CD ROM Paradise Collection 4/CD ROM Paradise Collection 4 1995 Nov.iso/hobby/gim_308.zip/GIM11E.DOC

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          CHAPTER 11     LISTS OF SIMILAR PERSONS, FAMILIES AND NOTES

                                      OR

          SEARCHING YOUR FOLDER FOR SPECIFIC INFORMATION


PART E:  CONJUNCTIONS USED IN WHERE COMMANDS

          In this section we assume that you know what a WHERE command
          is, how it's used, and what it's used for.  We also assume
          that you know what operators and operands in WHERE commands
          are, and how they are used.  If you haven't done so already,
          review the first section in this chapter, which is entitled
          "An Introduction with Basic Examples", and skim the sections
          entitled "Operators Used in WHERE Commands" and "Operands
          Used in WHERE Commands".

DEFINITION...

          DEFINITION:  A CONJUNCTION

          After reading those sections, you know how to make sense of
          the following where commands:

          WHERE birth date is after 1950
          WHERE birth date is before 1970

          But so far, we've kept quiet about how to search for dates
          that are BETWEEN 1950 and 1970.  That is done by combining
          these two expressions into a single WHERE command by using
          the word AND, as follows:

          WHERE birth date is after 1950 AND birth date is before 1970

          The word "AND" is a conjunction.  GIM LISTS provides three
          conjunctions:  AND, OR, and NOT.  This section explains how
          to use these conjunctions in WHERE commands.

ATOMS...

          DEFINITION:  AN ATOM IN A WHERE COMMAND

          All of the following are examples of atoms:

          any place contains "Denmark"
          full is "Abe Lincoln"
          birth date is before 1964
          general source note is missing
          children gt 5
          children le 7

          If you've read all of the GIM LISTS sections of this manual
          up to this point, you've seen dozens of atoms.

          An atom is the smallest piece of a WHERE command that still
          makes sense on its own.  It consists of a left-hand operand,
          an operator, and most of the time, a right-hand operand.

COMBINING...

          COMBINING ATOMS TOGETHER

          Atoms can be combined in all sorts of ways, as you will see.

          For example, we could combine the first two atoms with an OR,
          like this:

          WHERE any place contains "Denmark" OR full is "Abe Lincoln"

          Or we could combine the last two atoms with an AND, like this:

          WHERE children GT 5 AND children LE 7

          In fact, we can combine any number of atoms -- whatever they
          are -- with ANDs and ORs and NOTs.

ABBREVIATIONS...

          If you've read the previous several sections of this manual,
          you know what an atom is.  For the rest of this section, we
          want to ignore the atoms and focus on the conjunctions.

          To help with this, we're going to represent atoms with double
          lower case letters -- like aa, bb, cc, and so on. When you
          see an abbreviation like "aa", you should realize that "aa"
          means "any atom that you want to put here".  This will help
          us to ignore the atoms in order to focus on the conjunctions.

          In other words, we won't talk about these WHERE commands:

          WHERE any place contains "Denmark" OR full is "Abe Lincoln"
          WHERE children GT 5 AND children LE 7

          ... instead, we'll just refer to these:

          WHERE aa OR bb
          WHERE cc AND dd

          That way we can talk about ANDs, ORs, and NOTs, without the
          values of the atoms themselves distracting us from the
          discussion of conjunctions.

PUTTING SOME OF THE PIECES TOGETHER...

          You can join as many atoms together as you want, just as long
          as there is an AND or an OR between each of them.  For
          example, any sequence like the following is perfectly legal:

          WHERE aa AND bb OR cc
          WHERE aa OR bb AND cc AND dd
          WHERE aa AND bb AND cc AND dd OR ee AND ff AND gg

PRECEDENCE PROBLEM...

          A PRECEDENCE PREDICAMENT

          Now, this brings up an interesting predicament.  Suppose for
          a moment that "aa" is "blue eyes", "bb" is "blonde hair", and
          "cc" is "female".

          If I'm searching for "aa AND bb", then I'm searching for
          someone with blue eyes AND blonde hair, and anyone I pick has
          to have both in order to be selected.

