Linux Administration

I’ve compiled a list of handy MySQL commands that I use very frequently  in a headless server. I am not a MySQL guru, so I decided to braindump everything in here. This covers random issues from WordPress slowness, Query Cache Modification, User Creation, Database dumping and importing, rsync’ing large files across servers and many more.

Most things are in random order, do Ctrl+F to search this page for keywords or use it as required to create your own personal Wiki page. I’ve leeched a lot of commands from multiple pages/websites and tried to credit them as much I could remember. If you migrate websites to different servers a lot like I do, you will definitely find this article very interesting.

At the bottom are statements, clauses, and functions you can use in MySQL. Below that are PHP and Perl API functions you can use to interface with MySQL. To use those you will need to build PHP with MySQL functionality. To use MySQL with Perl you will need to use the Perl modules DBI and DBD::mysql.

Below when you see # it means from the unix shell. When you see mysql> it means from a MySQL prompt after logging into MySQL.

So here goes random mysql commands in no particular order :)

Improving MySQL Query Cache

Important Note: From MySQL 5.6.8,query_cache_type is set to OFF by default. So if you haven’t explicitly turned it ON on old version, it may not work anymore!

Check current status of query_cache

mysql -e "show variables like 'query_cache_%'"

Will output something like:

+------------------------------+-----------+
| Variable_name | Value |
+------------------------------+-----------+
| query_cache_limit | 2097152 |
| query_cache_min_res_unit | 4096 |
| query_cache_size | 268435456 |
| query_cache_strip_comments | ON |
| query_cache_type | ON |
| query_cache_wlock_invalidate | OFF |
+------------------------------+-----------+

Changing Query Cache size:

mysql> SET GLOBAL query_cache_size = 16777216;

mysql> SHOW VARIABLES LIKE 'query_cache_size';

Output:

+------------------+----------+
| Variable_name    | Value    |
+------------------+----------+
| query_cache_size | 16777216 |

Query Cache Config

Add something like below following to your /etc/my/my.cnf

query_cache_size =16777216
query_cache_type = 1 
query_cache_limit = 2M 
query_cache_strip_comments =1

Fixing slow Query for the wp_options table

The reason the query is being logged is it doesn’t use an index. The query time is 0, i.e. it actually executes fast. You can unset the “log-queries-not-using-indexes” option if you don’t want these to be logged.

The wp_options table has no index on autoload, so the query ends up doing a full table scan. In general that table shouldn’t get too large, so it’s not a problem, but I’m guessing that’s somehow happened in your case.

Adding an index might solve the problem, but as TheDeadMedic pointed out in the comments, it might not if the values of autoload are either majority yes, or evenly distributed between yes and no:

First, do this query to see what the distribution looks like:

SELECT COUNT(*), autoload FROM wp_options GROUP BY autoload;

if a large majority of them are set to ‘no’, you can solve the problem for now by adding an index on autoload.

ALTER TABLE wp_options ADD INDEX (`autoload`);

Dumping one particular MySQL Database

mysqldump -u username -p --databases database_name> /tmp/server1_database_name.sql

Importing one particular MySQL Database

First create user and database

mysql -u root -p
create database database_name;
show databases;
use database_name;
CREATE USER 'username'@'localhost' IDENTIFIED BY 'somepasswordhere';
GRANT ALL PRIVILEGES ON databse_name.* TO 'username'@'localhost';
FLUSH PRIVILEGES;

Move database dump from server 1 to server 2:

cd /tmp
rsync -avz --progress server1_username@server1_ip_address:/tmp/database_name.sql .

Dont forget the last . in the above command.
Finally import that database you dumped in server 1.

mysql -u username -p database_name < server1_database_name.sql

To login (from unix shell) use -h only if needed.

# [mysql dir]/bin/mysql -h hostname -u root -p

Create a database on the sql server.

mysql> create database [databasename];

List all databases on the sql server.

mysql> show databases;

Switch to a database.

mysql> use [db name];

To see all the tables in the db.

mysql> show tables;

To see database’s field formats.

mysql> describe [table “” not found /]

;

To delete a db.

mysql> drop database [database name];

To delete a table.

mysql> drop table [table “” not found /]

;

Show all data in a table.

mysql> SELECT * FROM [table “” not found /]

;

Returns the columns and column information pertaining to the designated table.

mysql> show columns from [table “” not found /]

;

Show certain selected rows with the value “whatever”.

mysql> SELECT * FROM [table “” not found /]

 WHERE [field name] = "whatever";

Show all records containing the name “Bob” AND the phone number ‘3444444’.

mysql> SELECT * FROM [table “” not found /]

 WHERE name = "Bob" AND phone_number = '3444444';

Show all records not containing the name “Bob” AND the phone number ‘3444444’ order by the phone_number field.

mysql> SELECT * FROM [table “” not found /]

 WHERE name != "Bob" AND phone_number = '3444444' order by phone_number;

Show all records starting with the letters ‘bob’ AND the phone number ‘3444444’.

mysql> SELECT * FROM [table “” not found /]

 WHERE name like "Bob%" AND phone_number = '3444444';

Show all records starting with the letters ‘bob’ AND the phone number ‘3444444’ limit to records 1 through 5.

mysql> SELECT * FROM [table “” not found /]

 WHERE name like "Bob%" AND phone_number = '3444444' limit 1,5;

Use a regular expression to find records. Use “REGEXP BINARY” to force case-sensitivity. This finds any record beginning with a.

mysql> SELECT * FROM [table “” not found /]

 WHERE rec RLIKE "^a";

Show unique records.

mysql> SELECT DISTINCT [column name] FROM [table “” not found /]

;

Show selected records sorted in an ascending (asc) or descending (desc).

mysql> SELECT [col1],[col2] FROM [table “” not found /]

 ORDER BY [col2] DESC;

Return number of rows.

mysql> SELECT COUNT(*) FROM [table “” not found /]

;

Sum column.

mysql> SELECT SUM(*) FROM [table “” not found /]

;

Join tables on common columns.

mysql> select lookup.illustrationid, lookup.personid,person.birthday from lookup left join person on lookup.personid=person.personid=statement to join birthday in person table with primary illustration id;

Creating a new user. Login as root. Switch to the MySQL db. Make the user. Update privs.

