- How to add column to mysql database table after the table has already been created

- How to delete column from mysql database table after the table has already been created

- How to add foreign key to table column after the table has already been created

Basically, all this can be done by ‘ALTER TABLE‘ statement. Here is an step-by-step tutorial on how we do it.

Create parent table

CREATE TABLE IF NOT EXISTS parent(
id INT(11) UNSIGNED NOT NULL AUTO_INCREMENT,
name VARCHAR(255) NOT NULL,
description TEXT DEFAULT NULL,
status TINYINT(1) NOT NULL,
PRIMARY KEY(id)
);


Create child table

CREATE TABLE IF NOT EXISTS child(
id INT(11) UNSIGNED NOT NULL AUTO_INCREMENT,
name VARCHAR(255) NOT NULL,
description TEXT DEFAULT NULL,
PRIMARY KEY(id)
);


After we create both parent and child table, we remember that:-

- We have added an extra unnecessary column in parent table.
- We have missed adding a column to child table.
- And this column in child table should be the foreign key for the parent table.

So, our step would be:-

First of all, delete the unncessary column from parent table

ALTER TABLE parent DROP status;

Secondly, add a new column in child table

ALTER TABLE child ADD parent_id int(11);

Finally, add foreign key contraint to the child table which relates to the parent table’s column (primary key)

ALTER TABLE child ADD CONSTRAINT child_FK_1
FOREIGN KEY (parent_id) REFERENCES parent (id)
ON DELETE CASCADE;

More detail on MySQL foreign key and its implementation can be found on: MySQL Database: Foreign Key Understanding and Implementation

Thanks.

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1. MySQL Database: Foreign Key Understanding and Implementation
2. SQL Error: Cannot insert the value NULL into column… column does not allow nulls. INSERT fails.
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A FOREIGN KEY in one table points to a PRIMARY KEY in another table.

A foreign key is a field in a relational table that matches the primary key of another table. The foreign key can be used to cross-reference tables. A table may have multiple foreign keys, and each foreign key can have a different referenced table.

A Foreign Key is a Referential Constraint between two tables. A Referential Constraint or Referential Integrity is a property of data which, when satisfied, requires every value of one attribute (column) of a relation (table) to exist as a value of another attribute in a different (or the same) relation (table).

For MySQL, to support foreign key, the table type should be ‘InnoDB‘. Therefore, when we are cross-referencing two tables through foreign key, both the tables must be InnoDB tables and they must not be Temporary tables.

InnoDB also supports foreign key references within a table. The referencing and referenced table may be the same table, i.e. the foreign key refers back to the same table. Such a foreign key is known as a self-referencing or recursive foreign key.

When the user attempts to delete or update a row from a parent table, and there are one or more matching rows in the child table, InnoDB handles this through “Referential Action“. There are five options regarding the action to be taken. If ON DELETE or ON UPDATE are not specified, the default action is RESTRICT.

CASCADE: Delete or update the row from the parent table and automatically delete or update the matching rows in the child table. Both ON DELETE CASCADE and ON UPDATE CASCADE are supported.

SET NULL: Delete or update the row from the parent table and set the foreign key column or columns in the child table to NULL. This is valid only if the foreign key columns do not have the NOT NULL qualifier specified. Both ON DELETE SET NULL and ON UPDATE SET NULL clauses are supported.

NO ACTION: In standard SQL, NO ACTION means no action in the sense that an attempt to delete or update a primary key value is not permitted to proceed if there is a related foreign key value in the referenced table. InnoDB rejects the delete or update operation for the parent table.

RESTRICT: Rejects the delete or update operation for the parent table. Specifying RESTRICT (or NO ACTION) is the same as omitting the ON DELETE or ON UPDATE clause. The main difference between NO ACTION and RESTRICT is that with NO ACTION the referential integrity check is done after trying to alter the table. RESTRICT does the check before trying to execute the UPDATE or DELETE statement. However, in MySQL, foreign key constraints are checked immediately, so NO ACTION is the same as RESTRICT.

SET DEFAULT: Similar to SET NULL, the foreign key values in the referencing row are set to the column default when the referenced row is updated or deleted. This action is recognized by the parser, but InnoDB rejects table definitions containing ON DELETE SET DEFAULT or ON UPDATE SET DEFAULT clauses.

Here, I will be implementing the foreign key CASCADE update and delete.

CASCADE:-

Whenever rows in the master (referenced) table are deleted, the respective rows of the child (referencing) table with a matching foreign key column will get deleted as well. This is called a cascade delete.

Example Tables: test_customer(customer_id, name, address) and test_order(order_id, customer_id, status)

test_customer is the master table and test_order is the child table, where ‘customer_id’ is the foreign key in test_order and represents the customer who placed the order. When a row of test_customer is deleted, any test_order row matching the deleted test_customer’s customer_id will also be deleted.

Create Tables

CREATE TABLE test_customer (
customer_id int(10) unsigned NOT NULL auto_increment,
name varchar(100),
address varchar(100),
PRIMARY KEY (customer_id)
) ENGINE=InnoDB;

CREATE TABLE test_order (
order_id int(10) unsigned NOT NULL auto_increment,
customer_id int(10) unsigned NOT NULL,
status varchar(30),
PRIMARY KEY (order_id)
) ENGINE=InnoDB;

Add Foreign Key

ALTER TABLE test_order
ADD CONSTRAINT FK_test_order
FOREIGN KEY (customer_id) REFERENCES test_customer(customer_id)
ON UPDATE CASCADE
ON DELETE CASCADE;

Alternative way

Alternatively we can directly add Foreign Key at the time of table creation. You may choose any one way (the following way or the way above) for creating tables and assigning foreign key.

SET foreign_key_checks = 0;

SET foreign_key_checks = 1;

Insert sample data to test_customer and test_order tables

INSERT INTO test_customer
( customer_id , name , address )
VALUES
( '1', 'John', 'New York' ),
( '2', 'Tom', 'London' );

INSERT INTO test_order
( order_id , customer_id , status )
VALUES
( '1', '1', 'pending' ),
( '2', '1', 'processing' ),
( '3', '2', 'complete' ),
( '4', '2', 'pending' );

Doing UPDATE CASCADE

When we update customer_id in test_customer (parent table), the customer_id field in test_order (child table) will also get updated.

UPDATE test_customer SET customer_id = 3 WHERE customer_id = 1;

Note:-

We CANNOT directly update the foreign key in child table.

Doing this will result in error saying “Cannot add or update a child row: a foreign key constraint fails“.

UPDATE test_order SET customer_id = 4 WHERE customer_id = 3;

Doing DELETE CASCADE

When we delete customer_id in test_customer (parent table), the customer_id field in test_order (child table) will also get deleted.

DELETE FROM test_customer WHERE customer_id = 2;

Note:-

We CAN directly delete rows from test_order (child table) like below:-

DELETE FROM test_order WHERE customer_id = 3;

Hope this helps. Thanks

References:-
1. http://dev.mysql.com/doc/refman/5.1/en/innodb-foreign-key-constraints.html
2. http://en.wikipedia.org/wiki/Foreign_key

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The JOIN keyword is used in an SQL statement to query data from two or more tables, based on a relationship between certain columns in these tables. [@via w3schools]

A SQL JOIN clause combines records from two or more tables in a database. It creates a set that can be saved as a table or used as is. A JOIN is a means for combining fields from two tables by using values common to each. ANSI standard SQL specifies four types of JOINs: INNER, OUTER, LEFT, and RIGHT. In special cases, a table (base table, view, or joined table) can JOIN to itself in a self-join. [@via wikipedia]

SQL Joins are helpful when we have to fetch data with a single query from two or more database tables.

Case:

I need to store news/article information. The news can also have comments and tags. News can be submitted/posted by multiple users.

According to the above scenario, I have created 5 tables and inserted some data into them :-

news - to store news
comments - to store comments for any particular news
tags - to store tags associated with any particular news
users - to store user information
tags_news – to store relationship between news and tags

–
– Table structure for table `comments`
–

CREATE TABLE `comments` (
`id` bigint(20) NOT NULL auto_increment,
`news_id` bigint(20) NOT NULL,
`user_id` bigint(20) NOT NULL,
`detail` text NOT NULL,
`status` tinyint(1) NOT NULL default '1',
`comment_date` int(11) NOT NULL,
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`),
KEY `news_id` (`news_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;


–
– Dumping data for table `comments`
–

INSERT INTO `comments` (`id`, `news_id`, `user_id`, `detail`, `status`, `comment_date`) VALUES
(1, 1, 2, 'nice post :)', 1, 1273431296),
(2, 2, 2, 'hahaha', 1, 1273431420),
(3, 3, 2, '3123123', 1, 1273431452),
(4, 1, 2, 'thank You', 1, 1273431475),
(5, 2, 1, 'congratulations!', 1, 1273431500);

– ——————————————————–

–
– Table structure for table `news`
–

CREATE TABLE `news` (
`id` bigint(20) NOT NULL auto_increment,
`user_id` bigint(20) NOT NULL,
`title` text NOT NULL,
`detail` text NOT NULL,
`visit` int(11) NOT NULL default '0',
`status` tinyint(1) NOT NULL default '1',
`created_date` int(11) NOT NULL,
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

–
– Dumping data for table `news`
–

INSERT INTO `news` (`id`, `user_id`, `title`, `detail`, `visit`, `status`, `created_date`) VALUES
(1, 1, 'Extra Content', 'fasdfasdf', 2, 1, 1273431296),
(2, 1, 'My My Question', 'this is jpt question.. :D', 16, 1, 1273431389),
(3, 2, 'Am I ram?', 'I am ram..yahoo !!', 3, 1, 1273431420);

– ——————————————————–

–
– Table structure for table `tags`
–

CREATE TABLE `tags` (
`id` bigint(20) NOT NULL auto_increment,
`name` varchar(255) NOT NULL,
`slug` varchar(255) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

–
– Dumping data for table `tags`
–

INSERT INTO `tags` (`id`, `name`, `slug`) VALUES
(1, 'sagarmatha', 'sagarmatha'),
(2, 'nepal', 'nepal'),
(3, 'gautam buddha', 'gautam-buddha'),
(4, 'testing', 'testing'),
(5, 'tags', 'tags'),
(6, 'tasty apple', 'tasty-apple'),
(7, 'banana', 'banana');

– ——————————————————–

–
– Table structure for table `tags_news`
–

CREATE TABLE `tags_news` (
`id` bigint(20) NOT NULL auto_increment,
`news_id` bigint(20) NOT NULL,
`tags_id` bigint(20) NOT NULL,
PRIMARY KEY (`id`),
KEY `news_id` (`news_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

–
– Dumping data for table `tags_news`
–

INSERT INTO `tags_news` (`id`, `news_id`, `tags_id`) VALUES
(1, 1, 1),
(2, 1, 2),
(3, 1, 3),
(4, 2, 4),
(5, 2, 5),
(6, 2, 6),
(7, 3, 6),
(8, 3, 7);

– ——————————————————–

–
– Table structure for table `users`
–

CREATE TABLE `users` (
`id` bigint(20) NOT NULL auto_increment,
`user_id` bigint(20) NOT NULL,
`firstname` varchar(100) NOT NULL,
`lastname` varchar(100) NOT NULL,
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

–
– Dumping data for table `users`
–

INSERT INTO `users` (`id`, `user_id`, `firstname`, `lastname`) VALUES
(1, 1, 'Mukesh', 'Chapagain'),
(2, 2, 'Christopher', 'Gayle'),
(3, 3, 'Brian', 'Lara');

– ——————————————————–

Now, by using SQL JOINS, I can fetch data from these tables for different conditions and in different ways. Here follows the different SQL Queries with the magical JOIN statements:-

1) SELECT NEWS WITH COMMENTS COUNT FOR EACH NEWS


SELECT n.*, ifnull(c.count,0) AS comment FROM news AS n
LEFT JOIN
(SELECT COUNT(comments.id) AS count, comments.news_id FROM comments GROUP BY comments.news_id) AS c
ON n.id = c.news_id
ORDER BY n.created_date DESC;


2) SELECT NEWS POSTED BY ANY PARTICULAR USER (HERE, user_id = 1)


SELECT n.* FROM news AS n
INNER JOIN
users AS u
ON n.user_id = u.id
WHERE u.id = 1;


3) SELECT NEWS POSTED BY ANY PARTICULAR USER (WITH USER’S FIRSTNAME AND LASTNAME) (HERE, user_id = 1)


SELECT n.*, u.firstname AS firstname, u.lastname AS lastname FROM news AS n
INNER JOIN
users AS u
ON n.user_id = u.id
WHERE u.id = 1;


4) SELECT NEWS POSTED BY ANY PARTICULAR USER (WITH COMMENTS COUNT FOR EACH NEWS) (HERE, user_id = 1)


SELECT n.*, ifnull(c.count,0) AS comment, u.firstname AS firstname, u.lastname AS lastname FROM news AS n
LEFT JOIN
(SELECT COUNT(comments.id) AS count, comments.news_id FROM comments GROUP BY comments.news_id) AS c
ON n.id = c.news_id
INNER JOIN
users AS u
ON n.user_id = u.id
WHERE u.id = 1;


5) SELECT COMMENTS FOR ANY PARTICULAR NEWS (HERE, news_id =1)


SELECT c.* FROM comments AS c
INNER JOIN
news AS n
ON c.news_id = n.id
WHERE c.news_id = 1;


6) SELECT COMMENTS FOR ANY PARTICULAR NEWS (ALONG WITH USER INFORMATION) (HERE, news_id = 1)


SELECT c.*, u.firstname AS firstname, u.lastname AS lastname FROM comments AS c
INNER JOIN
news AS n
ON c.news_id = n.id
INNER JOIN
users AS u
ON c.user_id = u.id
WHERE c.news_id = 1;


7) SELECT TAGS FOR ANY PARTICULAR NEWS (HERE, news_id = 1)


SELECT t.* FROM tags AS t
INNER JOIN
(SELECT tags_news.news_id AS news_id, tags_news.tags_id FROM tags_news WHERE tags_news.news_id = 1) AS tn
ON t.id = tn.tags_id;


8) SELECT NEWS BY TAG NAME (HERE, tag = ‘nepal’)


SELECT n.*, t.name AS tag_name, t.slug AS tag_slug FROM news AS n
INNER JOIN
tags_news as tn
ON n.id = tn.news_id
INNER JOIN tags AS t
ON tn.tags_id = t.id
WHERE t.slug = 'nepal';


9) SELECT NEWS BY TAG NAME (ALONG WITH COMMENTS COUNT) (HERE, tag = ‘nepal’)


SELECT n.*, ifnull(c.count,0) AS comment, t.name AS tag_name, t.slug AS tag_slug, t.id AS tag_id FROM news AS n
INNER JOIN
tags_news AS tn
ON (n.id = tn.news_id)
INNER JOIN tags AS t
ON (tn.tags_id = t.id)
LEFT JOIN
(SELECT COUNT(comments.id) AS count, comments.news_id FROM comments
GROUP BY comments.news_id) AS c
ON n.id = c.news_id
WHERE t.slug = 'nepal'
ORDER BY n.created_date DESC;


10) SELECT TAG CLOUD


SELECT t . * , IFNULL( tq.count, 0 ) AS count
FROM tags AS t
LEFT JOIN (
SELECT COUNT( tags_news.id ) AS count, tags_news.tags_id
FROM tags_news
GROUP BY tags_news.tags_id
) AS tq ON t.id = tq.tags_id
ORDER BY t.name ASC
LIMIT 15;

Hope this helps. And thanks for reading.

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Category ids are stored for product in a comma separated way. So, to filter product collection by more than one category, you have to use:
addAttributeToFilter(‘category_ids’,array(‘finset’=>$categoryIds));

$categoryIds can be a single category id or comma separated category ids.

The following code filters product collection by two category ids (36 and 37).

 $_productCollection = Mage::getResourceModel('reports/product_collection')
    ->addAttributeToSelect('*')
    ->addAttributeToFilter('category_ids',array('finset'=>'36,37'));

finset does so… when we use finset, the mysql function find_in_set is used in the sql query by Magento.

mysql function find_in_set:

FIND_IN_SET() looks for the first occurrence of a string within another string containing comma-separated values.

SELECT FIND_IN_SET(‘b’,'a,b,c,d’); // result = 2

Cheers!

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But i got feedback that it was a bit complex for novice users of PHP (users who have just started PHPing). It was a kind of full-fledge system with PHP, MySQL and session.

So in this article, I have simplified the same functionality of the above tutorial. I have not used session in this case. If you want tutorial about session, login, and register then you can refer to my previous article (the link which is provided above).

Files included in this tutorial/code:

users.sql = This contains sql queries to make MySQL database and table

config.php = contains the database connection code (this file is included in all the php pages with the include() function)

index.php = contains the display part (displaying data from database)

add.html = contains form to add data

add.php = contains php code to add data

edit.php = contains php code to edit data

delete.php = contains php code to delete data

Download full source code: Create, read, update, delete in php and mysql

Cheers,

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Concatenate and fetch data from two fields of database and then break it again.

Solution:

Posted below.

The description is included in the code.

You have to make a mysql database. The database part is as under:

create database test;

use test;

create table user(id int(9) not null auto_increment, first_name varchar(20), last_name varchar(20), primary key(id));

insert into user(first_name, last_name) values ("mukesh","chapagain");

PHP code below:

I have not used any loop (for, while, foreach, etc.) here to make the code simple. :-)

<html>
<head>
<title>Fun with strings</title>
</head>

<body>
<?php

// in my configuration, i am running mysql in localhost, my database username is 'root' and i don't have any database password
// i have a table called 'user' in the database 'test'

// connecting to the database server
mysql_connect("localhost","root","");

// connecting to the database
mysql_select_db("test");

// selecting data from table named 'user' where id = 1
$result = mysql_query("select concat(first_name,',',last_name) as name from user where id = '1'");

