mysql alter column not null

column that represents a particular meaning, not just a null value. For these special types of columns, the main reader is to remember two. One is the author of the above timestamp data type. If you insert a null value into a column of this data type, it represents the current time of the system. The other is a

MySQL field constraints: null, notnull, default, auto_increment bitsCN.com MySQL field constraints: null, not null, default, auto_increment In MySQL, each field definition contains additional constraints or modifiers, which can

Tags: des using ar file data problem amp time onI encountered a problem when I was developing a project within the company:Select student_quality_id from student_quality where Mark_status=0 and batch_stauts in (2,3)The result encountered a student_quality_id that had been unable to find a qualifying condition, and later found no consideration of the null value, modified to select student_quality_id from Student_quality where (Mark_ Status=0 or mark_st

Mysql adds a column to modify the column name attribute and deletes the column bitsCN.com First, I would like to recommend a book for beginners, a booklet: "mysql must know", which is practical and authoritative. it was a little expensive and I bought it yesterday. Http

Summary I haven't written SQL statements for a long time. I have a question in the Q A today and I will study it. Problem link: about Mysql's hierarchical output problem scenario overview the scores recorded in the school. each person chooses different courses and will add courses later, so you do not need to take all courses as a column. Data Overview I haven't written an SQL statement for a long time. I have a question in the Q A today. I'll take

In many cases, the database defaults are null, but the case is that the database values are empty rather than NULL when the program is processed. Note When a unique index is created, when the database will empty as multiple duplicate values, and the index failed to create the following example: Step 1: Mysql> Select Phone, COUNT (1) from User Group by phone;+--

Tags: primary key object des SYS exist ODI one-key cmd atiRecently in the collation of things about MySQL, to write down some things to record, in order to query and browse later, the following are some of the operational skills.Add a table fieldALTER TABLE table1 add transactor varchar (ten) not Null;ALTER TABLE table1 add ID int unsigned not

An interesting thing was found during the recent writing process. Mysql procedure has its own unique processing method when passing parameters. For example, if the parameter is defined as an int, the result is null.Mysql procedure runs normally.Database table structure:Create database db5;Use db5;Drop table if exists t;Create table t (Id int primary key auto_increment,Value int);Create table t2 (Id int primary key auto_increment,Value float);Create pr

Mysql| reference This appendix describes each column type provided by MySQL. See chapter 2nd for detailed instructions on taking advantage of each type. Unless otherwise stated, the listed types are already available in MySQL3.21.0. A description of the type name is given in the following conventions: square brackets ([]) optional information. M Maximum display w

of 8 are 1 = and are converted to 2 ^ 8-1 in decimal format. The computer can also store negative numbers and use the highest bit as the symbol bit. 0 indicates a positive number. 1 indicates a negative number. When the highest bit is 0: [0] [] [] [] [] [] [] [] [] => 0 ~ 2 ^ 7-1 convert to decimal 0 ~ 127 When the highest bit is 1 [1] [] [] [] [] [] [] [] [] => 0 ~ -2 ^ 7 to decimal 0 ~ -128 Column Bytes Unsigned Signed Ti

This article describes how to use dynamic row-to-column conversion in the MySQL stored procedure. For more information, see the examples described in this article. The following is an example of a simple database. Data table structure Here, I will use a simple example to illustrate it. it is also a classic example of row-to-column conversion, that is, the score

Indexes have an immediate effect on raising the query speed of select/update statements, indexed and non-indexed, and the query speed is often several orders of magnitude.This discussion uses the scenario for index (each column as an index, a single row index) and Multiple-column Indexes (multiple columns as an index, up to 16 columns, composite index).Common ways to create or add indexes:Way one, build the

This article describes how to insert duplicate values in the unique column in mysql. it is mainly based on the mysql platform. through these solutions, you can implement some new functions and applications. This article will be shared here with developers. When unique columns on a UNIQUE key to insert records containing duplicate values, we can control how

You need to update the field value to a null value of ' None ' The first way to think about it is Update ' client ' Set ' company ' = ' None ' WHERE ' company ' = NULL Error results The correct approach is to use the IS null condition Update ' client ' Set ' company ' = ' None ' WHERE ' Company ' is NULL Example

Mysql basics: mysql column type-string-CHAR, which is a synonym for CHAR (1. · [NATIONAL] VARCHAR (M) [BINARY] variable-length string. M indicates the maximum column length. M ranges from 0 to 65,535. (The maximum actual length of a VARCHAR is determined by the maximum row size and the character set used. The maximum v

Tongda OA mysql adds a column of commands at a certain position in the table to be judged by a program. You have to write a document and use a program in the middle to determine whether a Column exists. If it does not exist, add the document. If it does not exist, simply display a result. The written program is as follows: If exists (select * from information_s

Tags: sign 0.00 content Err char ror CTE Ann CreatNULL and NOT NULL mysql> CREATE TABLE tb2(-Usename VARCHAR (a) not NULL,-Age TINYINT UNSIGNED null (can be empty));Query OK, 0 rows affected (0.06 sec) Mysql> SHOW COLUMNS from TB2;+---------+---------------------+------+----

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 s

CIn MySQL, how to change the column declaration.Modify table-Modify column namesUse change column to modify the names of columns, and you must also set the data type of the columnsMysql> desc Test_tab//+-------+-------------+------+-----+---------+-------+|Field |Type |Null

Trap 1: NULL values are not necessarily emptyIn MySQL databases, null represents a special meaning for some special types of columns, not just a null value. For these special types of columns, you must remember two types. One is what I mentioned above.Times ***** p Data classType. If a