Kyle Smith wrote:
PJ wrote:
I hope somebody can make sense of this.
First, I don't understand what the difference is supposed to be between
& and && - is there a difference for php and/or mysql?
Second, I am trying to select all occurrences in a table.column where
the first letter of the name is I, J, or K but not I or J or K; I want
the all. Now, entering the following is no problem for any one letter:
SELECT * FROM book
WHERE id IN (SELECT bookID
FROM book_author WHERE authID IN (SELECT author.id
FROM author
WHERE LEFT(last_name, 1 ) = '$Auth' ));
But the following does not work:
SELECT * FROM book
WHERE id IN (SELECT bookID
FROM book_author WHERE authID IN (SELECT author.id
FROM author WHERE LEFT(last_name, 1 ) = '$Auth' &&
LEFT(last_name, 1 ) = '$Auth1' && LEFT(last_name, 1 ) = '$Auth2'));
Yet this gives off the wall results where the last names are nowhere
near what they should be.
Anyone have a rational explanation?
I wasn't aware that && was an option in MySQL, I've always used
"AND". If there is a difference it's probably the same as in PHP and
many other languages. Allow me to explain.
In the case of & the left-side expression is evaluated for true and if
not true, the right side expression is not evaluated. In the case of
&& both sides are evaluated regardless of the value of either side.
Here's an example:
$a = 1;
$b = 2;
$result = ($a == 2) && $b++; // In this case, $a does not equal two
and therefor the left-side is false, thus the right-side is never
evaluated. $b remains 2.
$result = ($a == 2) & $b++;
// In this case, $a still does not equal two and therefor the
left-side is still false, however because we use the '&' operator the
right side is evaluated anyway and $b now equals 3.
echo $result;
// In both cases, $result is false.
HTH,
Kyle
Oops, my first paragraph is backwards. I'm sure that was confusing! My
example is the correct logic for && and &, and the paragraph before that
is exactly wrong.
--
*Kyle Smith*
UNIX/Linux Systems Administrator
Inforonics, LLC
