Question

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Retrieve (or simulate) full query from PDO prepared statement

I can’t figure out why my query is returning 0 rows.. it implements some very dynamic search functionality, and a lot of if/loop statements etc. Therefor to debug it, I’d like to see EXACTLY what string is being sent to the server. Is there a way to do this through PHP?

Is there maybe a way to ask the server “what was the last query”, or tell PDO “show me what you sent”?

I saw one response using str_replace to manually enter the values in place of :fieldValue, but it’s likely a syntax problem (or maybe it’s going through an incorrect loop, etc), which this method doesn’t help with.

Using bindValue(":fieldValue", $value); if that makes a difference.

EDIT

Turns out it was a simple if ($var="true") { ... which should have been if ($var=="true") { .... PHP I guess is not the same as Java in that sense? Either way, the question still stands (as I run into this often). I had to use a series of echo "You are Here"; to find this error, as it was technically valid but not correct. If I had the final SQL statement, I could have seen “Oh, my code has added the where column = true, must have gone through the wrong IF…”.

Answer 1

That’s the single most common myth about SQL debugging. “I need to see the query after preparation to be able to tell if an error occurred”. The fact is, you don’t, and I’ll tell you why.

Once a query has been prepared, the placeholder can be considered as a valid string/integer. You don’t care what’s in it.

Also, if you set up PDO correctly, you’ll get a detailed PDOException detailing the error you’ve had along with a complete backtrace of where the error happened, plus you get the error string from MySQL, which makes syntax errors very easy to find.

To enable PDO Exceptions and disable emulated prepares:

$pdo = new PDO("mysql:host=localhost;dbname=database_name", "user", "password");
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);