well as a HAVING clause

SELECT      * 
FROM        CUSTOMERS 
WHERE       (Country = 'Finland' OR Country = 'Brazil')

SELECT      * 
FROM        CUSTOMERS 
WHERE       CompanyName = 'Island Trading'

--When using SQL on text data, "alfred" is greater than "a" (like in a dictionary).

SELECT      CompanyName 
            ,ContactName 
FROM        CUSTOMERS 
WHERE       CompanyName > 'g'

Note: Text strings should always be enclosed in quotes. if one of your values is a text string containing blanks or special characters you must enclose the swhole string in quotes.
Note: If you are comparing date values you must enclose the date constants in pound sign (#)

--This works in SQL Server despite Extension being a text column - doesn't work in Access

SELECT      * 
FROM        EMPLOYEES 
WHERE       EMPLOYEES.Extension > 3000

--This works in both Access and SQL server

SELECT      * 
FROM        EMPLOYEES 
WHERE       Extension = '428'

--This works in SQL Server (despite Extension being a text column) but not in Access

SELECT      * 
FROM        EMPLOYEES 
WHERE       Extension = 428

OR Clause

Operator Precedence

When multiple operators are used in a WHERE clause the operator precedence determines the order in which the operations are performed.
If two operators in an expression have the same operator precedence level then they are evaluated from left to right.
Since parentheses have the highest precedence level these can be used to override the default operator precedence.
The following tables are shown in the order of precedence.

Comparison Operators

>	Greater Than
>=	Greater Than or Equal To
=	Equal
<	Less Than
<=	Less Than or Equal To
<>	Not Equal

Logical Operators

Whenever you use both the AND and OR operators together always use parentheses to ensure that you get the correct results.

AND	Requires both expressions on either side of the AND operator to be true for data to be returned
OR	Requires at least one expression on either side of the OR operator to be true in order for data to be returned.
NOT	Used to match any condition opposite of the one defined.

??? Operators

BETWEEN	Used to determine whether a value of an expression falls within a specified range of values
IN	Used to match conditions in a list of expressions
LIKE	Used to match patterns in data
IS NULL	Used to determine if a field contains data
IS NOT NULL	Used to determine if a field does not contain data.

Arithmetic Operators

These are used to perform mathematical calculations.
These are listed in order of precedence.

-	Negation
^	Exponentiation (not available in SQL Server ?)
/	Divide
*	Multiple
%	Modulus
=	Plus
-	Minus


!=, ^=	Test for inequality
=	Test for equality
\|\|	String concatenation
:=	Assignment operator

String Operators

When using SQL on text data, "alfred" is greater than "a" (like in a dictionary).

SELECT     CompanyName 
           ,ContactName 
FROM       CUSTOMERS 
WHERE      CompanyName > 'g'