null mysql

MySQL administrator Guide MySQL database IntroductionMySQL is a real multi-user, multi-thread SQL database server. SQL (Structured Query Language) is the world's most popular and standardized database language. MySQL is implemented in a

Standard SQL mode matching SQL pattern matching allows you to use "_" to match any single character, while "%" matches any number of characters (including zero characters ). In MySQL, the SQL mode is case-insensitive by default. Some examples are

//. H /////////////////////////////////////// ///////////////////////////////////////// // Cppmysql-a c ++ wrapper around the MySQL database library. // // Copyright (c) 2009 Rob Groves. All Rights Reserved. lizp.net@gmail.com // // Permission to

Ifnull (expr1, expr2) If expr1 is not null, the returned value of ifnull () is expr1;Otherwise, the returned value is expr2. The returned value of ifnull () is a number or string, depending on the context in which it is used. Mysql> select ifnull (1,

Before introducing these SQL languages, list the common mysql Data Types and Data Type modifications for query reference.The length of a field of this type is indicated by a number. Numeric type: TINYINT 1, SMALLINT 2, MEDIUMINT 3, INT 4,

The last time Mysql5.6 was installed, an exception occurred after a period of time. The Mysql service could not start the process and ended: 1067. Many methods have been found on the internet, but this problem still could not be solved. I

MySQL Optimization-Delete Insert update and mysql optimization insert update Reprinted please indicate the source: http://blog.csdn.net/l1028386804/article/details/46581769 Insert Insert multi-row query results into the table Syntax INSERT INTO

Install Cacti on CentOS7 Cacti is an excellent open-source monitoring software that is implemented in php. It uses the snmp service to obtain data and then uses rrdtool to store and update data. When users view data, rrdtool generates charts and

MySQL supports Partitioned Tables from version 5.1.x to the latest version 5.1.56, which is also a stable MySQL version. In addition, MySQL 5.5 supports range columns and list columns partitions. That is to say, non-integer COLUMNS no longer need to

Experiment document: Process: Create a table-backup-simulate downtime (delete a table)-restore a backup-restore the current data.1. Restore and restore the experiment Create a database and use the drop statement to simulate database downtime. Use

Use case First, let's look at an example: There is a user table User (username VARCHAR (5) character set utf8COLLATE utf8_general_ci ); Record ('username'); and ('username '); Select * from User where username = 'username'; two data items are

I would like to summarize several other experiments on high-availability clusters. Let's look back at the content summarized by others. It's very fine, and a lot of content has been extended. Sorry, I just skipped it, haha ~~~ Here we will summarize

Before installing keepalived, check whether the host has been installed. If ps-ef | grep keepalive is not checked, it is easy to overwrite the previously installed content, in this case, the configuration files we used to get are not much

1. What is an index? Indexes are used to quickly search for records with specific values. All MySQL indexes are saved as B-trees. If no index exists, MySQL must scan all the records of the entire table from the first record until the required

// Primary key Alter table tablename add new_field_id int (5) unsigned default 0 not null anto_increment, add primary key (new_field_id ); // Add a new columnMysql> alter table tablename ADD fieldname fieldtypeFor example:Alter table t2 ADD d

When group_concat and Concat are mentioned above, if the column value is null, the value after Concat is null. Another problem is found. If Column A is null and column B is null, the sum of the number of records of column A <> B and A = B is not

Update the null field value to 'none'The first thought is:Update 'client' set 'Company' = 'none' WHERE 'Company' = NULLResult errorThe correct method IS to use the is null condition.Update 'client' set 'Company' = 'none' WHERE 'Company' IS

1. unix_timestampConverts a time to a timestamp. Convert data of the date type to the timestamp formatThe timestamp of the current time is used by default.Mysql> select unix_timestamp ();+ ------ +| Unix_timestamp () |+ ------ +| 1, 1436775578 |+ ---

File service:Ftp: Application layer, C/S, file sharing; file transfer protocol;Nfs, cifs: file system interface, network file system;Nfs: network file systemCifs: common internet file systemSambaNetwork Storage:NAS: Network Attached Storage, file

  From: http://waryist.javaeye.com/blog/104739Waryist blog    Alter [ignore] Table tbl_name alter_spec [, alter_spec...]Alter_specification:Add [column]