subquery workaround in MySQL 4

Last Updated on: September 16, 2022

One of the most valuable additions to MySQL 5 is the subquery, which lets you execute SQL like this:

SELECT username FROM users WHERE user_id IN (SELECT user_id FROM admins);

Unfortunately, MySQL 4 lacks this feature and is still prevalent in many web hosts. If you need to use a subquery, try the following workaround in PHP;

# Get the admin user_ids into an array
$sql = "SELECT user_id FROM admins";
$rs = mysql_query($sql);
$adminuserids = array();

while ($ra = mysql_fetch_array($rs)) {
  $adminuserids[] = $ra['user_id'];

# Now the clever bit - implode your array into a comma seperated string
$adminuserids = implode(',',$adminuserids);

# Our sql with "sub query"
$sql = "SELECT username FROM users WHERE user_id IN ($adminuserids)";

Step by step:

  • Get an array of user_ids from the “admins” table
  • implode the array, so we get a string similar to “1,5,7,8,4,9”
  • append the string to the SQL, so we get something like: “SELECT username FROM users WHERE user_id IN (1,5,7,8,4,9);”

This is a bit more code to do, but depending on the SQL you are writing, this may save you time and code if you are writing for MySQL 4.

Plus, the added benefit is that it is pretty easy to upgrade the code if/when you move to MySQL 4.

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