# mysql -u root -p
mysql> use mysql;
mysql> INSERT INTO user (Host,User,Password) VALUES('%','username',PASSWORD('password'));
mysql> flush privileges;

Change a users password from unix shell.

# [mysql dir]/bin/mysqladmin -u username -h hostname.blah.org -p password 'new-password'

Change a users password from MySQL prompt. Login as root. Set the password. Update privs.

# mysql -u root -p
mysql> SET PASSWORD FOR 'user'@'hostname' = PASSWORD('passwordhere');
mysql> flush privileges;

Recover a MySQL root password. Stop the MySQL server process. Start again with no grant tables. Login to MySQL as root. Set new password. Exit MySQL and restart MySQL server.

# /etc/init.d/mysql stop
# mysqld_safe --skip-grant-tables &
# mysql -u root
mysql> use mysql;
mysql> update user set password=PASSWORD("newrootpassword") where User='root';
mysql> flush privileges;
mysql> quit
# /etc/init.d/mysql stop
# /etc/init.d/mysql start

Set a root password if there is on root password.

# mysqladmin -u root password newpassword

Update a root password.

# mysqladmin -u root -p oldpassword newpassword

Allow the user “bob” to connect to the server from localhost using the password “passwd”. Login as root. Switch to the MySQL db. Give privs. Update privs.

# mysql -u root -p
mysql> use mysql;
mysql> grant usage on *.* to bob@localhost identified by 'passwd';
mysql> flush privileges;

Give user privilages for a db. Login as root. Switch to the MySQL db. Grant privs. Update privs.

# mysql -u root -p
mysql> use mysql;
mysql> INSERT INTO db (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv,Create_priv,Drop_priv) VALUES ('%','databasename','username','Y','Y','Y','Y','Y','N');
mysql> flush privileges;

or

mysql> grant all privileges on databasename.* to username@localhost;
mysql> flush privileges;

To update info already in a table.

mysql> UPDATE [table “” not found /]

 SET Select_priv = 'Y',Insert_priv = 'Y',Update_priv = 'Y' where [field name] = 'user';

Delete a row(s) from a table.

mysql> DELETE from [table “” not found /]

 where [field name] = 'whatever';

Update database permissions/privilages.

mysql> flush privileges;

Delete a column.

mysql> alter table [table “” not found /]

 drop column [column name];

Add a new column to db.

mysql> alter table [table “” not found /]

 add column [new column name] varchar (20);

Change column name.

mysql> alter table [table “” not found /]

 change [old column name] [new column name] varchar (50);

Make a unique column so you get no dupes.

mysql> alter table [table “” not found /]

 add unique ([column name]);

Make a column bigger.

mysql> alter table [table “” not found /]

 modify [column name] VARCHAR(3);

Delete unique from table.

mysql> alter table [table “” not found /]

 drop index [colmn name];

Load a CSV file into a table.

mysql> LOAD DATA INFILE '/tmp/filename.csv' replace INTO TABLE [table “” not found /]

 FIELDS TERMINATED BY ',' LINES TERMINATED BY '\n' (field1,field2,field3);

Dump all databases for backup. Backup file is sql commands to recreate all db’s.

# [mysql dir]/bin/mysqldump -u root -ppassword --opt >/tmp/alldatabases.sql

Dump one database for backup.

# [mysql dir]/bin/mysqldump -u username -ppassword --databases databasename >/tmp/databasename.sql

Dump a table from a database.

# [mysql dir]/bin/mysqldump -c -u username -ppassword databasename tablename > /tmp/databasename.tablename.sql

Restore database (or database table) from backup.

# [mysql dir]/bin/mysql -u username -ppassword databasename < /tmp/databasename.sql

Create Table Example 1.

mysql> CREATE TABLE [table “” not found /]

 (firstname VARCHAR(20), middleinitial VARCHAR(3), lastname VARCHAR(35),suffix VARCHAR(3),officeid VARCHAR(10),userid VARCHAR(15),username VARCHAR(8),email VARCHAR(35),phone VARCHAR(25), groups VARCHAR(15),datestamp DATE,timestamp time,pgpemail VARCHAR(255));

Create Table Example 2.

mysql> create table [table “” not found /]

 (personid int(50) not null auto_increment primary key,firstname varchar(35),middlename varchar(50),lastnamevarchar(50) default 'bato');

Sources:

• https://mariadb.com/kb/en/mariadb/sql-commands/
• https://www.pantz.org/software/mysql/mysqlcommands.html
• https://dev.mysql.com/doc/refman/5.7/en/mysql-commands.html
• https://easyengine.io/tutorials/mysql/query-cache/
• https://wordpress.stackexchange.com/questions/71691/slow-query-for-the-wp-options-table