mysql alter column not null

faster than string operations. Short strings run faster than long strings, and consume less disk. The fixed length type has better performance than the variable length type.What kind of comparison do you want? Strings can be case-sensitive or case-insensitive. The selection also affects sorting because it is based on comparison.Are you planning to index columns? If you plan to index a column, your selection of the

' 0000-00-00 00:00:00 ') Engine=innodb DEFAULT Charset=utf8; Mysql> select * from T1;Empty Set (0.00 sec) mysql> INSERT INTO T1 (create_time) values (null);ERROR 1048 (23000): Column ' create_time ' cannot be nullMysql> select * from T1;Empty Set (0.00 sec) Direct error, intercept statement insertion. In 5.6, this bug

In mysql, null, not null, default, and auto_increment are common content. Here I will detail the usage of null, not null, default, and auto_increment in msyql. NULL and not null modifie

group by phone;+ ----------------- + ---------- +| Phone | count (1) |+ ----------------- + ---------- +| NULL | 70 || 40 || + 86-13390889711 | 1 || + 86-13405053385 | 1 | In step 1, 70 null data records and 40 null data records are found in the database. Step 2: Mysql> select count (1) from User where phone is

Label:Believe that a lot of people who have been using MySQL for a long time, the concept of these two field properties is not very clear, the general will have the following questions: My field type is not NULL, why can I insert a null value is less efficient than null for a gross NOT

Believe that a lot of people who have been using MySQL for a long time, the concept of these two field properties is not very clear, the general will have the following questions:1, my field type is not NULL, why I can insert null value2, the efficiency of the wool NOT NULL is higher than null3, when the judgment field

/hw_libo/article/details/40097125 The following experiment confirms:2. MyISAM table MySQL [bosco]> CREATE TABLE `t5` ( -> `id` int(10) unsigned NOT NULL AUTO_INCREMENT, -> PRIMARY KEY (`id`) -> ) ENGINE=MyISAM;Query OK, 0 rows affected (0.05 sec)MySQL [bosco]> insert into t5 values(null);Query OK, 1 row af

Believe that a lot of people who have been using MySQL for a long time, the concept of these two field properties is not very clear, the general will have the following questions:1, my field type is not NULL, why I can insert null value2, the efficiency of the wool NOT NULL is higher than null3, when the judgment field

Before adding a column, mysql checks whether bitsCN.com exists in the column. Mysql checks whether the column exists before adding a column Mysql does not have a direct syntax to d

default character set utf8; // modify the character set of the table. If you want to change the default character set and all character columns (CHAR, VARCHAR, TEXT) of the table to the new character set, use the following statement: alter table tbl_name convert to character set charset_name; warning: the previous operation converted the column type between CHARACTER sets. If one

NULL and NOT NULL modifiers: You can add this null or NOT NULL modifier to each field to specify whether the field can be empty (null), or you must fill in the data (not NULL). mysql s

In MySQL, each field definition contains additional constraints or modifiers that can be used to increase the constraints on the input data. Today, let's take a look at MySQL field constraints:NULLAndNot NULLmodifiers,DEFAULTModifier,auto_incrementModifier.null and NOT NULL modifiers: You can add this null or NOT

Believe that a lot of people who have been using MySQL for a long time, the concept of these two field properties is not very clear, the general will have the following questions:1, my field type is not NULL, why I can insert null value2, the efficiency of the wool NOT NULL is higher than null3, when the judgment field

Tags: style http color using IO strong for AR dataAbout MySQLMySQL is an open source small relational database management system, the developer of the Swedish MySQL AB company. MySQL is widely used in small and medium-sized websites on the internet. Because of its small size, fast speed, low total cost of ownership, especially the open source, many small and medium-sized web sites in order to reduce the tot

Precautions for using the AUTO_INCREMENT column in MySQL: update auto-incrementing Column1. Description (1) For the MyISAM table, if you use UPDATE to UPDATE the auto-incrementing column, if the column value is the same as the existing value, an error occurs. If the column v

Explicit_defaults_for_timestampMySQL5.6 Version IntroductionExplicit_defaults_for_timestampto control the timestamp NULL handling of Values timestamp not NULL insert null value, no error, no warning if the my.cnf Zhong Explicit_defaults_for_timestamp =1The value will be inserted when the error indicates that the colum