Chapter 9 – Boolean values

In SQL, a Boolean value – either TRUE, FALSE or UNKNOWN – is a truth value. A Boolean value may be a <literal>, the value of a parameter or a host language variable or the result of any expression or argument (including a possibly qualified <Column name> or the result of an SQL predicate or search condition) that evaluates to a truth value. Boolean values are stored in the Boolean <data type>: BOOLEAN.

Table of Contents

<Boolean literal>s

A <Boolean literal> is one of these three words:

TRUE
FALSE
UNKNOWN

Its <data type> is BOOLEAN. A <Boolean literal> of TRUE represents the truth value TRUE, a <Boolean literal> of FALSE represents the truth value FALSE and a <Boolean literal> of UNKNOWN represents the truth value UNKNOWN.

If you want to restrict your code to Core SQL, don’t use <Boolean literal>s.

Boolean <data type>s

A Boolean <data type> is defined by a descriptor that contains one piece of information – the <data type>’s name: BOOLEAN.

BOOLEAN

The required syntax for a BOOLEAN <data type> specification is as follows.

BOOLEAN <data type> ::=
BOOLEAN

BOOLEAN defines a set of truth values: either TRUE, FALSE or UNKNOWN (as the null value).

The SQL Standard doesn’t differentiate between BOOLEAN’s null value (that is, UNKNOWN) and the UNKNOWN truth value that is returned by an SQL predicate, search condition or by any argument or expression that returns a Boolean value – it allows both to be used interchangeably to mean the same thing. Warning: by saying that UNKNOWN and NULL are the same thing, one is saying that the answers “I don’t know” and “I know that the data is missing” are the same thing. The drafters of the SQL Standard apparently forgot the distinction, and they have been justly criticized for this error.

If you want to restrict your code to Core SQL, don’t define any BOOLEAN <data type>s.

Now that we’ve described SQL’s Boolean <data type>, let’s look at some example SQL statements that put it to use.

These SQL statements make a Table with three Boolean Columns, insert a row, then search for a pair of equal Column values.

CREATE TABLE Logicals (
   boolean_1 BOOLEAN,
   boolean_2 BOOLEAN,
   boolean_3 BOOLEAN);

INSERT INTO Logicals (
   boolean_1,
   boolean_2,
   boolean_3)
VALUES (TRUE,FALSE,UNKNOWN);

SELECT boolean_1
FROM   Logicals
WHERE  boolean_1 = boolean_3;

Boolean Operations

A Boolean value is compatible with, and comparable to, all other Boolean values and all SQL truth values (for example, the truth values returned by an SQL predicate) – that is, all truth values are mutually comparable and mutually assignable. Truth values may not be directly compared with, or directly assigned to, any other <data type> class, though implicit type conversions can occur in expressions, SELECTs, INSERTs, DELETEs and UPDATEs. Explicit truth value type conversions can be forced with the CAST operator.

CAST

In SQL, CAST is a scalar operator that converts a given scalar value to a given scalar <data type>. The required syntax for the CAST operator is as follows.

CAST (<cast operand> AS <cast target>)
   <cast operand> ::= scalar_expression
   <cast target> ::= <Domain name> | <data type>

The CAST operator converts values of a source <data type> into values of a target <data type>, where each <data type> is an SQL pre-defined <data type> (data conversions between UDTs are done with a user-defined cast). The source <data type>, or <cast operand>, can be any expression that evaluates to a single value. The target <data type>, or <cast target>, is either an SQL predefined <data type> specification or the name of a Domain whose defined <data type> is the SQL predefined <data type> that you want to convert the value of “scalar_expression” into. (If you use CAST (... AS <Domain name>), your current <AuthorizationID> must have the USAGE Privilege on that Domain.)

