Operand1 & Operand2

Bit1	Bit2	Bit1 & Bit2
0	0	0
1	0	0
0	1	0
1	1	1

Imagine you have two byte values represented as 187 and 242. Based on our study of numeric systems, the binary value of decimal 187 is 1011 1011 (and its hexadecimal value is 0xBB). The binary value of decimal 242 is 1111 0010 (and its hexadecimal value is 0xF2). Let’s compare these two values bit by bit, using the bitwise AND operator:

Most of the times, you will want the interpreter to perform this operation and use the result in your program. This means that you can get the result of this operation and possibly display it to the user. The above operation can be performed by the following program:

PRINT 187 & 242

This would produce 178

Bits Comparison: The Bitwise OR Operator |

You can perform another type of comparison on bits using the bitwise OR operator that is represented by |. Its syntax is:

Value1 | Value2

Once again, the interpreter compares the corresponding bits of each operand. If at least one of the equivalent bits is 1, the comparison produces 1. The comparison produces 0 only if both bits are 0. This operation is resumed as follows:

Once again, let’s consider decimals 187 and 242. Their bitwise OR comparison would render the following result:

You can also let the compiler perform the operation and produce a result. Here is an example:

PRINT 187 | 242

This would produce 251

Bits Comparison: The Bitwise-Exclusive XOR Operator ^

Like the previous two operators, the bitwise-exclusive OR operator performs a bit comparison of two values. It syntax is:

Value1 ^ Value2

The compiler compares the bit of one value to the corresponding bit of the other value. If one of the bits is 0 and the other is 1, the comparison produces 1. In the other two cases, that is, if both bits have the same value, the comparison produces 0. This operation is resumed as follows:

We will again consider decimals 187 and 242. Their bitwise-exclusive XOR comparison would render the following result:

Binary Decimal N1 1 0 1 1 1 0 1 1 187 N2 1 1 1 1 0 0 1 0 242 N1 ^ N2 0 1 0 0 1 0 0 1 73

If the interpreter performs this operation, it can produce a result as in the following example:

PRINT 187 ^ 242;

This would produce 73

Logical Operators

Negation Operator: NOT

In reverse, you may want to use a rule that exclude your criterion and produces only those records that don't follow the rule. For example, you may want to see the list of your customers but excluding those who live in Mexico. To negate a condition, you can start it with the NOT operator. The condition in this case would become NOT (Country = 'Mexico'). Here is an example:

Pattern Operator: LIKE

The comparison and the negation operators we reviewed above usually expect exact matches during their comparisons. For example, when a WHERE condition is formulated as Country = 'Mexico', the interpreter would look for a string under the Country column that exactly display Mexico and Mexico only. In some cases, you know know only the starting characters of a string, only the middle characters of a string, or only the ending characters of a string. The SQL allows you to look for a string that begins, contains, or ends with a few characters. The character or group of characters you provide is referred to as a pattern.

To use a pattern as criterion to look for in the values of a column, you can use the LIKE operator in your WHERE condition. The formula used by this operator is:

ExpressionToMatch LIKE Pattern

The ExpressionToMatch placeholder of this formula can be the name of a column whose values would be examined. The Pattern placeholder is an expression that would be looked for. To create a pattern, the LIKE operator uses wildcards. A wildcard is a placeholder in which you provide one or more characters.

One of the wildcards used to create a pattern is %. This is used as a placeholder for any character. For example, imagine that you want to find a list of your customers who live in a city whose name starts with M. In this case, any (number of) characters on the right side of M can be represented with the % placeholder. The condition would be formulated as follows:

SELECT     *

FROM        Customers

WHERE     City LIKE 'M%'

In the same way, you can use the % wildcard to specify the starting or the middle characters of a string.

Another wildcard used is the underscore _. Like the % wildcard, the underscore character is used as a placeholder but for only one character. Here is an example:

Instead of the underscore, you can use a combination of the opening and the closing square brackets [] as a placeholder for a single character. While the underscore placeholder is used for any character, the difference with the square brackets is that, this time, you can specify the particular alphabetic-based ranged of characters you want to use. To specify the range or characters, between the square brackets, type both ending letters of the range separated by a dash character as in [a-e].

Another variance of the single character placeholder is the ^ used between the square brackets. It is used to exclude characters of the specified range. An example would be [^a-e]. This would produce a list of strings that don't start with either a, b, c, d, or e.

You can use combinations of these wildcards to create more precise patterns.

To negate a condition resulting from a LIKE pattern, you can use the NOT operator on the left side of LIKE as in NOT LIKE.

