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Introduction to SQL Expressions
 

Logical Comparisons
 

Introduction

For your databases, you can create expressions that represent a combination of values, variables, and operators. To support expressions, Transact-SQL provides operators other than, or in addition to, those we saw in Lesson 2.
Logical Comparisons

A comparison is a Boolean operation that produces a true or a false result, depending on the values on which the comparison is performed. A comparison is performed between two values of the same type; for example, you can compare two numbers, two characters, or the names of two cities. To support comparisons, Transact-SQL provides all necessary operators.

Equality Operator =

To compare two values for equality, use the = operator. Its formula is:

Value1 = Value2

The equality operation is used to find out whether two values are the same. From the above formula, the SQL interpreter would compare Value1 and Value2. If Value1 and Value2 are the same, the comparison produces a TRUE result. If they are different, the comparison renders FALSE.

The equality operation can be illustrated as follows:

Equalilty Flowchart

Practical LearningPractical Learning: Introducing Variables
  1. All Computers: Start the computer and log in using your domain account
  2. Server: Launch Microsoft SQL Server. In the Server Name combo box, make sure the name of the computer is selected. In the Authentication combo box, make sure Windows Authentication is selected. Make sure the account you are using is selected in the User Name combo box. Click Connect
  3. Each Student:
    1. Start Microsoft SQL Server
    2. In the Server Name combo box, select the name of the server or type it
    3. In the Authentication combo box, select SQL Server Authentication
    4. In the User Name combo box, type the name of the domain, followed by \, and followed by the login name you were given
    5. In the Password combo box, type your password
    6. Click Connect
  4. All Computers: On the Standard toolbar, click the New Query button New Query

Not Equal <>

As opposed to equality, to find out if two values are not equal, use the <> operator. Its formula is:

Value1 <> Value2

The <> is a binary operator (like all logical operators) proposed by the international standard (ISO). It is used to compare two values. The values can come from two variables as in Variable1 <> Variable2. Upon comparing the values, if both hold different values, the comparison produces a TRUE. Otherwise, the comparison renders FALSE or a null value.

It can be illustrated as follows:

Flowchart: Not Equal

Notice that the Not Equal operator <> is the opposite to the Equality operator =.

Besides the <> operator of the ISO SQL, Transact-SQL also supports the C language inequal operator, which is represented as !=. It essentially performs the same role as <> and can be used in the same circumstances.

Less Than <

To find out whether one value is lower than another, use the < operator. Its formula is:

Value1 < Value2

The value held by Value1 is compared to that of Value2. As it would be done with other operations, the comparison can be made between two variables, as in Variable1 < Variable2. If the value held by Value1 is lower than that of Value2, the comparison produces a true or positive result.

The Less Than operator "<" can be illustrated as follows:

Flowchart: Less Than

 

Less Than Or Equal To <=

The Equality and the Less Than operators can be combined to compare two values. This allows you to know if two values are the same or if the first is less than the second. The operator used is <= and its formula is:

Value1 <= Value2

If both Value1 and Value2 are the same, the result is true or positive. If the left operand, in this case Value1, holds a value lower than the second operand, in this case Value2, the result is still true. If the value of Value1 is strictly higher than that of Value, the comparison produces a FALSE result.

A <= operation can be illustrated as follows:

Flowchart

Greater Than >

To find out if one value is strictly greater than another, you can use the > operator. Its formula is:

Value1 > Value2

Both operands, in this case Value1 and Value2, can be variables or the left operand can be a variable while the right operand is a constant. If the value on the left of the > operator is greater than the value on the right side or a constant, the comparison produces a true or positive value. Otherwise, the comparison renders false or null.

The > operator can be illustrated as follows:

Greater Than

Notice that the > operator is the opposite to <=.

Greater Than or Equal To >=

The greater than and the equality operators can be combined to produce an operator as follows: >=. This is the "greater than or equal to" operator. Its formula is:

Value1 >= Value2

The comparison is performed on both operands: Value1 and Value2. If the value of Value1 and that of Value2 are the same, the comparison produces a true or positive value. If the value of the left operand is greater than that of the right operand, the comparison produces true or positive also. If the value of the left operand is strictly less than the value of the right operand, the comparison produces a false or null result. This can be illustrated as follows:

Flowchart: Greater Than Or Equal To

Notice that the >= operator is the opposite to <.

Not Greater Than !>

Transact-SQL supports an operator represented as !>. This means that a value is not greater than the indicated value. In most cases, this is equivalent to the Less Than Or Equal To operator "<=".

Not Less Than !<

Transact-SQL supports one more comparison operator represented as !<. It is used to find out whether one of the operands is less than the other. This operator is equivalent to >=.

Conditional Statements
 

Introduction
Conditional Statements

A condition statement is an expression you formulate to evaluate it. Most of the time,  the statement is written so that, when evaluated, it can produce a result of true or false, then, depending on the outcome, you can take action. A condition is usually written as simple as possible to make it clear to you and the SQL interpreter. Although the interpreter never gets confused, if you create a difficult statement, you may receive an unpredictable result.