// fetching the result and assigning it to certain variables
while ($row = mysql_fetch_assoc($result))
{
$name = $row['name'];
echo "$name <br/>"; // you will get the result as first_name,last_name
}
$newName = explode(",",$name); // this will make an array with each name as values

//print_r($newName);

// print the first name as
echo "First Name: " . $newName[0] . "<br/>";

// print the second name as
echo "Second Name: " . $newName[1] . "<br/>";

// you can find about how many elements/items are there with the commas
$count = count($newName);
echo "Count: $count <br/>";

// there is a function called 'implode' which will join array elements with a string
$newNameTwo = implode(" ",$newName); // this will print the names with spaces in between... before the names were separated with comma
echo "$newNameTwo <br/>";

?>
</body>
</html>

Happy PHPing!

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The description is present in the code.. in comments :D I hope those comments are sufficient to describe the code. The database part is in the sql file named ‘database.sql’.

I have attached the code with this post. Download it and enjoy PHPing…

Download full source code: PHP code for add,edit,delete,login, and register

Cheers,

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You have to make backup before you enter inside your MySQL system i.e. by typing mysql –u username –p password . (Here, username and password indicates your MySQL server username and password.)

Hence, go to your MySQL root path.

Type “mysqldump –u username –p password database_name > backup.sql” (without quote)

Here, database_name is the name of the database of which you want to create backup and backup.sql is the backup file name. You can name it as yourname.sql or whatever you like.

The file backup.sql created from the above procedure will be saved inside the ‘bin’ directory (root directory) of the MySQL server.

You can specify other location as well for the file backup.sql .

Type “mysqldump –u username –p password database_name > E:/backup.sql” (without quote)

This will save the file to the drive E [for windows OS]

Hope, unix, linux users know it well to specify location path.

Incase, your MySQL server password is blank, you need not have to type –p password.

You can type

mysqldump –u username database_name > backup.sql

Or, if your MySQL username and password both are blank than you can type

mysqldump database_name > backup.sql

2) To make a backup of multiple MySQL database

Type

mysqldump –u username –p password –databases db1 db2 db3 > threedb.sql

Here, db1, db2, db3 are three different database name. You can use more databases, similarly.

3) To make a backup of all MySQL databases

Type

mysqldump –u username –p password –all-databases > alldb.sql

4) To read back the backup we have made i.e. to create tables and databases from the backup file

If you had made a backup file of a single database, then you should create a database in your MySQL server. Then type “mysql –u username –p password db_name < backup.sql”

Here, db_name is the database name in your MySQL server. All tables are created inside this database from the backup.sql file.

If you had made a backup file of multiple databases, then you don’t have to create database beforehand. The backup file consists of the query to create all the databases and tables. Note it here, that you don’t have to specify the database name. You just have to type “mysql –u username –p password < backup.sql” and that’s all.

Note: Be careful to specify the proper location of your backup.sql file.

Enjoy!!!

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“Fails to start the service. Please wait 30 seconds to try again.”

I faced this problem 2 years back when I was a newbie to MySQL. I was just starting to learn PHP and MySQL at that time.

[PS: The problem was faced and tackled in Windows platform.]

Solution:

Well, there is my.ini file to control the server. It might be in your

C:/Documents and Settings/Administrator/WINDOWS/my.ini

[I suppose that your Operating System is installed in C: drive]

Here, “Administrator” is the user with the administrator privilege. This could be different as you might have the admin user with your name.

Copy the my.ini file from the above location to

C:/WINDOWS/my.ini

And you are done! Your mysql server will start quite happily.

Cheers,

Mukesh

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I had this problem and had a very tough and frustrating time with it. While I was installing a fresh copy of MySQL 5 in Windows XP, I got this problem. (I was installing it from the installer i.e. setup file.) The server was not starting and ‘error 0′ was displayed. I tried to start the server from the command line by typing “mysqld-nt –console” and then got the error “can’t create pid file: no such file or directory”. It was also displaying the message like “cannot create/write file C:\Program Files\MySQL\MySQL Server 5.0\data\com2.pid”.

Solution:

The main problem was my computer name. My computer name was ‘com2′. MySQL makes the file with the computer name in the directory of it’s data (in my case C:\Program Files\MySQL\MySQL Server 5.0\data\) . Now, it was unable to create a file named com2 (which was my computer name) because in WINDOWS you cannot make file/folder with the name com1,com2,com3… and so on.

Hence, I changed my computer name, restarted my computer and then reconfigured MySQL. All well done and MySQL started running.

Cheers,

Mukesh

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