MYSQL - PRIMARY KEY DESCRIPTION

A PRIMARY KEY is a unique index where all key columns must be defined as NOT NULL. If they are not explicitly declared as NOT NULL, MySQL declares them so implicitly (and silently). A table can have only one PRIMARY KEY. The name of a PRIMARY KEY is always PRIMARY, which thus cannot be used as the name for any other kind of index.
If you do not have a PRIMARY KEY and an application asks for the PRIMARY KEY in your tables, MySQL returns the first UNIQUE index that has no NULL columns as the PRIMARY KEY.

Comments

Post a Comment

Popular posts from this blog

Physical Representation of MYSQL Tables

MySQL represents each table by an  .frm  table format (definition) file in the database directory. The storage engine for the table might create other files as well. For  InnoDB  tables, the file storage is controlled by the  innodb_file_per_table  configuration option. When this option is turned off, all  InnoDB  tables and indexes are stored in the  system tablespace , represented by one or more  ibdata* files . For each  InnoDB  table created when this option is turned on, the table data and all associated indexes are stored in a  .ibd file  located inside the database directory. For  MyISAM  tables, the storage engine creates data and index files. Thus, for each  MyISAM  table  tbl_name , there are three disk files. File Purpose tbl_name .frm Table format (definition) file tbl_name .MYD Data file tbl_name .MYI Index file
Read more
  18 Commands to Monitor Network Bandwidth on Linux server https://www.binarytides.com/linux-commands-monitor-network/ Follow the above link .
Read more