Transact-SQL Expressions

 Logical Comparisons

 Introduction

Besides using functions as we saw in the previous two lessons, 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 4.

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 variables for equality, use the = operator. Its formula is: `Value1 = Value2` The equality operation is used to find out whether two variables (or one variable and a constant) hold the same value. From our syntax, the compiler would compare the value of Value1 with that of Value2. If Value1 and Value2 hold the same value, the comparison produces a true result. If they are different, the comparison renders false or 0. The equality operation can be illustrated as follows:

 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) that 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 or positive value. Otherwise, the comparison renders false or a null value. It can be illustrated as follows:

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

 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 Variable1 is lower than that of Variable2, the comparison produces a true or positive result. The Less Than operator "<" can be illustrated as follows:

 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 hold the same value, 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. A <= operation can be illustrated as follows:

 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:

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` A 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:

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

 Conditional Statements

 Introduction

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 formula 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 indication 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. Here is an example:

```DECLARE @DateHired As DateTime,
@CurrentDate As DateTime
SET @DateHired = '1996/10/04'
SET @CurrentDate  = GETDATE()
IF @DateHired < @CurrentDate
PRINT 'You have the experience required for a new promotion in this job'
GO```

This would produce:

 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. Consider the following code: ```DECLARE @DateHired As DateTime, @CurrentDate As DateTime SET @DateHired = '1996/10/04' SET @CurrentDate = GETDATE() IF @DateHired > @CurrentDate PRINT 'You have the experience required for a new promotion' GO``` This would produce: 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: ```DECLARE @CharGender Char(1), @Gender Varchar(20) SET @CharGender = 'F' SET @Gender = CASE @CharGender WHEN 'm' THEN 'Male' WHEN 'M' THEN 'Male' WHEN 'f' THEN 'Female' WHEN 'F' THEN 'Female' END SELECT 'Student Gender: ' + @Gender GO``` Here is the result of executing it:

 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 = '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 'Student Gender: ' + @Gender GO``` This would produce: 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: 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:
 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: This time, the statement would be implemented as follows: ```DECLARE @Number As int SET @Number = 1 WHILE @Number < 5 BEGIN SELECT @Number AS Number SET @Number = @Number + 1 END GO``` This would produce: