SQL = IF NOT EXISTS in a prepared statement? - php

Question

I'm trying to check if a record already exists in my table and if it doesn't I want to execute an insert... using a prepared statement. Can anyone tell me what's wrong below? I've written the code with error checking and it basically says the query is poo :) I am atrocious when it comes to SQL and pretty much anything programming related so I really do appreciate any wisdom that shared on this dodgy looking quest... Thanks!
$mysqli = mysqli_connect($config['host'], $config['user'], $config['pass'], $config['db']);
$timestamp = time();
$stmt = $mysqli->prepare("IF NOT EXISTS (SELECT id FROM course_licence_cart WHERE userid = ? AND courseid = ? AND lmsid = ?) BEGIN INSERT INTO course_licence_cart (lmsid, userid, courseid, assigned_by, assigned_on) VALUES (?, ?, ?, ?, ?) END");
foreach($_POST['assignTo'] as $assignTo){
$stmt->bind_param('iiiiiiii', $assignTo, $_POST['course'], $core['id'], $core['id'], $assignTo, $_POST['course'], $userInfo['id'], $timestamp);
$stmt->execute();
}
FYI: This takes place after a form submission, I've checked ALL of the variables and they're all good, and the process itself works perfectly (I have it working without the IF NOT EXISTS), it's just this new query type mixed with prepared statements that has totally thrown me off.
-- UPDATE --
I ran the following directly:
IF NOT EXISTS (SELECT `id` FROM `course_licence_cart` WHERE `userid` = '175' AND `courseid` = '1' AND `lmsid` = '1') BEGIN INSERT INTO `course_licence_cart` (`lmsid`, `userid`, `courseid`, `assigned_by`) VALUES ('1', '175', '1', '175') END
In which I get the error:
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'IF NOT EXISTS (SELECT `id` FROM `course_licence_cart` WHERE `userid` = '175' AND' at line 1

Answer 1

First of all the root of the error has nothing to do with prepared statements. It's just you can't use IF, BEGIN ... END blocks and other constructs out of the scope of a stored routine (procedure, function, trigger, event).
To prevent duplicates you can leverage INSERT IGNORE like so
$stmt = $mysqli->prepare("INSERT IGNORE INTO course_licence_cart (lmsid, userid, courseid, assigned_by, assigned_on) VALUES (?, ?, ?, ?, ?)");
foreach($_POST['assignTo'] as $assignTo){
$stmt->bind_param('iiiiiiii', $assignTo, $_POST['course'], $core['id'], $core['id'], $assignTo, $_POST['course'], $userInfo['id'], $timestamp);
$stmt->execute();
}
In order for this to work you have to have a unique constraint defined.
CREATE UNIQUE INDEX index_name
ON course_licence_cart (userid, courseid, lmsid);
Here is SQLFiddle demo
Now your code (after correcting syntax) could've worked in a stored procedure like so
DELIMITER //
CREATE PROCEDURE add_to_cart(IN _lmsid INT, IN _userid INT, _courseid INT, IN _assigned_by INT, IN _assigned_on DATETIME)
BEGIN
IF NOT EXISTS (SELECT *
FROM course_licence_cart
WHERE userid = _userid
AND courseid = _courseid
AND lmsid = _lmsid) THEN
INSERT INTO course_licence_cart (lmsid, userid, courseid, assigned_by, assigned_on)
VALUES (_lmsid, _userid, _courseid, _assigned_by, _assigned_on);
END IF;
END//
DELIMITER ;
Here is SQLFiddle demo
In this case php code would look like
$stmt = $mysqli->prepare("CALL add_to_cart (?, ?, ?, ?, ?)");
foreach($_POST['assignTo'] as $assignTo){
$stmt->bind_param('iiiiiiii', $assignTo, $_POST['course'], $core['id'], $core['id'], $assignTo, $_POST['course'], $userInfo['id'], $timestamp);
$stmt->execute();
}

Answer 2

Since you're coding in PHP, one thing to consider is doing a sql query to SELECT from that table, if nothing is returned, run an insert query.
I'm sure it can be doing through SQL, but I personally don't know how.
Good luck, sorry if this doesn't help.

Answer 3

in MYSQL there are 4 types of inserts available.
1)INSERT 2)INSERT IGNORE 3)INSERT ON DUPLICATE KEY 4)REPLACE
Please explore on them.
As far as I can understand your specific case can be handled by INSERT IGNORE command.
Note : I am assuming here that Mysqli is same as MYSQL
Update: now I know that mysqli is interface to MYSQL.But the concept of insert ignore will still be same.
example to understand use of INSERT IGNORE
consider below table.
CREATE TABLE person_tbl ( first_name CHAR(20) NOT NULL, last_name CHAR(20) NOT NULL, sex CHAR(10), PRIMARY KEY (last_name, first_name) );
here firstname&lastname forms primary key
now we run query
mysql> INSERT INTO person_tbl VALUES( 'Jay', 'Thomas');
it will add one row in table
if we will run the above sql it will fail with duplicate record exception.to prevent this exception we have 2 options
1)check if record already exists,if not then insert record.... we have to fire select and then insert query.
2) fire INSERT IGNORE SQL.. it will check the if record exist in table if exist then it will not insert record ,if not then inserts record.
for e.g
if we run query
mysql> INSERT IGNORE INTO person_tbl VALUES( 'Jay', 'Thomas');
this sql will not insert any record ..as record already exists
but
mysql> INSERT IGNORE INTO person_tbl VALUES( 'Vijay', 'Thomas');
it will insert one record in table.