In the next few sections, we will review the keywords and formulas that Transact-SQL provides to help you formulate clear expressions. Expressions usually start with a keyword, followed by the expression itself. After the expression, you can tell the interpreter what to do. The statement may appear as follows:

Keyword Expression
	Statement

BEGIN...END

With the above formula, we will always let you know what keyword you can use, why, and when. After the expression, you can write the statement in one line. This is the statement that would be executed if/when the Expression of our formula is satisfied. In most cases, you will need more than one line of code to specify the Statement.

As it happens, the interpreter considers whatever comes after the Statement as a unit but only the line immediately after the Expression. To indicate that your Statement covers more than one line, start it with the BEGIN keyword. Then you must use the END keyword to indicate where the Statement ends. In this case, the formula of a conditional statement would appear as follows:

Keyword Expression
BEGIN
	Statement Line 1
	Statement Line 2
	
	Statement Line n
END

You can still use the BEGIN...END combination even if your Statement covers only one line:

Keyword Expression
BEGIN
	Statement
END

Using the BEGIN...END combination makes your code easier to read because it clearly indicates the start and end of the Statement.

IF a Condition is True

Probably the primary comparison you can perform on a statement is to find out whether it is true. This operation is performed using an IF statement in Transact-SQL. Its basic formula is:

IF Condition
	Statement

When creating an IF statement, first make sure you provide a Condition expression that can be evaluated to produce true or false. To create this Condition, you can use variables and the logical comparison operator reviewed above.

When the interpreter executes this statement, it first examines the Condition to evaluate it to a true result. If the Condition produces true, then the interpreter executes the Statement.

Practical LearningPractical Learning: Checking a Condition

From here to the end of this lesson, all instructions are intended for All Computers:

  1. In the Query window, type the following:
    DECLARE @DateHired As datetime2,
	@CurrentDate As datetime2
SET @DateHired = N'1996/10/04'
SET @CurrentDate  = N'2007/04/11'
IF @DateHired < @CurrentDate
	PRINT N'You have the experience required for a new promotion in this job';
  2. To execute the statement, press F5.
    This would produce:

IF

IF...ELSE

The IF condition we used above is appropriate when you only need to know if an expression is true. There is nothing to do in other alternatives.

Practical LearningPractical Learning: Creating an IF...ELSE Condition
  1. Click inside the top section of the Query window and press Ctrl + A to select everything
  2. Type the following: 
    DECLARE @DateHired As datetime2,
	@CurrentDate As datetime2
SET @DateHired = N'1996/10/04'
SET @CurrentDate  = N'2007/04/16'
IF @DateHired > @CurrentDate
	PRINT 'You have the experience required for a new promotion'
GO
  3. To execute, press F5. This would produce:

    IF...ELSE

    Notice that, in case the expression to examine produces a false result, there is nothing to do. Sometimes this will happen.

CASE...WHEN...THEN

The CASE keyword is used as a conditional operator that considers a value, examines it, and acts on an option depending on the value. The formula of the CASE statement is:

CASE Expression
	WHEN Value1 THEN Result
	WHEN Value2 THEN Result

	WHEN Value_n THEN Result
END

In the following example, a letter that represents a student is provided. If the letter is m or M, a string is created as Male. If the value is provided as f or F, a string is created as Female.
 
 
 

Practical LearningPractical Learning: Creating a CASE Condition
  1. Click inside the top section of the Query window and press Ctrl + A to select everything
  2. Type: 
    DECLARE @CharGender Char(1),
	@Gender  Varchar(20);
SET @CharGender = N'F';
SET @Gender = 
	CASE @CharGender
		WHEN 'm' THEN 'Male'
		WHEN 'M' THEN 'Male'
		WHEN 'f' THEN 'Female'
		WHEN 'F' THEN 'Female'
	END;

SELECT N'Student Gender: ' + @Gender;
GO
  3. To execute, press F5

    CASE

CASE...WHEN...THEN...ELSE

In most cases, you may know the only types of value that would be submitted to a CASE statement. In some other cases, an unpredictable value may be submitted. If you anticipate a value other than those you are aware of, the CASE statement provides a "fit-all' alternative by using the last statement as ELSE. In this case, the formula of the CASE statement would be:

CASE Expression
	WHEN Value1 THEN Result
	WHEN Value2 THEN Result
	WHEN Value_n THEN Result
	
	ELSE Alternative
END

The ELSE statement, as the last, is used when none of the values of the WHEN statements fits. Here is an example: 

DECLARE @CharGender Char(1),
	@Gender  Varchar(20);
SET @CharGender = N'g';
SET @Gender = 
	CASE @CharGender
		WHEN 'm' THEN 'Male'
		WHEN 'M' THEN 'Male'
		WHEN 'f' THEN 'Female'
		WHEN 'F' THEN 'Female'
		ELSE 'Unknown'
	END;

SELECT N'Student Gender: ' + @Gender;
GO

This would produce:

CASE...WHEN...THEN...ELSE

If you don't produce an ELSE statement but a value not addressed by any of the WHEN statements is produced, the result would be NULL. Here is an example:

CASE...WHEN...THEN...ELSE

This means that it is a valuable safeguard to always include an ELSE sub-statement in a CASE statement.

