How to SELECT one field form one table which is part of string contained in one field of another table?

Question

use PHP and MYsql. I have 2 tables like

• table 'tag': has field 'id' and 'tag' ('tag' is varchar)

• table 'entry': has fields 'tags' and other fields ('tags' is varchar that stores many words as tag, each word is separated by comma. Then I use function 'explode' to extract tags from string to store in array '$tags'. All tags from table 'entry' are stored in table 'tag'.)

What I want is to select all 'tag's from table 'tag' which are tags in the same string of 'tags' of table 'entry' that has a given tag (variable $tag). But no idea, I just try to SELECT tag from table 'entry' which string in 'tags' contains a specified tag (variable $tag).My current code is

$count=0;
$sql="SELECT tags FROM entry WHERE tag LIKE '%$tag%'";
$result = mysql_query($sql);
while($data=mysql_fetch_array($result)){
  if(mysql_num_rows($result)!=0){
    $tags1=explode(',',$data['tags']);
    for($loop=0; $loop<=count($tags1)-1; $loop++){
      if(!in_array($tags1[$loop], $tags2)){
        $count+=1; 
        $tags2[$count]=$tags1[$loop];
      }
    }
  }
}

Answer 1

The super inefficient, SQL injection attack prone way of doing it as one statement would be:

$sql="SELECT tags FROM entry WHERE 1=1 ";

Then add each tag to the query before running it:

$sql.="AND tag LIKE '" . $tags1[$loop] . "' ";

If you want to match any tag instead of all tags replace AND with OR .

I'd really suggest normalizing the tags and looking into not building SQL queries that way though.

Answer 2

The difficulties you are having solving this problem all stem from the fact that you're storing a list of tags in a single column.

You really need to add an intersection table, to contain the data for the many-to-many relationship between tags and entries. This is the proper way to design a relational database, and many SQL queries will become more efficient if you do that.

For example, to get all the tags from an entry that matches tag $tag :

<?php

$sql = "SELECT t1.tag FROM tag AS t1
INNER JOIN entries_tags AS e1 ON e1.tag = t1.id
INNER JOIN entries_tags AS e2 ON e2.entry = e1.entry
INNER JOIN tag AS t2 ON t2.id = e2.tag
WHERE t2.tag = ?";

$stmt = $pdo->prepare($sql);
$result = $stmt->execute( array($tag) );

$data = $result->fetchAll();
$count = count($data);

I wrote a chapter to detail the practical problems caused by this design in my book, SQL Antipatterns: Avoiding the Pitfalls of Database Programming .