mysql alter column not null

Note for processing NULL values in Mysql integer operations: mysqlnull When CleverCode was recently exporting a report, it was found that the integer was null after the integer was subtracted. This is a matter of detail. I hope you will pay attention to it later. Total and used values in Table 1 are all integer values, which can be

Today, I found in mysql that there is such a select * from bb order by null sentence that cannot be seen, later, I found out that order by null is used to forcibly disable sorting of query results. Order by null is used to forcibly disable sorting of query results. When you use explain to check the query statement, if

This is the case: There are several fields in the user tableusernamechar (+) not NULL COMMENT ' user name ',passwordchar (+) not NULL COMMENT ' password ',emailchar (+) not NULL COMMENT ' user mailbox ',mobilechar (+) not NULL COMMENT ' user phone ', Username Email Mobile is unique, so I want to create a unique index

Note 1 For mysql count distinct null: An example is used to explain the problem. Now another database table is hello. The table content is as follows: www.2cto.com id name 1 Null2 Null3 Null4 Null5 Null hello table has two fields: id and name, name is null. The first SQL statement www.2cto.com: SELECT COUNT (id) FROM h

Tags: restrict check CAD ICT allow NOT NULL setting mode. Cascade WaySynchronize update/delete out a matching record of a child table when Update/delete records on the parent table. Set NULL modeSet the column of the matching record on the child table to NULL when the record is update/delete on the parent tableNote tha

Tags: share integer http SSO get img tab Color tail Clevercode recently when exporting a report, when the integer is subtracted, the integer minus null is found to be null. This is a matter of detail, I hope you will pay attention later. 1 The data total,used in the table are all shaped and agreed to be empty. 2 problematic arithmetic statement subtraction problemSelect ID, total , used, (total-used)

Tags: style http color io strong for AR Art Div www.169it.comAdd a table fieldALTER TABLE table1 add transactor varchar (ten) not Null;ALTER TABLE table1 add ID int unsigned not Null auto_increment primary keyModify the field type of a table and specify empty or non-emptyALTER TABLE name change field Name f

Reprinted from: http://blog.163.com/inflexible_simple/blog/static/1676946842011616102543931/ FOREIGN key constraint meaning for child table: If a candidate key is not found in the parent table, INSERT/UPDATENBSP is not allowed on the child table; The meaning of the foreign key constraint to the parent table: when update/delete on a parent table to update or delete a candidate key that has one or more matching rows in the child table, the behavior of the parent table is determined by the

The ORDER by null purpose is to force the ordering to be disabled for query results. When you check the query by explain, if you see a using Filesort in the extra column, this has a certain effect on performance, especially when group by is used, even if the specified order By,mysql you do not display is sorted by default by group field. In some cases it is not n

Implementation of dynamic row-to-column conversion in MySQL: bitsCN.com The web pages are static and implemented using the case when structure. So I wrote a dynamic one. SP code: DELIMITER $ Drop procedure if exists 'test'. 'sp _ row_column_wrap '$ Create definer = 'root' @ 'localhost' PROCEDURE 'sp _ row_column_wrap '(IN $ schema_name varchar (64 ),IN $ table_name varchar (64 ))BEGINDeclare cnt int (11 );D

MySQL column type: integer http://blog.csdn.net/jk110333/article/details/9342283 MySQL column type -- time and date http://blog.csdn.net/jk110333/article/details/9342319 · Char This is a synonym for char (1. · [National] varchar (M) [Binary] Variable-length string.MMaximum column