Only Insert if unique(check 2 rows) - php

Question

I am learning SQL and I am in situation where I have to INSERT values into database only if it do not exist already.
My tableview structure is like this:
+----------+-----------+-----+-----+
| first_id | second_id | timestamp |
+----------+-----------+-----------+
I Want to insert only if there is not same first_id and second_id e.g if in table there is first_id 1 and second_id 2 and I am adding it again, I do not want to add it anymore. So if the first_id and second_id rows already have values 1 and 2 then do not add but if first_id is 3 and second_id is 1 then I would allow the inserting.
This is my query ATM:
INSERT INTO `testtable`.`ids` (`first_id`, `second_id`) VALUES (:first_id, :second_id)
And like this I tried with NOT EXISTS but it is not working:
NOT EXISTS (SELECT first_id, second_id FROM `testtable`.`ids` WHERE first_id = : first_id AND second_id = : second_id) INSERT INTO `testtable`.`ids` (`first_id `, `second_id `) VALUES (: first_id, : second_id)
The last mentioned query gives me Syntax error but once I even got the integrity violation and it told me to check documentation.
I am executing my queries using PHP ->query(""); function.
I tried to do like IF NOT EXISTS and NOT EXISTS but those didn't work. How should I approach to this?

Answer 1

This is simple. Declare first_id and second_id as composite key. I would prefer not to make any changes in your PHP Code but make your DB structure versatile so that it doesn't accept any duplicate values how-so-ever you are inserting it.
CREATE TABLE `demo` (
`first_id` smallint(6) DEFAULT NULL,
`second_id` smallint(6) DEFAULT NULL,
`timestamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
UNIQUE KEY `first_id` (`first_id`,`second_id`)
)
Now first_id and second_id will never accept duplicate values.
^ table contains values (1,2). Now inserting (1,3).
^the table accepts (1,3). Now inserting (1,2) again.
the insert statement throws an error. Now the table will never accept duplicate values for the key (first_id,second_id).
If the table already exists and you're not creating it from scratch, simply execute:
alter table `table_name` add unique key (first_id, second_id);
This will prevent the duplicate values henceforth.

Answer 2

You may try this if you are using PHP and MySql:
<?php
//Added database connection code here
$first_id = $_POST['first_id '];
$second_id = $_POST['second_id '];
$sql = "select * from ids where first_id = ".$first_id ." and second_id ='".$second_id."'" ;
$result = $mysqli->query($sql);
$row = $result->fetch_row();
if($row[0]) {
$mysqli->close();
} else {
//preapare an insert statement
$stmt = $mysqli->prepare(" INSERT INTO `ids` (first_id, second_id) VALUES (?,?)");
$stmt->bind_param("ii", $first_id, $second_id);
//execute the statement
if( $stmt->execute() ) {
unset($first_id);
unset($second_id);
} else {
echo $mysqli->error;
}
//close statement
$stmt->close();
$mysqli->close();
}
?>

Answer 3

Checking for existence for the first and second ids and additionally declare 2 sql variables for first and second ids and set them as per required.
DECLARE #Exists int; #first_id int; #second_id int;
SET #first_id = 1;
SET #second_id = 2;
SELECT #Exists = COUNT(*) FROM [testtable] where [first_id] = #first_id and [second_id] = #second_id;
Condition to insert if count of matched records is 0:
IF(#Exists = 0)
BEGIN
INSERT INTO [testtable](first_id, second_id)
VALUES(#first_id,#second_id)
END