Logical Conjunction: The AND Operator

Imagine that you are reviewing the students’ records of a school and you want to make sure that you know whom to contact in case of emergency for each student. For example, imagine that you are trying to make sure the record of each student provides an emergency telephone number. The statement to evaluate would be:

• This student's record provides an emergency phone number

If the record of the student referred to contains an emergency phone number, the statement is true. If the record does not provide this information, our goal is not met and the statement is false. Imagine that, on the other hand, we want to find out if the record of each student displays the name to contact in case of emergency. The statement considered would be:

• This student's record provides an emergency name

If the record considered displays an emergency name, the statement is true.

Imagine that a student's record indicates an emergency telephone number but does not provide an emergency name. What if a student's record indicates an emergency name but no emergency telephone number? It appears that in the absence of one of these pieces of information, it would be hard to perform an effective contact. To make a record complete or to accomplish our goal, we want each record to have both an emergency name and an emergency telephone number. We can combine both of the above statements as follows:

• "This student's record provides an emergency phone number" AND "This student's record provides an emergency name"

Evaluating the combined statement would produce the following results:

• If the record of the student referred to does not provide an emergency phone number, the record of the student is not complete and our goal is not met, regardless of the second statement. Therefore, the combined statement is false. 
• If the record of the student referred to provides an emergency phone number, then we would consider the second statement.
  If the record does not provide a name in case of emergency, the record is not complete and our goal is not met. Therefore, the combined statement is false. 
• If the record of the student being considered provides neither an emergency phone number nor an emergency name, the record is still not complete and our goal is not met. Therefore, the combined statement is false. 
• If the record of the student being considered provides both an emergency telephone number and an emergency name, we consider that the record is complete and our goal is met. Therefore, the combined statement is true.

To resume, our goal is met only if BOTH the first and the second statement are TRUE. This is done using the AND operator.

Logical Disjunction: The OR Operator

Imagine that our main goal is to make sure that we have a number we can use to contact somebody for the student. In this case, either the home phone number or the emergency number would be fine. We already have the following two statements:

1. This student's record indicates a home phone number
2. This student's record provides an emergency phone number

To get either number, we can concatenate these two statements and evaluate the truthfulness of either one of them. We create a new statement as follows:

"This student's record indicates a home phone number" OR "This student's record provides an emergency phone number"

The comparative evaluation produces the following results:

• If the student's record indicates a home phone number, our goal is met and we do not need to evaluate the second statement. Therefore, the combined statement is true.
• If the student's record does not indicate a home phone number, we can then consider the second statement.
  If the student's record provides an emergency phone number, we have a number we can use. This means that the result of the first statement is not important. Therefore, the combined statement is true. 
• If the student's record indicates a home phone number and the student's record provides an emergency phone number, our goal is still met; we can use either number. Therefore, the combined statement is true. 
• If the student's record does not indicate a home phone number, then we consider the second statement. If the student's record does not provide an emergency phone number, our goal is not met: we do not have any number to contact somebody for the student in case of emergency. Therefore, the combined statement is false.

The logical disjunction shows us that a combined statement is true if EITHER of its two statements IS TRUE. The logical disjunction produces a false result if BOTH of its two statements are FALSE. To perform a logical disjunction, you can use the OR operator.

Range Test: BETWEEN

If you have a logical range of values and you want to know if a certain exact value is contained in that range, you use the BETWEEN operator. The BETWEEN operator is usually combined with the AND operator to get a list of records between two values. The basic formula of this operator is:

Expression BETWEEN Start AND End

The Expression placeholder of our formula is usually the name of the column whose values you want to examine. The Start factor is the starting value of the range to consider. The End factor is the highest value to consider in the range. After this condition is executed, it produces the list of values between Start and End. As an example, if you are creating some groups of students for activities, imagine that one group would have students from ID 54 to 72. You would write the WHERE condition as follows:

SELECT     StudentID, StudentNbr, Gender, LastName, FirstName, DateOfBirth

FROM         Students

WHERE     StudentID BETWEEN 54 AND 72

The IS Operator

To validate something as being possible, you can use the IS operator. For example, to acknowledge that something is NULL, you can use the IS NULL expression. In the same way, to validate that something is not null, you can use the expression IS NOT NULL.

Here is an example that displays the list of students whose records don't show the emergency name:

Comparison Operators: IN

If you have a series of records and want to find a record or a group of records among them, you can use the IN operator.

SQL provides many and many other operators that we have not reviewed here.

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