如何在SQL中搜索列中的确切字符串

My goal is to search column A for string B and if it is found, return which row it was found in, and if it wasn't found, I would need to know that as well in order to take a different course of action.

My current PHP code:

$string = "teststring";
$searchquery = "SELECT *
                  FROM AllStringsTable 
                 WHERE `Column_A` LIKE '$string'"

$searchresult = mysql_query($searchquery) or die(mysql_error());
$row = mysql_fetch_row($searchresult);
echo "The returned row was: $row";

This just breaks and does nothing, so I think I'm way off here. Also, I have read that for exact string searching that doesn't require wildcard substrings, etc, LIKE is not needed. So I'm not sure what I would use instead...

You're almost there. You need the % wildcards:

// First, prevent sql injection with mysql_real_escape_string
$string = mysql_real_escape_string($string);

$searchquery = "SELECT * FROM AllStringsTable WHERE `Column_A` LIKE '%{$string}%'";
//  ----------------------------------------------------------------^^^-------^^^

$searchresult = mysql_query($searchquery) or die(mysql_error());

if (mysql_num_rows($searchresult) == 0) {
  echo "no rows found";
}
else {

  // You need to loop over the result resource to get all the rows.
  // Better to use mysql_fetch_array()
  while ($row = mysql_fetch_array($searchresult)) {
    $print_r $row;
}

If you want to do an exact match, use = instead of LIKE:

SELECT ... WHERE Column_A = '$string';

If you want to do a substring match (which I suspect is more what you want), use LIKE with the % wildcards:

SELECT ... WHERE Column_A = '%$string%';

The difference is that the first query requires that the entire Column_A matches exactly. The second query requires only that the exact word is found somewhere in the column.