Query By Form Definition

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In Query By Form (QBF) format, the columns of ONE table can be queried. In its simplest form, the field values determine the query condition:

ABC

 

which results in this query:

SELECT     *

FROM       <table>

WHERE      <column> = 'ABC'

 

Wildcards, operators, and special characters can be used to refine the query:

Token

Description

Example

Resulting where clause

%

Wildcard

H%

<column> LIKE 'H%'

*

Wildcard

H*

<column> LIKE 'H%'

_

Wildcard

H_H

<column> LIKE 'H_H'

?

Wildcard

H?H

<column> LIKE 'H_H'

NULL

NULL

NULL

<column> IS NULL

\

Escape

\<TAG\>

<column> = '<TAG>'

\\

\

\\AND\%

<column> = '\AND%'

The following table describes the operators that can be used:

Token

Description

Field filling

Resulting where clause

<

Less than

< 5

<column> < 5

<=

Less equal

<= 5

<column> <= 5

=

Equal

= 5

<column> = 5

>

Greater

> 5

<column> > 5

>=

Greater equal

>= 5

<column> >= 5

!

Not

!ABC

NOT (<column> = 'ABC')

 

 

!>5

NOT(<column> > 5)

 

 

!ABC%

NOT(<column> LIKE 'ABC%')

&

And

>5 & <10

<column> > 5 AND <column> < 10

|

Or

5 | 6

<column> = 5 OR <column> = 6

()

Nesting of conditions

!(5 | 6)

NOT (<column> = 5 OR <column> = 6)

 

 

(>5 & < 7)|

(>10 & < 20)

( <column> > 5 AND <column> < 7) OR

(<column> > 10 AND <column> < 20)

AND

And

 

 

OR

Or

 

 

•'AND' and 'OR' are aliases for the '&' and '|' operators and can be used everywhere where these operators can be used.

•<, <=, =, > are comparison operators. These operators are followed by a value. In combination with this value they form a Boolean expression.

•A value on itself without a comparison operator is also a Boolean expression. It implicitly uses the '=' operator.

•&, |, AND, OR are logical operators. These operators are preceded and followed by a Boolean expression. With these expressions they form a new Boolean expression.

•! is a unary operator. This operator expects a Boolean expression on the right side. With this expression it forms a Boolean expression.

•() encapsulates a Boolean expression.

•In a YACC (Yet Another Compiler-Compiler) parser-like notation the definition of the syntax allowed is:

expression : VALUE

        |    < VALUE

        |    <= VALUE

        |    = VALUE

        |    > VALUE

        |    >= VALUE

        |    ! expression

        |    expression & expression

        |    expression | expression

        |    ( expression )

        |    expression AND expression

        |    expression OR expression

        ;