7.20.9 Logical predicate

(1) Specification format

logical-predicate ::= value-expression IS [NOT] {TRUE|FALSE|UNKNOWN}

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(2) Explanation of specification format

value-expression:

Specifies the value expression to be evaluated as a logical predicate. For details about value expressions, see 7.21 Value expression.

The rules are as follows:

• The data type of the value expression result must be BOOLEAN type.

• If the result of the value expression is a null value, the logical value of that value expression becomes equivalent to unknown.

• If a dynamic parameter is specified in the value expression, the data type assumed for the dynamic parameter is BOOLEAN type.

The result of the logical predicate is true when the value specified in the logical predicate (TRUE, FALSE, or UNKNOWN) matches the logical value of the value expression. The following table shows a list of logical predicate results.

Table 7‒16: List of logical predicate results

Specified value in logical predicate	Logical value of value expression
	True	False	Unknown
IS TRUE	True	False	False
IS FALSE	False	True	False
IS UNKNOWN	False	False	True

Legend:

True: The logical predicate result is true.

False: The logical predicate result is false.

NOT:

If NOT is specified, the logical predicate result is true when the value specified in the logical predicate (TRUE, FALSE, or UNKNOWN) does not match the logical value of the value expression. The True and False values in the table above are reversed.

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(3) Rules

HADB converts the specified logical predicates are converted to comparison predicates and NULL predicates as shown in the following table and executes them.

Table 7‒17: Relationship between specified logical predicates and converted predicates

Specified logical predicates	Converted predicates
value-expression IS TRUE	value-expression IS NOT NULL AND value-expression = 'T'
value-expression IS FALSE	value-expression IS NOT NULL AND value-expression = 'F'
value-expression IS UNKNOWN	value-expression IS NULL
value-expression IS NOT TRUE	value-expression IS NULL OR value-expression != 'T'
value-expression IS NOT FALSE	value-expression IS NULL OR value-expression != 'F'
value-expression IS NOT UNKNOWN	value-expression IS NOT NULL

In HADB, logical data in value expressions is treated as CHAR(1) character string data. TRUE is regarded as 'T', and FALSE is regarded as 'F'.

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(4) Examples

Example 1

Retrieves employees from the employee table (EMPLIST) who are currently employed and permitted to work remotely.

SELECT "ID","NAME" FROM "EMPLIST"
    WHERE "IS_ACTIVE" IS TRUE AND "IS_REMOTE_ALLOWED" IS TRUE

The underlined portions show the specification of the logical predicate.

Example 2

Retrieves employees from the employee table (EMPLIST) whose remote work permission has not been set.

SELECT "ID","NAME" FROM "EMPLIST"
    WHERE "IS_REMOTE_ALLOWED" IS UNKNOWN

The underlined portions show the specification of the logical predicate.

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