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Closing the cursor before running another query

From: Accessing Databases with Object-Oriented PHP

Video: Closing the cursor before running another query

In the previous video, we created a transaction On line nine is a new prepared statement with a named If bal is less than 0, the conditional statement So, we'll load it into a browser.

Closing the cursor before running another query

In the previous video, we created a transaction to transfer money from one account to another. But there was nothing to prevent the transaction from going ahead if the payer ran out of funds. This is pdo_check_balance.php which you can find in the Chapter 3, 03_07 folder of the exercise files. The script has been updated to roll back the transaction if the balance in the outgoing account falls below zero.

Adding this extra step results in some databases displaying the wrong balance after the transaction has been rolled back. The fix is very easy but first let's take a look at the code and see what's happening. On line nine is a new prepared statement with a named placeholder that gets the balance from the savings account for the payer. Then on lines 16 and 17 the statement is prepared and the payer's name is bound to the named placeholder.

The third statement is executed a bit further down, here on line 31. And then, on line 32, the fetchColumn method is used to get the result. There's only one result. So that's used to get it and it's assigned bal. If bal is less than 0, the conditional statement a bit further down, here on lines 35 to 38. That rolls back the transaction and sets an appropriate error message. The rest of the transaction is inside the else block.

All this looks fairly straightforward, but let's test it with SQLite. So, we'll load it into a browser. And John White has gone down to 600. Black's gone fine there. 1,400 for Jane Black. Refresh. That seems to be okay. Refresh again. 200. He's now at zero. The next time, it should fail. Transaction failed: insufficient funds in John White's account. But look, it says that John White's account is now at minus $200. That's clearly quite wrong. This gives the impression that the roll back failed, but that's not the case. So let's go back to the code. The check prepared statement that's executed on line 31 gets the pair's balance as part of the transaction.

Then, further down. This for each loop on line 71 runs another select query to get the balance again. What's happening is that the result from the first select statement isn't being cleared before the second one is run. The solution is to clear the first statement using the closeCursor method. This frees the database connection to execute the subsequent query correctly. So, let's go back up to where the check statement is executed, and then here, we need to call check, closeCursor.

So, if we save the page, and go back to the browser, and refresh it. This time we get transaction failed, insufficient funds in John White's account. But we've now got the correct balance of 0, and 2,000 for Jane Black. And just to prove that it works correctly, let's go back, and we'll reverse the payments. So, we'll make John White the payee. And we'll make Jane Black the payer. We'll save that.

Go back to the browser and refresh. We'll have do this quite a few times, but as we keep on going, now we've got her at zero. We don't have the minus 200 at all. Incidentally, if you run the original script using MySQL as your database, it'll work correctly without needing to call closeCursor. However, using closeCursor makes the code more portable, so if we just go back, this is what we have done. Here we have used closeCursor to free the connection to the server to allow other queries to be run, but it leaves the statement object in a state that allows it to be executed again if necessary.

It's not required by all databases, but certainly, calling this method doesn't do any harm.

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This video is part of

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Accessing Databases with Object-Oriented PHP

47 video lessons · 3315 viewers

David Powers
Author

 
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  1. 13m 33s
    1. Welcome
      1m 4s
    2. What you should know before watching this course
      2m 8s
    3. Using the exercise files
      4m 56s
    4. Setting SQLite permissions
      1m 11s
    5. A quick primer on using PHP objects
      4m 14s
  2. 10m 12s
    1. Overview of PHP database APIs
      4m 5s
    2. Using prepared statements
      4m 24s
    3. Using transactions
      1m 43s
  3. 48m 57s
    1. Creating a database source name
      2m 3s
    2. Connecting to a database with PDO
      7m 27s
    3. Looping directly over a SELECT query
      3m 49s
    4. Fetching a result set
      8m 3s
    5. Finding the number of results from a SELECT query
      7m 14s
    6. Checking if a SELECT query contains results
      3m 32s
    7. Executing simple non-SELECT queries
      6m 2s
    8. Getting error messages
      7m 17s
    9. Using the quote() method to sanitize user input
      3m 30s
  4. 39m 51s
    1. Binding input and output values
      2m 36s
    2. Using named parameters
      9m 51s
    3. Using question marks as anonymous placeholders
      2m 35s
    4. Passing an array of values to the execute() method
      5m 20s
    5. Binding results to variables
      7m 53s
    6. Executing a transaction
      6m 54s
    7. Closing the cursor before running another query
      4m 42s
  5. 21m 20s
    1. Generating an array from a pair of columns
      2m 44s
    2. Setting an existing object's properties with a database result
      4m 42s
    3. Creating an instance of a specific class with a database result
      6m 1s
    4. Reusing a result set
      7m 53s
  6. 38m 14s
    1. Connecting to a database with MySQLi
      5m 57s
    2. Setting the character set
      1m 57s
    3. Submitting a SELECT query and getting the number of results
      4m 4s
    4. Fetching the result
      7m 35s
    5. Rewinding the result for reuse
      3m 20s
    6. Handling non-SELECT queries
      5m 27s
    7. Getting error messages
      5m 47s
    8. Sanitizing user input with real_escape_string()
      4m 7s
  7. 27m 49s
    1. Initializing and preparing a statement
      4m 17s
    2. Binding parameters and executing a prepared statement
      5m 55s
    3. Binding output variables
      5m 6s
    4. Executing a MySQLi transaction
      7m 5s
    5. Dealing with "commands out of sync" in prepared statements
      5m 26s
  8. 24m 7s
    1. Buffered and unbuffered queries
      4m 19s
    2. Using real_query()
      6m 1s
    3. Freeing resources that are no longer needed
      2m 31s
    4. Submitting multiple queries
      6m 41s
    5. Creating an instance of a class from a result set
      4m 35s
  9. 3m 31s
    1. PDO and MySQLi compared
      3m 31s

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