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Igor
Igor

Reputation: 6255

How to check if the value in the mySQL DB exist?

I have a mySQL database and I have a Perl script that connects to it and performs some data manipulation. One of the tables in the DB looks like this:

CREATE  TABLE `mydb`.`companies` (
  `company_id` INT NOT NULL AUTO_INCREMENT,
  `company_name` VARCHAR(100) NULL ,
  PRIMARY KEY (`company_id`) );

I want to insert some data in this table. The problem is that some companies in the data can be repeated.

The question is: How do I check that the "company_name" already exist? If it exist I need to retrieve "company_id" and use it to insert the data into another table. If it does not, then this info should be entered in this table, but I already have this code.

Here is some additional requirement: The script can be run multiple times simultaneously, so I can't just read the data into the hash and check if it already exist.

I can throw an additional "SELECT" query, but it will create an additional hit on the DB.

I tried to look for an answer, but every question here or the thread on the web talks about using the primary key checking. I don't need this. The DB structure is already set but I can make changes if need to be. This table will be used as an additional table.

Is there another way? In both DB and Perl.

Upvotes: 3

Views: 2382

Answers (3)

Buttle Butkus
Buttle Butkus

Reputation: 9456

"The script can be run multiple times simultaneously, so I can't just read the data into the hash and check if it already exist."

It sounds like your biggest concern is that one instance of the script may insert a new company name while another script is running. The two scripts may check the DB for the existence of that company name when it doesn't exist, and then they might both insert the data, resulting in a duplicate.

  1. Assuming I'm understanding your problem correctly, you need to look at transactions. You need to be able to check for the data and insert the data before anyone else is allowed to check for that data. That will keep a second instance of the script from checking for data until the 1st instance is done checking AND inserting.

    Check out: http://dev.mysql.com/doc/refman/5.1/en/innodb-transaction-model.html

    And: http://dev.mysql.com/doc/refman/5.1/en/commit.html

    MyISAM doesn't support transactions. InnoDB does. So you need to make sure your table is InnoDB. Start your set of queries with START TRANSACTION.

  2. Alternatively, you could do this, if you have a unique index on company_name (which you should).

    $query_string = "INSERT INTO `companies` (NULL,'$company_name')";

    This will result in an error if the company_name already exists. Try a sample run attempting to insert a duplicate company name. In PHP, 

    $result = mysql_query($query_string);

    $result will equal false on error. So,

    if(!$result) {
  $query2 = "INSERT INTO `other_table` (NULL,`$company_name`)";
  $result2 = mysql_query($query2);
}

    If you have a unique key on company_name in both tables, then MySQL will not allow you to insert duplicates. Your multiple scripts may spend a lot of time trying to insert duplicates, but they will not succeed.

EDIT: continuing from the above code, and doing your work for you, here is what you would do if the insert was successful.

if(!$result) {
  $query2 = "INSERT INTO `other_table` (NULL,`$company_name`)";
  $result2 = mysql_query($query2);
} else if($result !== false) { // must use '!==' rather than '!=' because of loose PHP typing
  $last_id = mysql_insert_id();
  $query2 = "UPDATE `other_table` SET `some_column` = 'some_value' WHERE `id` = '$last_id'";
  // OR, maybe you want this query
  // $query2a = "INSERT INTO `other_table` (`id`,`foreign_key_id`) VALUES (NULL,'$last_id');
}

Upvotes: 4

Alexandr Ciornii
Alexandr Ciornii

Reputation: 7392

For InnoDB use transaction. For MyISAM lock table, do modifications, unlock.

Upvotes: 0

Balu
Balu

Reputation: 128

I suggest you write a stored procedure(STP), which takes input as company name. In this STP, first check existing company name. If it exists, return id. Otherwise, insert and return id.

This way, you hit DB only once

Upvotes: 0

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