It isn’t, of course, possible to convert the values of every <data type> into the values of every other <data type>. For Boolean values, the rules are:

  • CAST (NULL AS <data type>) and CAST (Boolean_source_is_a_null_value AS <data type>) both result in a CAST result of NULL.
  • You can CAST a Boolean source to these targets: fixed length character string, variable length character string, CLOB, NCLOB and Boolean. You can also CAST a Boolean source to a UDT target or a <reference type> target if a user-defined cast exists for this purpose and your current <AuthorizationID> has the EXECUTE Privilege on that user-defined cast.

When you CAST a Boolean value to a Boolean target, the result of the CAST is the source value.

When you CAST a Boolean value to a fixed length character string target, there are four possibilities:

  1. The source value is TRUE and the fixed length of the target is at least four characters. In this case, the result of the CAST is 'TRUE', padded on the right with spaces, if necessary, to make it the exact fixed length of the target.
  2. The source value is FALSE and the fixed length of the target is at least five characters. In this case, the result of the CAST is 'FALSE', padded on the right with spaces, if necessary, to make it the exact fixed length of the target.
  3. The source value is UNKNOWN. As already stated, the result of the CAST is NULL.
  4. The fixed length of the target is less than the length of the source value. In this case, the CAST will fail: your DBMS will return the SQLSTATE error 22018 "data exception-invalid character value for cast".

When you CAST a Boolean value to a variable length character string, CLOB or NCLOB target, there are four possibilities:

  1. The source value is TRUE and the maximum length of the target is at least four characters. In this case, the result of the CAST is 'TRUE'.
  2. The source value is FALSE and the fixed length of the target is at least five characters. In this case, the result of the CAST is 'FALSE'.
  3. The source value is UNKNOWN. As already stated, the result of the CAST is NULL.
  4. The maximum length of the target is less than the length of the source value. In this case, the CAST will fail: your DBMS will return the SQLSTATE error 22018 "data exception-invalid character value for cast".

[Obscure Rule] The result of a CAST to a character string target has the COERCIBLE coercibility attribute; its Collation is the default Collation for the target’s Character set.

When you CAST a Boolean value to a UDT or a <reference type> target, your DBMS invokes the user defined cast routine, with the source value as the routine’s argument. The CAST result is the value returned by the user defined cast.

If you want to restrict your code to Core SQL, don’t use <Domain name> as a CAST target: CAST only to a <data type>.

Assignment

In SQL, the TRUE and FALSE truth values may be assigned to any Boolean target and the UNKNOWN truth value may be assigned to any Boolean target that isn’t constrained by a NOT NULL Constraint.

Comparison

SQL provides the usual scalar comparison operators – = and <> and < and <= and > and >= – to perform operations on truth values. All of them will be familiar; there are equivalent operators in other computer languages. In SQL, TRUE is greater than FALSE. If any of the comparands are the UNKNOWN truth value or are NULL, the result of the operation is UNKNOWN. For example:

TRUE = {result is FALSE}

returns FALSE.

TRUE <> {result is NULL}

returns UNKNOWN.

SQL also provides three quantifiers – ALL, SOME, ANY – which you can use along with a comparison operator to compare a truth value with the collection of truth values returned by a <table subquery>. Place the quantifier after the comparison operator, immediately before the <table subquery>. For example:

SELECT occurrence_boolean
FROM   Boolean_Example
WHERE  occurrence_boolean = ALL (
   SELECT char_column
   FROM   Table_1
   WHERE  char_column LIKE '%e');

ALL returns TRUE either (a) if the collection is an empty set (i.e.: if it contains zero rows) or (b) if the comparison operator returns TRUE for every value in the collection. ALL returns FALSE if the comparison operator returns FALSE for at least one value in the collection.

SOME and ANY are synonyms. They return TRUE if the comparison operator returns TRUE for at least one value in the collection. They return FALSE either (a) if the collection is an empty set or (b) if the comparison operator returns FALSE for every value in the collection. (The search condition = ANY (collection) is equivalent to IN (collection).)

Other Operations

With SQL, you have several other operations that you can perform on truth values, or on other values to get a truth value result.

Boolean operators

SQL provides the usual scalar Boolean operators – AND and OR and NOT and IS - - to perform operations on Boolean operands. Each returns a Boolean result, or truth value. All of them will be familiar; there are equivalent operators in other computer languages. If any of the operands are the UNKNOWN truth value or are NULL, the result of the operation is UNKNOWN. Here is the syntax allowed for Boolean expressions:

<boolean value expression> ::=
<boolean term> |
<boolean value expression> OR <boolean term>

   <boolean term> ::=
   [ NOT ] <boolean test> |
   <boolean term> AND [ NOT ] <boolean test>

      <boolean test> ::=
      boolean_argument [ IS [ NOT ] {TRUE | FALSE | UNKNOWN} ]

A Boolean expression operates on one or more operands that evaluate to a truth value – that is, the boolean_argument shown in this syntax diagram is either a <Boolean literal>, the value of a parameter or a host language variable or the result of any expression or argument (including a possibly qualified <Column name> or – most often – the result of an SQL predicate or search condition) that evaluates to a truth value. The result is also a truth value, derived by applying the given Boolean operator(s) to the boolean_argument result.

IS is a monadic Boolean operator. It tests for a condition: is the result of the expression TRUE, is it FALSE or is it UNKNOWN? You use the <Boolean test> to influence a search condition result, since its effect is to change a Boolean value (which is TRUE or FALSE or UNKNOWN) to either TRUE or FALSE. For example, consider these SQL statements, which create a Table that contains four rows:

CREATE TABLE Boolean_Test (
   column_1 SMALLINT);

INSERT INTO Boolean_Test (column_1)
VALUES (5);

INSERT INTO Boolean_Test (column_1)
VALUES (NULL);

INSERT INTO Boolean_Test (column_1)
VALUES (0);

INSERT INTO Boolean_Test (column_1)
VALUES (10);

Row 1 of the Table BOOLEAN_TEST contains 5, row 2 contains NULL, row 3 contains 0 and row 4 contains 10. Normally, of course, a search for equality doesn’t find NULLs – so the result of this SQL statement:

SELECT column_1
FROM   Boolean_Test
WHERE  column_1 = 5;

is row 1 and the result of this SQL statement:

SELECT column_1
FROM   Boolean_Test
WHERE  column_1 <> 5;

is row 3 and row 4. If you add a <Boolean test>, though, you can override the comparison’s usual result. Thus, the result of this SQL statement:

SELECT column_1
FROM   Boolean_Test
WHERE  (column_1 = 5 IS UNKNOWN);

is row 2 – it returns the rows where the search condition is UNKNOWN, rather than the rows where it is TRUE. The result of this SQL statement:

SELECT column_1
FROM   Boolean_Test
WHERE (column_1 = 5 IS FALSE);

is row 3 and row 4 – it returns only the rows where the search condition is FALSE. The result of this SQL statement:

SELECT column_1
FROM   Boolean_Test
WHERE (column_1 = 5 IS TRUE);

is row 1 – it returns the rows where the search condition is TRUE. Since this is the same result you’d get without the <Boolean test>, adding it is redundant. Finally, the result of this SQL statement:

SELECT column_1
FROM   Boolean_Test
WHERE (column_1 = 5 IS NOT FALSE);

is row 1 and row 2 – it returns the rows where the search condition is either TRUE or UNKNOWN. Table 9-1 shows how the result of a Boolean IS operation is determined.

