Logical expressions are expressions yielding the Boolean values True, False, or Unknown (for NULL values). 4GL includes the Boolean comparison operators and the logical operators AND, OR, and NOT.

The Boolean comparison operators join numeric or string expressions into a logical expression. The following table summarizes the Boolean comparison operators:

The result of a comparison with a nullable value can be True, False or Unknown. The result is Unknown if one or both operands of the expression are NULL, except for the IS NULL and IS NOT NULL operations.

4GL tests the resulting expressions in if and while statements. 4GL also uses them in the where clauses of query statements.

The condition that a 4GL if or while statement tests must be a Boolean expression. For example, if Status is a character field in the form currently being displayed, then the following condition yields a Boolean value:

The if statement below leads to a call on NewProject when the Status field has "n" as its value:

In if and while statements, if the result of an expression is Unknown, then the flow of control occurs exactly as if the Boolean expression evaluates to False. For examples, see Nulls in Expressions.

4GL allows you to compare two strings to see whether they resemble each other in specific ways.

To do this, specify a pattern--a string of characters with special formatting characters--that shows what the compared string must look like, along with the LIKE pattern-matching operation. If the string you specify is an instance of the pattern string, then the comparison evaluates to True. The syntax for the LIKE operation is as follows:

Variable is the name of a character string field or variable, pattern is a character string literal or character string variable, and escapechar is a character string literal or character string variable of length 1. Pattern must not be NULL.

Pattern strings can include the following special characters:

In comparisons using the arithmetic operators, pattern-matching characters do not have any effect. For example, in the following statement, the comparison tests the value of name to see whether it exactly equals the constant literal "Leslie%." If it does, the object of the conditional is executed.

To make the pattern-matching characters take on their special meanings, use the LIKE operator as any other comparison operator is used. For example, the following statement tests whether a text value ends in the letter e:

The following statement tests the value of variable emp_name against the pattern, to see if it starts with "Leslie", and it makes sure that the last name of the employee is "Smith":

You can use the escape clause with the LIKE operator in two ways.

For example, suppose you want to match all strings ending with X, Y or Z. Use the following phrase:

In the following example, the pattern matches "ABC," "ACC," and "FCC Fairness Doctrine," and does not match "FDC Yellow #42" or "Access."

The next example matches any string beginning with a left bracket and whose second character is "1," "2," "3" or a right bracket:

An escape character must be followed in the pattern by an underscore, percent, left or right bracket, or another escape character.

The logical operators (AND, OR, and NOT) join logical expressions into new logical expressions. The following truth tables show the result of comparisons made with the AND, OR, and NOT operators:

The following table lists the AND operators:

The following table lists the OR operators:

The following table lists the NOT operators:

The example below illustrates the use of the IF and AND operators:

The frame newemployee is called only if both conditions are True. The current value of empnum must be greater than zero and status must have any value other than 3.

This example illustrates the use of the NOT and OR operators:

The frame newemployee is called if neither of the conditions are True. The current value of status cannot be 1 or 2.

The logical operators have the following precedence (in descending order):

NOT
AND or OR

Precedence is handled as follows:

You can use parentheses to change the default order of evaluation, or simply to make it more explicit; for example, the following expression:

is equivalent to:

The order of precedence is different for 4GL logical operators than for SQL logical operators. To avoid confusion when using logical operators in 4GL, use parentheses in expressions that use a mixture of the operators.