What is the default Maximum file upload size and how to change it?

By default Maximum file upload size is 128MB.

php_value upload_max_filesize 128M
php_value post_max_size 128M

If you want to change the default Maximum file upload size from 128MB to 1GB, then it can be done by changing in the these files..

1.) In .htaccess file (Apache + mod_php) add the following two lines to a .htaccess in the installed directory:

php_value upload_max_filesize 1G
php_value post_max_size 1G

2.) In php.ini file, which is located in the php folder (like C:/xampp/php/php.ini)

upload_max_filesize = 1G
post_max_size = 1G

After modifying the global php.ini you will need to restart the webserver to have changes take effect.

In MySQL, what are the differences between primary, foreign, and unique keys?

The one thing that primary, unique, and foreign keys all have in common is the fact that each type of key can consist of more than just one column from a given table. In other words, foreign, primary, and unique keys are not restricted to having just one column from a given table – each type of key can cover multiple columns. Of course, the database programmer is the one who will define which columns are covered by a foreign, primary, or unique key. That is one similarity all those keys share, but there are some major differences that exist between primary, unique, and foreign keys. Let’s go over those differences. We also give a thorough explanation of why foreign keys are necessary in some situations.

Can a table have multiple unique, foreign, and/or primary keys?

A table can have multiple unique and foreign keys. However, a table can have only one primary key.

Can a unique key have NULL values? Can a primary key have NULL values?

Unique key columns are allowed to hold NULL values. The values in a primary key column, however, can never be NULL.

Can a foreign key reference a non-primary key?

Yes, a foreign key can actually reference a key that is not the primary key of a table. But, a foreign key must reference a unique key.

Can a foreign key contain null values?

Yes, a foreign key can hold NULL values. Because foreign keys can reference unique, non-primary keys – which can hold NULL values – this means that foreign keys can themselves hold NULL values as well. Continue reading

What is the differences between InnoDB and MyISAM?

InnoDB

MyISAM

InnoDB implements row-level locking. MyISAM implements table-level locking.
Automatic Crash Recovery. No Crash Recovery
InnoDB doesn’t have FULLTEXT search indexes. MyISAM have FULLTEXT search indexes.
InnoDB also implements transactions, foreign keys and relationship constraints. MyISAM does not support transactions, foreign keys and relationship constraints.
InnoDB stores both data and indexes in one file. MyISAM stores indexes in one file and data in another.
InnoDB uses a buffer pool to cache both data and indexes. MyISAM uses key buffers for caching indexes and leaves the data caching management to the operating system.

In InnoDB the COUNT(*)s (when WHERE, GROUP BY, or JOIN is not used) execute slower than in MyISAM because the row count is not stored internally.

Overall I would recommend InnoDB for most purposes and MyISAM for specialized uses only. InnoDB is now the default engine in new MySQL versions.