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Reusing a result set

From: Accessing Databases with Object-Oriented PHP

Video: Reusing a result set

{QTtext}{width:960}{textColor:65280,65280,65280}{justify:center}{timescale:1000}{backColor:0,0,0}{plain}{font:Verdana}{size:20} Sometimes you need to reuse a result set. In the body of the page, there's a while loop on lines 27 to 29, that uses the The unordered list is displayed correctly, but the definition list, isn't.

Reusing a result set

Sometimes you need to reuse a result set. There are several ways of doing so. Chosing the right one, depends on the database system you're using. In the body of the page, there's a while loop on lines 27 to 29, that uses the This is pdo_reuse.php which you can find in the chapter 04_04_04 folder of the exercise files, along with the other files used in this video. The try block at the top of the page, submits a select query, and stores the result set in result. fetch column method to get the values from the first column, and displays an unordered list of links.

first column, and displays an unordered list of links. Further down on line 33 to 36, there's another while loop, that uses the same result object, to display a definition list. So let's load this in a browser to see what happens. The unordered list is displayed correctly, but the definition list, isn't. This is because a result set uses an internal cursor to keep track of the next row. Displaying the unordered list puts the cursor at the end of the result set.

And before trying to use the results again, we need to move the cursor back to the beginning. In PDO, if you're using a database such as post PostgreSQL that supports scrollable cursors, there's a way of doing that. Let's see how you do it. We need to open pdo_scrollable.php. And on line six, the setAttribute method has been called on the database connection object, before submitting the query. And this sets the cursor to scroll.

The set attribute method, takes two arguments, both of them PDO constants. The first one is the type of attribute that you want to control, in this case cursor. And the second one is what you want to do with it, in other words, make the cursor scroll. So, once you've got a scrollable cursor, you can reset the result set before reusing it. Let's see how that's done. We need to scroll down to the bottom. And here, on line 36, the fetch method is called with three arguments.

The first argument is a PDO constant which sets it as an associative array, sets the result set as an associative array. The second argument, sets the cursor. And what this is does is it sets the orientation of the cursor, to an absolute position. And the final argument is the position you're setting it to, which is zero, in other words, the beginning of the results set.

So this resets it back to the first row. The first row is stored as row, so you need to use a do while loop. And then the while condition also uses the fetch method, this time with the argument for the associative array, and the second one to fetch the orientation next. The problem with using this method, is that it doesn't work in all databases. So it's not portable. Let's just try this. I'm currently using SQLite.

And again, we've got the unordered list, we don't have the definition list. This won't work in MySQL either. So let's see if we can find an alternative solution that will work in all database systems. Go back to the original page, pdo_reuse.php, and the very simple way to do this, is to store all the results, as an array, so after getting the results set, we'll add a new line on line six.

And we'll call the new array names. And we'll use the result object, with the fetchAll method. By default, fetchAll returns each value twice, indexed both with a number and with a column name. Because we're getting a complete array, we need to keep memory use to a minimum, so it's a good idea to specify just one type of array. So we'll get an associative array. We do that by passing a PDO constant as an argument to fetchAll. So PDO, then two colons, and FETCH_ASSOC.

So, all the results in the result set are now in an associative array called names. So, we need to change the way in which everything is displayed. So, I'll go down to the first while loop, and we need to replace this with a for each loop. Replace that entirely, for each. And the array that we're going through is called names, and we'll refer to each item in there as name. So, instead of col here, we need a names array, and it is the name element that we want from the names array.

it is the name element that we want from the names array. And the same here. I've just noticed I've got names there. It should be the alias that we're using, so it should be name, name. Looks a bit strange, but this will work. And then to reuse, we need to change this also so a for reach loop. But arrays work in exactly the same way that they've got a sort of internal cursor. And we need to reset the array before we can use it.

So let's add a new line here. And we use the reset method, this is a PHP function. And we just pass it the array that we're working with, so it's names. And this will automatically set the internal cursor for the names array back to the beginning. So, this needs to be, again, for each names is name and then instead of row, we are using name.

And again here. And in the last row. So, if we save that, go back to the browser, go back to this PDO reuse page, and refresh it, we now get the definition list at the bottom of the page. So this works very well. It's totally portable. There's only one potential disadvantage, and that's that it will tie up a lot of memory, if you're working with a very large dataset.

So, there is another way in which you can approach this. If we open pdo_resubmit.php, here on line 35. The query has simply been submitted again. So, the result set is available for a second time. I won't bother to go through all the code. It's extremely simple, and you can see it for yourself, in the exercise files. But, if we load this page into a browser. We get the unordered list at the top, and the definition list at the bottom. So it works just fine, the only minor disadvantage, is that it does involve a second round trip to the database server.

it does involve a second round trip to the database server. So, to reuse a result set, you have three options. The first one that we looked at, was using a scrollable cursor, but this works only with some databases. The other one is to use fetchAll to get everything as an array. And finally, to resubmit the query. And with that, we come to the end of our review of accessing databases with PDO Database Abstraction layer.

The remaining chapters, are dedicated to the object oriented interface of MySQL Improved, which works only with MySQL and MARIADB

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This video is part of

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Accessing Databases with Object-Oriented PHP

47 video lessons · 1946 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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