WHILE

To examine a condition and evaluate it before taking action, you can use the WHILE operator. The basic formula of this statement is:

WHILE Expression 
    Statement

When implementing this statement, first provide an Expression after the WHILE keyword. The Expression must produce a true or a false result. If the Expression is true, then the interpreter executes the Statement. After executing the Statement, the Expression is checked again. AS LONG AS the Expression is true, it will keep executing the Statement. When or once the Expression becomes false, it stops executing the Statement. This scenario can be illustrated as follows:

WHILE

Here is an example: 

DECLARE @Number As int

WHILE @Number < 5
	SELECT @Number AS Number
GO

To effectively execute a while condition, you should make sure you provide a mechanism for the interpreter to get a referenced value for the condition, variable, or expression being checked. This is sometimes in the form of a variable being initialized although it could be some other expression. Such a while condition could be illustrated as follows:

WHILE

Practical LearningPractical Learning: Creating a WHILE Statement

  1. Click inside the top section of the Query window and press Ctrl + A to select everything
  2. Type: 
    DECLARE @Number As int
SET @Number = 1
WHILE @Number < 5
    BEGIN
	SELECT @Number AS Number
	SET @Number = @Number + 1
    END
GO
  3. To see the result, press F5. This would produce:

    WHILE

Boolean Constants
 

Introduction

Databases and other programming environments provide operators you can use to perform data analysis. The operators used are called logical operators because they are used to perform comparisons that produce a result of true or false (there is no middle result; in other words, something is not half true or half false or "Don't Know": either it is true or it is false).

The TRUE and FALSE Constants

In Boolean algebra, something is considered TRUE when it holds a value. The value is also considered as 1 or Yes. By contrast, if something doesn't hold a value, it is considered non-existent and non-worthy of consideration. Such a thing has a value of FALSE, 0, or No. To retrieve such a value, you can just find out if the value of a field is existent or not.

The comparison for a True or False value is mostly performed on Boolean fields, such a case is the SingleParentHome (which specifies whether a student lives in a single parent home) field of the Students table of the HighSchool database. If a record has a value of 1, the table considers that such a field is True. If the field has a 0 value, then it holds a FALSE value.

The NULL Constant

After you have declared a variable, the SQL interpreter reserves a space in the computer memory for it but doesn't put anything in that memory space. At that time, that area of memory doesn't hold a significant value. Also at that time, the variable is considered null.

Here is note to be careful about: when a variable is said to hold a null value, it doesn't mean its value is 0. It doesn't even mean that the variable's memory space is empty. It actually means that we cannot clearly determine the current value that the variable is holding.

 Boolean Constants

To support the null value, Transact-SQL provides a constant named NULL. The NULL constant is mostly used for comparison purposes. For example, you can use an IF statement to check the nullity of a variable.

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.

 

Practical LearningPractical Learning: Creating an IS Statement

  1. Click inside the top section of the Query window and press Ctrl + A to select everything
  2. Type: 
    -- Square Calculation
DECLARE @Side As Decimal(10,3),
        @Perimeter As Decimal(10,3),
        @Area As Decimal(10,3);

SET     @Perimeter = @Side * 4;
SET     @Area = @Side * @Side;
IF @Side IS NULL
	PRINT N'A null value is not welcome'
ELSE IF @Side > 0
    BEGIN
	SELECT @Side AS Side;
	SELECT @Perimeter AS Perimeter ;
	SELECT @Area AS Area;
    END;
ELSE
	PRINT N'You must provide a positive value';
GO
  3. To see the result, press F5. This would produce:

  4. To avoid having a NULL value, you can either initialize the variable or you can assign it a value. As an example, change the statement as follows: 
    -- Square Calculation
DECLARE @Side As Decimal(10,3),
        @Perimeter As Decimal(10,3),
        @Area As Decimal(10,3);
SET     @Side = 48.126;
SET     @Perimeter = @Side * 4;
SET     @Area = @Side * @Side;
IF @Side IS NULL
	PRINT N'A null value is not welcome'
ELSE IF @Side > 0
    BEGIN
		SELECT @Side AS Side;
		SELECT @Perimeter AS Perimeter ;
		SELECT @Area AS Area;
    END;
ELSE
	PRINT N'You must provide a positive value';
GO
  5. To execute, press F5. This would produce:

    NULL Value

The NOT Operator

To deny the presence, the availability, or the existence of a value, you can use the NOT operator. This operator is primarily used to reverse a Boolean value. For example, we have learned that FALSE is the opposite of TRUE. In the same way, TRUE is the opposite of FALSE. If you want to compare a value as not being TRUE, the NOT TRUE would produce the same result as the FALSE value. For the same reason, the expression NOT FALSE is the same as TRUE.

Practical LearningPractical Learning: Ending the Lesson
  1. Close Microsoft SQL Server
  2. If asked whether you want to save the file, click No
 
 
   
 
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