Privileges Provided by MySQL

Interview Questions

MySQL and SQL

Privileges Provided by MySQL

Information about user privileges is stored in the user, db, host, tables_priv, and columns_priv tables in the mysql database (that is, in the database named mysql).

The names used in this manual to refer to the privileges provided by MySQL are shown below, along with the table column name associated with each privilege in the grant tables and the context in which the privilege applies:

Privilege Column Context
select Select_priv tables
insert Insert_priv tables
update Update_priv tables
delete Delete_priv tables
index Index_priv tables
alter Alter_priv tables
create Create_priv databases, tables, or indexes
drop Drop_priv databases or tables
grant Grant_priv databases or tables
references References_priv databases or tables
reload Reload_priv server administration
shutdown Shutdown_priv server administration
process Process_priv server administration
file File_priv file access on server

The select, insert, update, and delete privileges allow you to perform operations on rows in existing tables in a database.
SELECT statements require the select privilege only if they actually retrieve rows from a table. You can execute certain SELECT statements even without permission to access any of the databases on the server. For example, you could use the mysql client as a simple calculator:

mysql> SELECT 1+1;
mysql> SELECT PI()*2;

The index privilege allows you to create or drop (remove) indexes.

The alter privilege allows you to use ALTER TABLE.

The create and drop privileges allow you to create new databases and tables, or to drop (remove) existing databases and tables.

Note that if you grant the drop privilege for the mysql database to a user, that user can drop the database in which the MySQL access privileges are stored!

The grant privilege allows you to give to other users those privileges you yourself possess.

The file privilege gives you permission to read and write files on the server using the LOAD DATA INFILE and SELECT ... INTO OUTFILE statements. Any user to whom this privilege is granted can read or write any file that the MySQL server can read or write.

The remaining privileges are used for administrative operations, which are performed using the mysqladmin program. The table below shows which mysqladmin commands each administrative privilege allows you to execute:

Privilege Commands permitted to privilege holders reload reload, refresh, flush-privileges, flush-hosts, flush-logs, and flush-tables
shutdown shutdown
process processlist, kill

The reload command tells the server to re-read the grant tables. The refresh command flushes all tables and opens and closes the log files. flush-privileges is a synonym for reload. The other flush-* commands perform functions similar to refresh but are more limited in scope, and may be preferable in some instances. For example, if you want to flush just the log files, flush-logs is a better choice than refresh.

The shutdown command shuts down the server.

The processlist command displays information about the threads executing within the server. The kill command kills server threads. You can always display or kill your own threads, but you need the process privilege to display or kill threads initiated by other users.

It is a good idea in general to grant privileges only to those users who need them, but you should exercise particular caution in granting certain privileges:

The grant privilege allows users to give away their privileges to other users. Two users with different privileges and with the grant privilege are able to combine privileges.
The alter privilege may be used to subvert the privilege system by renaming tables.
The file privilege can be abused to read any world-readable file on the server into a database table, the contents of which can then be accessed using SELECT. This includes the contents of all databases hosted by the server!
The shutdown privilege can be abused to deny service to other users entirely, by terminating the server.
The process privilege can be used to view the plain text of currently executing queries, including queries that set or change passwords.
Privileges on the mysql database can be used to change passwords and other access privilege information. (Passwords are stored encrypted, so a malicious user cannot simply read them to know the plain text password). If they can access the mysql.user password column, they can use it to log into the MySQL server for the given user. (With sufficient privileges, the same user can replace a password with a different one.)
There are some things that you cannot do with the MySQL privilege system:

You cannot explicitly specify that a given user should be denied access. That is, you cannot explicitly match a user and then refuse the connection. You cannot specify that a user has privileges to create or drop tables in a database but not to create or drop the database itself.

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