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.

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:

 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. 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 Value1 is lower than that of Value2, 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 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:

 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`

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:

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 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. Here is an example:

```DECLARE @DateHired As DateTime,
@CurrentDate As DateTime
SET @DateHired = '1996/10/04'
SET @CurrentDate  = '2007/04/11'
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  = '2007/04/16'
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:

 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 SPHome (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.

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. Here is an example:

```-- 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 '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 'You must provide a positive value';
GO```

This would produce:

To avoid having a NULL value, you can either initialize the variable or you can assign it a value. Here is an example:

```-- 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 '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 'You must provide a positive value';
GO```

This would produce:

 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.

 Lesson Summary

 Miscellaneous Exercises
1. Write a statement that can find out and display each number that is divisible by 3, between 1 and 30
2. Write a statement that, when given the yearly salary of a person, can evaluate the hourly salary (consider that the yearly salary is based on 40 hours a week). Dismiss any negative yearly salary
3. Write a statement so that, if given the radius of a circle, it calculates and displays: the diameter, the circumference, and the area
4. Write a statement so that, given the radius and the height of a cylinder, it calculates and displays: the diameter of the base, the circumference of the base, the base area, the side area, the total area, and the volume