Boolean expressions

This section covers boolean expression evaluation rules.

Boolean expressions are a combination of AND, OR, NOT boolean operators, as well as comparison operators such as ==, >= or !=.

The result of a boolean expression is a TRUE or FALSE boolean value, but it can also be NULL if one of the operands is NULL.

A boolean value is typically used in an IF block, WHILE block, or the WHEN clause in a CASE block.

The language provides the TRUE and FALSE predefined constants to initialize boolean variables or return boolean values from functions.

There are three kind of boolean expressions:
  • expr AND expr
  • expr OR expr
  • NOT expr

The (expr) operands of boolean expressions are boolean values.

Important:

The syntax and semantics of boolean expressions in Genero BDL programs is not the same as Boolean conditions in SQL, as SQL statements are executed by the database engine.

If both operands are NULL, the result is NULL. If one of the operands is NULL and the other is non-null, the result depends on the operator type and the non-null value. For more details, see AND, OR, NOT operators.

Make sure that both operands using with a boolean or comparison operator are not null.

The following example shows a simple boolean expression using the AND operator:

IF a AND b THEN
   DISPLAY "Both a and b are TRUE"
END IF
In the next example, a boolean expression uses two comparison expressions:
IF (a == b) AND (a == c) THEN
   DISPLAY "a, b and c are equal"
END IF
The NOT operator will negate a boolean expression:
IF NOT a THEN
   DISPLAY "a is FALSE"
END IF

Use a BOOLEAN variable to store the result of a boolean expression:

MAIN
  DEFINE b BOOLEAN
  LET b = ( "a" == "b" )  -- result is FALSE
END MAIN
Important:
It is bad practive to use non-boolean operands in boolean expressions, as in the following example:
DEFINE s STRING, cnt INTEGER
IF s AND cnt>0 THEN
    ...
END IF
Good pratice (for character strings for example) is to do the following: 
IF LENGTH(s)>0 AND cnt>0 THEN
or:
IF s IS NOT NULL AND cnt>0 THEN

If the operand is not of type boolean, it has to be converted to a boolean. If a conversion is required:

  • Any numeric value evaluates to FALSE, if and only if the value is 0.
  • Any character string value (STRING, CHAR, VARCHAR) follows the next rules:
    • If the string starts with a digit, then this conversion evaluates to FALSE, if and only if the string to integer conversion returns 0.
    • If the string does not start with a digit, then this conversion evaluates to FALSE if and only if the string has a length of 0.

    Consider using the expression (LENGTH(string)>0) or string IS NOT NULL, to check that a string contains characters, or convert the string to a numeric variable and then test the numeric value.

  • DATE values can be converted to integers. MDY(12,31,1899) = 0 and evaluates to FALSE. Any other date value is different from zero and evaluates to TRUE.
  • Any other data type produces a conversion error and raises the runtime error -1260.

Below a more complex example of boolean expressions:

MAIN
  DEFINE r BOOLEAN, c INTEGER
  LET c = 4
  LET r = ( c!=5 ) AND ( c==2 OR c==4 )
  IF ( r AND canReadFile("config.txt") ) THEN
     DISPLAY "OK"
  END IF
END MAIN

If an expression that returns NULL is the operand of the IS NULL operator, the value of the boolean expression is TRUE:

MAIN
  DEFINE r INTEGER
  LET r = NULL
  IF r IS NULL THEN
     DISPLAY "TRUE"
  END IF
END MAIN

Boolean expressions in CASE, IF, or WHILE statements evaluate to FALSE, if any element of the comparison is NULL, except for operands of the IS NULL and the IS NOT NULL operator.

If you include a boolean expression in a context where the runtime system expects a number, the expression is evaluated, and is then converted to an integer by the rules TRUE=1 and FALSE=0.

MAIN
  DEFINE r INTEGER
  LET c = 4
  LET r = 4 + (1==0)    -- result is 4.
END MAIN