My SQL Interview Questions and Answers

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.

MySQL - How the Privilege System Works
The MySQL privilege system ensures that all users may do exactly the things that they are supposed to be allowed to do. When you connect to a MySQL server, your identity is determined by the host from which you connect and the user name you specify. The system grants privileges according to your identity and what you want to do.

MySQL considers both your hostname and user name in identifying you because there is little reason to assume that a given user name belongs to the same person everywhere on the Internet. For example, the user bill who connects from whitehouse.gov need not be the same person as the user bill who connects from microsoft.com. MySQL handles this by allowing you to distinguish users on different hosts that happen to have the same name: you can grant bill one set of privileges for connections from whitehouse.gov, and a different set of privileges for connections from microsoft.com.

MySQL access control involves two stages:

Stage 1: The server checks whether or not you are even allowed to connect.
Stage 2: Assuming you can connect, the server checks each request you issue to see whether or not you have sufficient privileges to perform it. For example, if you try to select rows from a table in a database or drop a table from the database, the server makes sure you have the select privilege for the table or the drop privilege for the database. The server uses the user, db, and host tables in the mysql database at both stages of access control. The fields in these grant tables are shown below:

Table name user db host
Scope fields Host Host Host
User Db Db
Password User
Privilege fields Select_priv Select_priv Select_priv
Insert_priv Insert_priv Insert_priv
Update_priv Update_priv Update_priv
Delete_priv Delete_priv Delete_priv
Index_priv Index_priv Index_priv
Alter_priv Alter_priv Alter_priv
Create_priv Create_priv Create_priv
Drop_priv Drop_priv Drop_priv
Grant_priv Grant_priv Grant_priv
References_priv
Reload_priv
Shutdown_priv
Process_priv
File_priv

For the second stage of access control (request verification), the server may, if the request involves tables, additionally consult the tables_priv and columns_priv tables. The fields in these tables are shown below:

Table name tables_priv columns_priv
Scope fields Host Host
Db Db
User User
Table_name Table_name
Column_name
Privilege fields Table_priv Column_priv
Column_priv
Other fields Timestamp Timestamp
Grantor

Each grant table contains scope fields and privilege fields.

Scope fields determine the scope of each entry in the tables, that is, the context in which the entry applies. For example, a user table entry with Host and User values of 'thomas.loc.gov' and 'bob' would be used for authenticating connections made to the server by bob from the host thomas.loc.gov. Similarly, a db table entry with Host, User, and Db fields of 'thomas.loc.gov', 'bob' and 'reports' would be used when bob connects from the host thomas.loc.gov to access the reports database. The tables_priv and columns_priv tables contain scope fields indicating tables or table/column combinations to which each entry applies.

For access-checking purposes, comparisons of Host values are case insensitive. User, Password, Db, and Table_name values are case sensitive. Column_name values are case insensitive in MySQL Version 3.22.12 or later.

Privilege fields indicate the privileges granted by a table entry, that is, what operations can be performed. The server combines the information in the various grant tables to form a complete description of a user's privileges.

Scope fields are strings, declared as shown below; the default value for each is the empty string:

Field name Type
Host CHAR(60)
User CHAR(16)
Password CHAR(16)
Db CHAR(64) (CHAR(60) for the tables_priv and columns_priv tables)
Table_name CHAR(60)
Column_name CHAR(60)

In the user, db and host tables, all privilege fields are declared as ENUM('N','Y') -- each can have a value of 'N' or 'Y', and the default value is 'N'.

In the tables_priv and columns_priv tables, the privilege fields are declared as SET fields:

Table name Field name Possible set elements
tables_priv Table_priv 'Select', 'Insert', 'Update', 'Delete', 'Create', 'Drop', 'Grant', 'References', 'Index', 'Alter'
tables_priv Column_priv 'Select', 'Insert', 'Update', 'References'
columns_priv Column_priv 'Select', 'Insert', 'Update', 'References'

Briefly, the server uses the grant tables like this:

The user table scope fields determine whether to allow or reject incoming connections. For allowed connections, any privileges granted in the user table indicate the user's global (superuser) privileges. These privileges apply to all databases on the server.
The db and host tables are used together:
The db table scope fields determine which users can access which databases from which hosts. The privilege fields determine which operations are allowed.
The host table is used as an extension of the db table when you want a given db table entry to apply to several hosts. For example, if you want a user to be able to use a database from several hosts in your network, leave the Host value empty in the user's db table entry, then populate the host table with an entry for each of those hosts.

The tables_priv and columns_priv tables are similar to the db table, but are more fine-grained: they apply at the table and column levels rather than at the database level.
Note that administrative privileges (reload, shutdown, etc.) are specified only in the user table. This is because administrative operations are operations on the server itself and are not database-specific, so there is no reason to list such privileges in the other grant tables. In fact, only the user table need be consulted to determine whether or not you can perform an administrative operation.

The file privilege is specified only in the user table, too. It is not an administrative privilege as such, but your ability to read or write files on the server host is independent of the database you are accessing.

When you modify the contents of the grant tables, it is a good idea to make sure that your changes set up privileges the way you want.

A useful diagnostic tool is the mysqlaccess script, which Yves Carlier has provided for the MySQL distribution. Invoke mysqlaccess with the --help option to find out how it works. Note that mysqlaccess checks access using only the user, db and host tables. It does not check table- or column-level privileges.

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