Table 9.1 Truth values for the Boolean IS operator

If a Boolean value
is:

… and the operator is:

… then the result is:
TRUE IS TRUE TRUE
TRUE IS FALSE FALSE
TRUE IS UNKNOWN FALSE
FALSE IS TRUE FALSE
FALSE IS FALSE TRUE
FALSE IS UNKNOWN FALSE
UNKNOWN IS TRUE FALSE
UNKNOWN IS FALSE FALSE
UNKNOWN IS UNKNOWN TRUE
TRUE IS NOT TRUE FALSE
TRUE IS NOT FALSE TRUE
TRUE IS NOT UNKNOWN TRUE
FALSE IS NOT TRUE TRUE
FALSE IS NOT FALSE FALSE
FALSE IS NOT UNKNOWN TRUE
UNKNOWN IS NOT TRUE TRUE
UNKNOWN IS NOT FALSE TRUE
UNKNOWN IS NOT UNKNOWN FALSE

NOT is a monadic Boolean operator. It negates the result of a Boolean expression (except in the case of NULLs) – that is:

  • NOT ( TRUE ) returns FALSE.
  • NOT ( FALSE ) returns TRUE.
  • NOT ( UNKNOWN ) returns UNKNOWN.

AND is a dyadic Boolean operator. It increases the number of conditions that must be met by a value to be included in a search: the result is TRUE only if both conditions are TRUE. For example, the result of this SQL statement:

SELECT column_1
FROM   Boolean_Test
WHERE (column_1 > 0 AND column_1 < 10);

is row 1. Table 9.2 shows how the result of a Boolean AND operation is determined.

Table 9.2 Truth for the Boolean AND operator

If the first Boolean
value is:
… and the second
Boolean value is:
… then the result is:

TRUE TRUE TRUE
TRUE FALSE FALSE
TRUE UNKNOWN UNKNOWN
FALSE TRUE FALSE
FALSE FALSE FALSE
FALSE UNKNOWN FALSE
UNKNOWN TRUE UNKNOWN
UNKNOWN FALSE FALSE
UNKNOWN UNKNOWN UNKNOWN

OR is a dyadic Boolean operator. It decreases the number of conditions that must be met by a value to be included in a search: the result is TRUE if either condition is TRUE. For example, the result of this SQL statement:

SELECT column_1
FROM   Boolean_Test
WHERE (column_1 > 0 OR column_1 < 10);

is row 1 and row 3. Table 9.3 shows how the result of a Boolean OR operation is determined.

If the first Boolean
value is:
… and the second
Boolean value is:
… then the result is:

TRUE TRUE TRUE
TRUE FALSE TRUE
TRUE UNKNOWN TRUE
FALSE TRUE TRUE
FALSE FALSE FALSE
FALSE UNKNOWN UNKNOWN
UNKNOWN TRUE TRUE
UNKNOWN FALSE UNKNOWN
UNKNOWN UNKNOWN UNKNOWN

The precedence of the Boolean operators and their effect on Boolean values is as follows:

Precedence Operator Effect on Boolean value(s)
IS overrides normal result
NOT negates result
AND combines, with logical AND
OR combines, with logical inclusive OR

The precedence shown determines evaluation order, unless you use parentheses to force a different order. Although SQL’s three-valued logic can complicate things if we use contrived and unlikely examples, the normal situation is straightforward for any speaker of a human tongue. When we hear the English expression “Martians are vicious and dishonest but not stupid”, we know we could search them with the SQL expression x = 'vicious' AND x = 'dishonest' AND x <> 'stupid'. The correct application of the Boolean operators turns out to be an intuitive calculation for most people. The most common error is to forget what the operator precedence is, and that can be corrected easily: always use parentheses if the search condition contains two different Boolean operators.

If you want to restrict your code to Core SQL, don’t use the optional truth value Boolean test (i.e.: don’t use the constructs boolean_argument IS TRUE, boolean_argument IS FALSE or boolean_argument IS UNKNOWN) and don’t use boolean_argument unless it’s an SQL predicate or it’s enclosed in parentheses.

Scalar Operations

SQL provides no scalar functions that return or operate on a Boolean value.

Set Functions

SQL provides eight set functions that operate on Booleans: EVERY, ANY, SOME, COUNT, MAX, MIN and GROUPING. We’ll discuss them all in our chapter on set functions.

Predicates

Every SQL predicate returns a Boolean value. Since none of them operate strictly on truth values, we won’t discuss them here. Look for them in our chapters on search conditions and the various other <data type>s.