          On the other hand, if I'm searching for "aa OR bb", then I'm
          searching for someone with EITHER blue eyes OR blonde hair;
          it doesn't matter which -- I'll take someone with either one
          or the other.

          But what do I mean if I say "aa AND bb OR cc"?  This could be
          interpreted in either of two ways.

          On the one hand, it could mean that I'm looking for EITHER
          someone with blonde hair and blue eyes OR someone who's
          female.  That's one way to look at it, and it means that I'll
          take any female, and I'll also take any blue-eyed, blonde-
          haired male.

          On the other hand, it could mean that I'm looking for someone
          with blue eyes, and who also EITHER has blonde hair OR is
          female.  This is another way to look at it, and it means that
          I'll take any blue-eyed, blonde-haired male, or any blue-eyed
          female.

          This confusion arises from a problem that is solved by
          defining what's called "precedence".  In other words, there
          are "rules of precedence" that govern how GIM LISTS should
          resolve ambiguous situations like this one.

PRECEDENCE SOLUTION...

          THE PRECEDENCE PREDICAMENT SOLVED

          The simplest of these "rules of precedence" is this:  all
          other things being equal, AND takes precedence over OR.
          (If you are a programmer, this rule comes as no surprise.)

          As a result, WHERE aa AND bb OR cc, from our example above,
          should be thought of as:

          WHERE      aa AND bb     OR     cc

          ... rather than:

          WHERE      aa     AND     bb OR cc

PARENTHESIS...

          PARENTHESIS PRECEDENCE

          Now, if that's not what we mean, we can override the rules of
          precedence by using parentheses.  We can surround two or more
          atoms and their conjunctions with parentheses.  GIM LISTS
          will evaluate everything inside parentheses before evaluating
          anything outside of them.  (Again, to a programmer, this rule
          comes as no surprise.)

          Therefore, if we really mean to say:

          WHERE      aa     AND     bb OR cc

          ... we can say this:

          WHERE aa AND (bb OR cc)



          AN EXAMPLE OF PARENTHESES

          As another example, consider the following:

          WHERE aa AND bb OR cc AND dd AND ee OR ff AND gg

          Because all of the ANDs are grouped together first, GIM LISTS
          looks at this WHERE command and interprets it like this:

          WHERE aa AND bb

                   OR

                cc AND dd AND ee

                   OR

                ff AND gg

          But you can change this with parentheses.  For example, by
          adding parentheses to our example, like this:

          WHERE aa AND ((bb OR cc) AND dd) AND (ee OR ff) AND gg

          ... then we instruct GIM LISTS to interpret this as follows:

          WHERE aa

                AND

                bb OR cc     AND     dd

                AND

                ee OR ff

                AND

                gg

          Study this for a while, and you'll see what I mean.  The use
          of parentheses can get awfully intricate awfully fast, so
          it's usually best to keep your WHERE commands as simple as
          possible, just for your own sake.  But bear in mind that GIM
          LISTS can keep up with as many sets of parentheses as you can
          dish out, which can be useful and reassuring in those cases
          where you need your WHERE commands to be intricate.

NOT...

          THE NOT MODIFIER

          Any place where you can put an AND or an OR, you can put an
          AND NOT or an OR NOT.  So for example, you can say:

          WHERE aa AND NOT bb

          or

          WHERE aa AND bb OR NOT cc

          or

          WHERE aa AND NOT (bb AND cc)

          Doing so means what you think it means.  If "aa" is "blue
          eyes" and "bb" is "blonde hair", then "aa AND NOT bb" means
          "anybody who has blue eyes but who also DOESN'T have blonde
          hair."  (Hint:  if you find yourself trying to use the word
          "BUT" in a where clause, use "AND NOT" instead.)

          NOT may also be used by itself, without AND and OR, but this
          usage is somewhat rare.  For example, you can say:

          WHERE NOT aa

          ... as for example:

          WHERE NOT surname contains "Smith"

          ... but this simply means:

          WHERE surname doesn't contain "Smith"

          Therefore, although NOT may be used by itself, there are
          almost always other ways to say the same thing.